10405 Implement ZFS sorted scans

Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: Kody Kantor <kody.kantor@joyent.com>
Approved by: Dan McDonald <danmcd@joyent.com>

45 files changed, 3258 insertions, 924 deletions

diff --git a/usr/src/cmd/zdb/zdb.c b/usr/src/cmd/zdb/zdb.c
index 3d0490b73e..16c8c60df1 100644
--- a/usr/src/cmd/zdb/zdb.c
+++ b/usr/src/cmd/zdb/zdb.c
@@ -2386,8 +2386,6 @@ dump_dir(objset_t *os)
 		max_slot_used = object + dnode_slots - 1;
 	}
 
-	ASSERT3U(object_count, ==, usedobjs);
-
 	(void) printf("\n");
 
 	(void) printf("    Dnode slots:\n");
@@ -2410,6 +2408,8 @@ dump_dir(objset_t *os)
 		    leaked_objects);
 		leaked_objects = 0;
 	}
+
+	ASSERT3U(object_count, ==, usedobjs);
 }
 
 static void
@@ -2964,7 +2964,7 @@ zdb_blkptr_done(zio_t *zio)
 	abd_free(zio->io_abd);
 
 	mutex_enter(&spa->spa_scrub_lock);
-	spa->spa_scrub_inflight--;
+	spa->spa_load_verify_ios--;
 	cv_broadcast(&spa->spa_scrub_io_cv);
 
 	if (ioerr && !(zio->io_flags & ZIO_FLAG_SPECULATIVE)) {
@@ -3035,9 +3035,9 @@ zdb_blkptr_cb(spa_t *spa, zilog_t *zilog, const blkptr_t *bp,
 			flags |= ZIO_FLAG_SPECULATIVE;
 
 		mutex_enter(&spa->spa_scrub_lock);
-		while (spa->spa_scrub_inflight > max_inflight)
+		while (spa->spa_load_verify_ios > max_inflight)
 			cv_wait(&spa->spa_scrub_io_cv, &spa->spa_scrub_lock);
-		spa->spa_scrub_inflight++;
+		spa->spa_load_verify_ios++;
 		mutex_exit(&spa->spa_scrub_lock);
 
 		zio_nowait(zio_read(NULL, spa, bp, abd, size,
diff --git a/usr/src/cmd/zpool/zpool_main.c b/usr/src/cmd/zpool/zpool_main.c
index 240e74f6da..013ef1c699 100644
--- a/usr/src/cmd/zpool/zpool_main.c
+++ b/usr/src/cmd/zpool/zpool_main.c
@@ -1692,7 +1692,7 @@ print_status_config(zpool_handle_t *zhp, status_cbdata_t *cb, const char *name,
 	(void) nvlist_lookup_uint64_array(root, ZPOOL_CONFIG_SCAN_STATS,
 	    (uint64_t **)&ps, &c);
 
-	if (ps && ps->pss_state == DSS_SCANNING &&
+	if (ps != NULL && ps->pss_state == DSS_SCANNING &&
 	    vs->vs_scan_processed != 0 && children == 0) {
 		(void) printf(gettext("  (%s)"),
 		    (ps->pss_func == POOL_SCAN_RESILVER) ?
@@ -4707,11 +4707,13 @@ static void
 print_scan_status(pool_scan_stat_t *ps)
 {
 	time_t start, end, pause;
-	uint64_t elapsed, mins_left, hours_left;
-	uint64_t pass_exam, examined, total;
-	uint_t rate;
+	uint64_t total_secs_left;
+	uint64_t elapsed, secs_left, mins_left, hours_left, days_left;
+	uint64_t pass_scanned, scanned, pass_issued, issued, total;
+	uint_t scan_rate, issue_rate;
 	double fraction_done;
-	char processed_buf[7], examined_buf[7], total_buf[7], rate_buf[7];
+	char processed_buf[7], scanned_buf[7], issued_buf[7], total_buf[7];
+	char srate_buf[7], irate_buf[7];
 
 	(void) printf(gettext("  scan: "));
 
@@ -4725,30 +4727,37 @@ print_scan_status(pool_scan_stat_t *ps)
 	start = ps->pss_start_time;
 	end = ps->pss_end_time;
 	pause = ps->pss_pass_scrub_pause;
+
 	zfs_nicenum(ps->pss_processed, processed_buf, sizeof (processed_buf));
 
 	assert(ps->pss_func == POOL_SCAN_SCRUB ||
 	    ps->pss_func == POOL_SCAN_RESILVER);
+
 	/*
 	 * Scan is finished or canceled.
 	 */
 	if (ps->pss_state == DSS_FINISHED) {
-		uint64_t minutes_taken = (end - start) / 60;
-		char *fmt = NULL;
+		total_secs_left = end - start;
+		days_left = total_secs_left / 60 / 60 / 24;
+		hours_left = (total_secs_left / 60 / 60) % 24;
+		mins_left = (total_secs_left / 60) % 60;
+		secs_left = (total_secs_left % 60);
 
 		if (ps->pss_func == POOL_SCAN_SCRUB) {
-			fmt = gettext("scrub repaired %s in %lluh%um with "
-			    "%llu errors on %s");
+			(void) printf(gettext("scrub repaired %s "
+			    "in %llu days %02llu:%02llu:%02llu "
+			    "with %llu errors on %s"), processed_buf,
+			    (u_longlong_t)days_left, (u_longlong_t)hours_left,
+			    (u_longlong_t)mins_left, (u_longlong_t)secs_left,
+			    (u_longlong_t)ps->pss_errors, ctime(&end));
 		} else if (ps->pss_func == POOL_SCAN_RESILVER) {
-			fmt = gettext("resilvered %s in %lluh%um with "
-			    "%llu errors on %s");
+			(void) printf(gettext("resilvered %s "
+			    "in %llu days %02llu:%02llu:%02llu "
+			    "with %llu errors on %s"), processed_buf,
+			    (u_longlong_t)days_left, (u_longlong_t)hours_left,
+			    (u_longlong_t)mins_left, (u_longlong_t)secs_left,
+			    (u_longlong_t)ps->pss_errors, ctime(&end));
 		}
-		/* LINTED */
-		(void) printf(fmt, processed_buf,
-		    (u_longlong_t)(minutes_taken / 60),
-		    (uint_t)(minutes_taken % 60),
-		    (u_longlong_t)ps->pss_errors,
-		    ctime((time_t *)&end));
 		return;
 	} else if (ps->pss_state == DSS_CANCELED) {
 		if (ps->pss_func == POOL_SCAN_SCRUB) {
@@ -4763,19 +4772,15 @@ print_scan_status(pool_scan_stat_t *ps)
 
 	assert(ps->pss_state == DSS_SCANNING);
 
-	/*
-	 * Scan is in progress.
-	 */
+	/* Scan is in progress. Resilvers can't be paused. */
 	if (ps->pss_func == POOL_SCAN_SCRUB) {
 		if (pause == 0) {
 			(void) printf(gettext("scrub in progress since %s"),
 			    ctime(&start));
 		} else {
-			char buf[32];
-			struct tm *p = localtime(&pause);
-			(void) strftime(buf, sizeof (buf), "%a %b %e %T %Y", p);
-			(void) printf(gettext("scrub paused since %s\n"), buf);
-			(void) printf(gettext("\tscrub started on   %s"),
+			(void) printf(gettext("scrub paused since %s"),
+			    ctime(&pause));
+			(void) printf(gettext("\tscrub started on %s"),
 			    ctime(&start));
 		}
 	} else if (ps->pss_func == POOL_SCAN_RESILVER) {
@@ -4783,50 +4788,68 @@ print_scan_status(pool_scan_stat_t *ps)
 		    ctime(&start));
 	}
 
-	examined = ps->pss_examined ? ps->pss_examined : 1;
+	scanned = ps->pss_examined;
+	pass_scanned = ps->pss_pass_exam;
+	issued = ps->pss_issued;
+	pass_issued = ps->pss_pass_issued;
 	total = ps->pss_to_examine;
-	fraction_done = (double)examined / total;
 
-	/* elapsed time for this pass */
+	/* we are only done with a block once we have issued the IO for it  */
+	fraction_done = (double)issued / total;
+
+	/* elapsed time for this pass, rounding up to 1 if it's 0 */
 	elapsed = time(NULL) - ps->pss_pass_start;
 	elapsed -= ps->pss_pass_scrub_spent_paused;
-	elapsed = elapsed ? elapsed : 1;
-	pass_exam = ps->pss_pass_exam ? ps->pss_pass_exam : 1;
-	rate = pass_exam / elapsed;
-	rate = rate ? rate : 1;
-	mins_left = ((total - examined) / rate) / 60;
-	hours_left = mins_left / 60;
-
-	zfs_nicenum(examined, examined_buf, sizeof (examined_buf));
+	elapsed = (elapsed != 0) ? elapsed : 1;
+
+	scan_rate = pass_scanned / elapsed;
+	issue_rate = pass_issued / elapsed;
+	total_secs_left = (issue_rate != 0) ?
+	    ((total - issued) / issue_rate) : UINT64_MAX;
+
+	days_left = total_secs_left / 60 / 60 / 24;
+	hours_left = (total_secs_left / 60 / 60) % 24;
+	mins_left = (total_secs_left / 60) % 60;
+	secs_left = (total_secs_left % 60);
+
+	/* format all of the numbers we will be reporting */
+	zfs_nicenum(scanned, scanned_buf, sizeof (scanned_buf));
+	zfs_nicenum(issued, issued_buf, sizeof (issued_buf));
 	zfs_nicenum(total, total_buf, sizeof (total_buf));
+	zfs_nicenum(scan_rate, srate_buf, sizeof (srate_buf));
+	zfs_nicenum(issue_rate, irate_buf, sizeof (irate_buf));
 
-	/*
-	 * do not print estimated time if hours_left is more than 30 days
-	 * or we have a paused scrub
-	 */
+	/* do not print estimated time if we have a paused scrub */
 	if (pause == 0) {
-		zfs_nicenum(rate, rate_buf, sizeof (rate_buf));
-		(void) printf(gettext("\t%s scanned out of %s at %s/s"),
-		    examined_buf, total_buf, rate_buf);
-		if (hours_left < (30 * 24)) {
-			(void) printf(gettext(", %lluh%um to go\n"),
-			    (u_longlong_t)hours_left, (uint_t)(mins_left % 60));
-		} else {
-			(void) printf(gettext(
-			    ", (scan is slow, no estimated time)\n"));
-		}
+		(void) printf(gettext("\t%s scanned at %s/s, "
+		    "%s issued at %s/s, %s total\n"),
+		    scanned_buf, srate_buf, issued_buf, irate_buf, total_buf);
 	} else {
-		(void) printf(gettext("\t%s scanned out of %s\n"),
-		    examined_buf, total_buf);
+		(void) printf(gettext("\t%s scanned, %s issued, %s total\n"),
+		    scanned_buf, issued_buf, total_buf);
 	}
 
 	if (ps->pss_func == POOL_SCAN_RESILVER) {
-		(void) printf(gettext("    %s resilvered, %.2f%% done\n"),
+		(void) printf(gettext("\t%s resilvered, %.2f%% done"),
 		    processed_buf, 100 * fraction_done);
 	} else if (ps->pss_func == POOL_SCAN_SCRUB) {
-		(void) printf(gettext("    %s repaired, %.2f%% done\n"),
+		(void) printf(gettext("\t%s repaired, %.2f%% done"),
 		    processed_buf, 100 * fraction_done);
 	}
+
+	if (pause == 0) {
+		if (issue_rate >= 10 * 1024 * 1024) {
+			(void) printf(gettext(", %llu days "
+			    "%02llu:%02llu:%02llu to go\n"),
+			    (u_longlong_t)days_left, (u_longlong_t)hours_left,
+			    (u_longlong_t)mins_left, (u_longlong_t)secs_left);
+		} else {
+			(void) printf(gettext(", no estimated "
+			    "completion time\n"));
+		}
+	} else {
+		(void) printf(gettext("\n"));
+	}
 }
 
 /*
diff --git a/usr/src/cmd/ztest/ztest.c b/usr/src/cmd/ztest/ztest.c
index ef62671b3f..c0054cddbe 100644
--- a/usr/src/cmd/ztest/ztest.c
+++ b/usr/src/cmd/ztest/ztest.c
@@ -397,15 +397,15 @@ ztest_info_t ztest_info[] = {
 	{ ztest_fzap,				1,	&zopt_sometimes	},
 	{ ztest_dmu_snapshot_create_destroy,	1,	&zopt_sometimes	},
 	{ ztest_spa_create_destroy,		1,	&zopt_sometimes	},
-	{ ztest_fault_inject,			1,	&zopt_sometimes	},
+	{ ztest_fault_inject,			1,	&zopt_incessant	},
 	{ ztest_ddt_repair,			1,	&zopt_sometimes	},
 	{ ztest_dmu_snapshot_hold,		1,	&zopt_sometimes	},
 	{ ztest_mmp_enable_disable,		1,	&zopt_sometimes	},
 	{ ztest_reguid,				1,	&zopt_rarely	},
-	{ ztest_scrub,				1,	&zopt_rarely	},
+	{ ztest_scrub,				1,	&zopt_often	},
 	{ ztest_spa_upgrade,			1,	&zopt_rarely	},
 	{ ztest_dsl_dataset_promote_busy,	1,	&zopt_rarely	},
-	{ ztest_vdev_attach_detach,		1,	&zopt_sometimes	},
+	{ ztest_vdev_attach_detach,		1,	&zopt_incessant	},
 	{ ztest_vdev_LUN_growth,		1,	&zopt_rarely	},
 	{ ztest_vdev_add_remove,		1,
 	    &ztest_opts.zo_vdevtime				},
diff --git a/usr/src/lib/libfakekernel/common/mapfile-vers b/usr/src/lib/libfakekernel/common/mapfile-vers
index 69d0055a27..2a307c7a88 100644
--- a/usr/src/lib/libfakekernel/common/mapfile-vers
+++ b/usr/src/lib/libfakekernel/common/mapfile-vers
@@ -216,6 +216,7 @@ SYMBOL_VERSION SUNWprivate_1.1 {
 	taskq_empty;
 	taskq_member;
 	taskq_wait;
+	taskq_wait_id;
 
 	thread_create;
 	thread_join;
diff --git a/usr/src/lib/libfakekernel/common/taskq.c b/usr/src/lib/libfakekernel/common/taskq.c
index 76962f5a64..e3922f6c37 100644
--- a/usr/src/lib/libfakekernel/common/taskq.c
+++ b/usr/src/lib/libfakekernel/common/taskq.c
@@ -234,6 +234,12 @@ taskq_wait(taskq_t *tq)
 	mutex_exit(&tq->tq_lock);
 }
 
+void
+taskq_wait_id(taskq_t *tq, taskqid_t id __unused)
+{
+	taskq_wait(tq);
+}
+
 static void *
 taskq_thread(void *arg)
 {
diff --git a/usr/src/lib/libzfs/common/libzfs_status.c b/usr/src/lib/libzfs/common/libzfs_status.c
index 7a701b78c6..975309c423 100644
--- a/usr/src/lib/libzfs/common/libzfs_status.c
+++ b/usr/src/lib/libzfs/common/libzfs_status.c
@@ -225,7 +225,7 @@ check_status(nvlist_t *config, boolean_t isimport)
 	 */
 	(void) nvlist_lookup_uint64_array(nvroot, ZPOOL_CONFIG_SCAN_STATS,
 	    (uint64_t **)&ps, &psc);
-	if (ps && ps->pss_func == POOL_SCAN_RESILVER &&
+	if (ps != NULL && ps->pss_func == POOL_SCAN_RESILVER &&
 	    ps->pss_state == DSS_SCANNING)
 		return (ZPOOL_STATUS_RESILVERING);
 
diff --git a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/cleanup.ksh b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/cleanup.ksh
index 80af13ba67..b8674764bc 100644
--- a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/cleanup.ksh
+++ b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/cleanup.ksh
@@ -26,7 +26,9 @@
 #
 
 . $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/cli_root/zpool_scrub/zpool_scrub.cfg
 
 verify_runnable "global"
 
+log_must set_tunable64 zfs_scan_vdev_limit $ZFS_SCAN_VDEV_LIMIT_DEFAULT
 destroy_mirrors
diff --git a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/setup.ksh b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/setup.ksh
index 5db12ba164..abf118828e 100644
--- a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/setup.ksh
+++ b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/setup.ksh
@@ -37,11 +37,8 @@ verify_disk_count "$DISKS" 2
 
 default_mirror_setup_noexit $DISK1 $DISK2
 
-mntpnt=$(get_prop mountpoint $TESTPOOL)
-typeset -i i=0
-while ((i < 10)); do
-	log_must mkfile 500M $mntpnt/bigfile.$i
-	((i += 1))
-done
+mntpnt=$(get_prop mountpoint $TESTPOOL/$TESTFS)
 
+# Create 256M of data
+log_must file_write -b 1048576 -c 256 -o create -d 0 -f $mntpnt/bigfile
 log_pass
diff --git a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub.cfg b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub.cfg
index 93e4e171c3..fdf2f42847 100644
--- a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub.cfg
+++ b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub.cfg
@@ -28,5 +28,8 @@
 # Copyright (c) 2012, 2016 by Delphix. All rights reserved.
 #
 
-export DISK1=$(echo $DISKS | awk '{print $1}')
+export DISK1=${DISKS%% *}
 export DISK2=$(echo $DISKS | awk '{print $2}')
+
+export ZFS_SCAN_VDEV_LIMIT_SLOW=$((128*1024))
+export ZFS_SCAN_VDEV_LIMIT_DEFAULT=$((4*1024*1024))
diff --git a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_002_pos.ksh b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_002_pos.ksh
index 26c22fd98a..712097bb1c 100644
--- a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_002_pos.ksh
+++ b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_002_pos.ksh
@@ -46,9 +46,9 @@
 #	6. Verify zpool scrub -s succeed when the system is scrubbing.
 #
 # NOTES:
-#	A 10ms delay is added to the ZIOs in order to ensure that the
-#	scrub does not complete before it has a chance to be cancelled.
-#	This can occur when testing with small pools or very fast hardware.
+#	Artificially limit the scrub speed by setting the zfs_scan_vdev_limit
+#	low and adding a 50ms zio delay in order to ensure that the scrub does
+#	not complete early.
 #
 
 verify_runnable "global"
@@ -56,13 +56,21 @@ verify_runnable "global"
 function cleanup
 {
 	log_must zinject -c all
+	log_must set_tunable64 zfs_scan_vdev_limit $ZFS_SCAN_VDEV_LIMIT_DEFAULT
+	log_must rm -f $mntpnt/biggerfile
 }
 
 log_onexit cleanup
 
 log_assert "Verify scrub, scrub -p, and scrub -s show the right status."
 
-log_must zinject -d $DISK1 -D20:1 $TESTPOOL
+# Create 1G of additional data
+mntpnt=$(get_prop mountpoint $TESTPOOL/$TESTFS)
+log_must file_write -b 1048576 -c 1024 -o create -d 0 -f $mntpnt/biggerfile
+log_must sync
+
+log_must zinject -d $DISK1 -D50:1 $TESTPOOL
+log_must set_tunable64 zfs_scan_vdev_limit $ZFS_SCAN_VDEV_LIMIT_SLOW
 log_must zpool scrub $TESTPOOL
 log_must is_pool_scrubbing $TESTPOOL true
 log_must zpool scrub -p $TESTPOOL
diff --git a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_003_pos.ksh b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_003_pos.ksh
index 2af3efcff7..c52ad84bc5 100644
--- a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_003_pos.ksh
+++ b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_003_pos.ksh
@@ -43,23 +43,22 @@
 #	2. Kick off a second scrub and verify it fails
 #
 # NOTES:
-#	A 10ms delay is added to the ZIOs in order to ensure that the
-#	scrub does not complete before it has a chance to be restarted.
-#	This can occur when testing with small pools or very fast hardware.
+#	Artificially limit the scrub speed by setting the zfs_scan_vdev_limit
+#	low in order to ensure that the scrub does not complete early.
 #
 
 verify_runnable "global"
 
 function cleanup
 {
-	        log_must zinject -c all
+	log_must set_tunable64 zfs_scan_vdev_limit $ZFS_SCAN_VDEV_LIMIT_DEFAULT
 }
 
 log_onexit cleanup
 
 log_assert "Scrub command fails when there is already a scrub in progress"
 
-log_must zinject -d $DISK1 -D10:1 $TESTPOOL
+log_must set_tunable64 zfs_scan_vdev_limit $ZFS_SCAN_VDEV_LIMIT_SLOW
 log_must zpool scrub $TESTPOOL
 log_must is_pool_scrubbing $TESTPOOL true
 log_mustnot zpool scrub $TESTPOOL
diff --git a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_004_pos.ksh b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_004_pos.ksh
index a2065d477b..492c7b20aa 100644
--- a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_004_pos.ksh
+++ b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_004_pos.ksh
@@ -42,16 +42,21 @@
 #	3. Verify scrub failed until the resilver completed
 #
 
+function cleanup
+{
+	log_must set_tunable64 zfs_scan_vdev_limit $ZFS_SCAN_VDEV_LIMIT_DEFAULT
+}
+
 verify_runnable "global"
 
 log_assert "Resilver prevent scrub from starting until the resilver completes"
 
+log_must set_tunable64 zfs_scan_vdev_limit $ZFS_SCAN_VDEV_LIMIT_SLOW
 log_must zpool detach $TESTPOOL $DISK2
 log_must zpool attach $TESTPOOL $DISK1 $DISK2
 log_must is_pool_resilvering $TESTPOOL
 log_mustnot zpool scrub $TESTPOOL
 
-# Allow the resilver to finish, or it will interfere with the next test.
 while ! is_pool_resilvered $TESTPOOL; do
 	sleep 1
 done
diff --git a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_005_pos.ksh b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_005_pos.ksh
index 5bc5eedb64..8db6ae9802 100644
--- a/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_005_pos.ksh
+++ b/usr/src/test/zfs-tests/tests/functional/cli_root/zpool_scrub/zpool_scrub_005_pos.ksh
@@ -58,4 +58,8 @@ log_must zpool scrub $TESTPOOL
 log_must zpool detach $TESTPOOL $DISK1
 log_must zpool attach $TESTPOOL $DISK2 $DISK1
 
+while ! is_pool_resilvered $TESTPOOL; do
+	sleep 1
+done
+
 log_pass "When scrubbing, detach device should not break system."
diff --git a/usr/src/test/zfs-tests/tests/perf/scripts/prefetch_io.d b/usr/src/test/zfs-tests/tests/perf/scripts/prefetch_io.d
index 03e6c4111e..5f49e4046e 100644
--- a/usr/src/test/zfs-tests/tests/perf/scripts/prefetch_io.d
+++ b/usr/src/test/zfs-tests/tests/perf/scripts/prefetch_io.d
@@ -48,9 +48,9 @@ arc_read:arc-demand-hit-predictive-prefetch
 	@c["prefetched_demand_reads"] = count();
 }
 
-arc_read:arc-sync-wait-for-async
+arc_read:arc-upgrade-sync
 {
-	@c["sync_wait_for_async"] = count();
+	@c["async_upgrade_sync"] = count();
 }
 
 arc_read_done:entry
diff --git a/usr/src/uts/common/fs/zfs/arc.c b/usr/src/uts/common/fs/zfs/arc.c
index 844abbcd5d..8cd31e7c7c 100644
--- a/usr/src/uts/common/fs/zfs/arc.c
+++ b/usr/src/uts/common/fs/zfs/arc.c
@@ -348,7 +348,8 @@ int			arc_no_grow_shift = 5;
  * minimum lifespan of a prefetch block in clock ticks
  * (initialized in arc_init())
  */
-static int		arc_min_prefetch_lifespan;
+static int		zfs_arc_min_prefetch_ms = 1;
+static int		zfs_arc_min_prescient_prefetch_ms = 6;
 
 /*
  * If this percent of memory is free, don't throttle.
@@ -695,8 +696,9 @@ typedef struct arc_stats {
 	kstat_named_t arcstat_meta_limit;
 	kstat_named_t arcstat_meta_max;
 	kstat_named_t arcstat_meta_min;
-	kstat_named_t arcstat_sync_wait_for_async;
+	kstat_named_t arcstat_async_upgrade_sync;
 	kstat_named_t arcstat_demand_hit_predictive_prefetch;
+	kstat_named_t arcstat_demand_hit_prescient_prefetch;
 } arc_stats_t;
 
 static arc_stats_t arc_stats = {
@@ -780,8 +782,9 @@ static arc_stats_t arc_stats = {
 	{ "arc_meta_limit",		KSTAT_DATA_UINT64 },
 	{ "arc_meta_max",		KSTAT_DATA_UINT64 },
 	{ "arc_meta_min",		KSTAT_DATA_UINT64 },
-	{ "sync_wait_for_async",	KSTAT_DATA_UINT64 },
+	{ "async_upgrade_sync",		KSTAT_DATA_UINT64 },
 	{ "demand_hit_predictive_prefetch", KSTAT_DATA_UINT64 },
+	{ "demand_hit_prescient_prefetch", KSTAT_DATA_UINT64 },
 };
 
 #define	ARCSTAT(stat)	(arc_stats.stat.value.ui64)
@@ -878,22 +881,23 @@ typedef struct arc_callback arc_callback_t;
 
 struct arc_callback {
 	void			*acb_private;
-	arc_done_func_t		*acb_done;
+	arc_read_done_func_t	*acb_done;
 	arc_buf_t		*acb_buf;
 	boolean_t		acb_compressed;
 	zio_t			*acb_zio_dummy;
+	zio_t			*acb_zio_head;
 	arc_callback_t		*acb_next;
 };
 
 typedef struct arc_write_callback arc_write_callback_t;
 
 struct arc_write_callback {
-	void		*awcb_private;
-	arc_done_func_t	*awcb_ready;
-	arc_done_func_t	*awcb_children_ready;
-	arc_done_func_t	*awcb_physdone;
-	arc_done_func_t	*awcb_done;
-	arc_buf_t	*awcb_buf;
+	void			*awcb_private;
+	arc_write_done_func_t	*awcb_ready;
+	arc_write_done_func_t	*awcb_children_ready;
+	arc_write_done_func_t	*awcb_physdone;
+	arc_write_done_func_t	*awcb_done;
+	arc_buf_t		*awcb_buf;
 };
 
 /*
@@ -1019,6 +1023,8 @@ struct arc_buf_hdr {
 #define	HDR_IO_IN_PROGRESS(hdr)	((hdr)->b_flags & ARC_FLAG_IO_IN_PROGRESS)
 #define	HDR_IO_ERROR(hdr)	((hdr)->b_flags & ARC_FLAG_IO_ERROR)
 #define	HDR_PREFETCH(hdr)	((hdr)->b_flags & ARC_FLAG_PREFETCH)
+#define	HDR_PRESCIENT_PREFETCH(hdr)	\
+	((hdr)->b_flags & ARC_FLAG_PRESCIENT_PREFETCH)
 #define	HDR_COMPRESSION_ENABLED(hdr)	\
 	((hdr)->b_flags & ARC_FLAG_COMPRESSED_ARC)
 
@@ -3249,6 +3255,8 @@ arc_evict_hdr(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
 {
 	arc_state_t *evicted_state, *state;
 	int64_t bytes_evicted = 0;
+	int min_lifetime = HDR_PRESCIENT_PREFETCH(hdr) ?
+	    zfs_arc_min_prescient_prefetch_ms : zfs_arc_min_prefetch_ms;
 
 	ASSERT(MUTEX_HELD(hash_lock));
 	ASSERT(HDR_HAS_L1HDR(hdr));
@@ -3301,8 +3309,7 @@ arc_evict_hdr(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
 	/* prefetch buffers have a minimum lifespan */
 	if (HDR_IO_IN_PROGRESS(hdr) ||
 	    ((hdr->b_flags & (ARC_FLAG_PREFETCH | ARC_FLAG_INDIRECT)) &&
-	    ddi_get_lbolt() - hdr->b_l1hdr.b_arc_access <
-	    arc_min_prefetch_lifespan)) {
+	    ddi_get_lbolt() - hdr->b_l1hdr.b_arc_access < min_lifetime * hz)) {
 		ARCSTAT_BUMP(arcstat_evict_skip);
 		return (bytes_evicted);
 	}
@@ -4613,13 +4620,15 @@ arc_access(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
 		 * - move the buffer to the head of the list if this is
 		 *   another prefetch (to make it less likely to be evicted).
 		 */
-		if (HDR_PREFETCH(hdr)) {
+		if (HDR_PREFETCH(hdr) || HDR_PRESCIENT_PREFETCH(hdr)) {
 			if (zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) == 0) {
 				/* link protected by hash lock */
 				ASSERT(multilist_link_active(
 				    &hdr->b_l1hdr.b_arc_node));
 			} else {
-				arc_hdr_clear_flags(hdr, ARC_FLAG_PREFETCH);
+				arc_hdr_clear_flags(hdr,
+				    ARC_FLAG_PREFETCH |
+				    ARC_FLAG_PRESCIENT_PREFETCH);
 				ARCSTAT_BUMP(arcstat_mru_hits);
 			}
 			hdr->b_l1hdr.b_arc_access = now;
@@ -4650,10 +4659,13 @@ arc_access(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
 		 * MFU state.
 		 */
 
-		if (HDR_PREFETCH(hdr)) {
+		if (HDR_PREFETCH(hdr) || HDR_PRESCIENT_PREFETCH(hdr)) {
 			new_state = arc_mru;
-			if (zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) > 0)
-				arc_hdr_clear_flags(hdr, ARC_FLAG_PREFETCH);
+			if (zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) > 0) {
+				arc_hdr_clear_flags(hdr,
+				    ARC_FLAG_PREFETCH |
+				    ARC_FLAG_PRESCIENT_PREFETCH);
+			}
 			DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, hdr);
 		} else {
 			new_state = arc_mfu;
@@ -4674,11 +4686,6 @@ arc_access(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
 		 * If it was a prefetch, we will explicitly move it to
 		 * the head of the list now.
 		 */
-		if ((HDR_PREFETCH(hdr)) != 0) {
-			ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
-			/* link protected by hash_lock */
-			ASSERT(multilist_link_active(&hdr->b_l1hdr.b_arc_node));
-		}
 		ARCSTAT_BUMP(arcstat_mfu_hits);
 		hdr->b_l1hdr.b_arc_access = ddi_get_lbolt();
 	} else if (hdr->b_l1hdr.b_state == arc_mfu_ghost) {
@@ -4689,12 +4696,11 @@ arc_access(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
 		 * MFU state.
 		 */
 
-		if (HDR_PREFETCH(hdr)) {
+		if (HDR_PREFETCH(hdr) || HDR_PRESCIENT_PREFETCH(hdr)) {
 			/*
 			 * This is a prefetch access...
 			 * move this block back to the MRU state.
 			 */
-			ASSERT0(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt));
 			new_state = arc_mru;
 		}
 
@@ -4760,21 +4766,26 @@ arc_buf_access(arc_buf_t *buf)
 	    demand, prefetch, !HDR_ISTYPE_METADATA(hdr), data, metadata, hits);
 }
 
-/* a generic arc_done_func_t which you can use */
+/* a generic arc_read_done_func_t which you can use */
 /* ARGSUSED */
 void
-arc_bcopy_func(zio_t *zio, arc_buf_t *buf, void *arg)
+arc_bcopy_func(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
+    arc_buf_t *buf, void *arg)
 {
-	if (zio == NULL || zio->io_error == 0)
-		bcopy(buf->b_data, arg, arc_buf_size(buf));
+	if (buf == NULL)
+		return;
+
+	bcopy(buf->b_data, arg, arc_buf_size(buf));
 	arc_buf_destroy(buf, arg);
 }
 
-/* a generic arc_done_func_t */
+/* a generic arc_read_done_func_t */
 void
-arc_getbuf_func(zio_t *zio, arc_buf_t *buf, void *arg)
+arc_getbuf_func(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
+    arc_buf_t *buf, void *arg)
 {
 	arc_buf_t **bufp = arg;
+
 	if (buf == NULL) {
 		ASSERT(zio == NULL || zio->io_error != 0);
 		*bufp = NULL;
@@ -4809,7 +4820,6 @@ arc_read_done(zio_t *zio)
 	arc_callback_t	*callback_list;
 	arc_callback_t	*acb;
 	boolean_t	freeable = B_FALSE;
-	boolean_t	no_zio_error = (zio->io_error == 0);
 
 	/*
 	 * The hdr was inserted into hash-table and removed from lists
@@ -4835,7 +4845,7 @@ arc_read_done(zio_t *zio)
 		ASSERT3P(hash_lock, !=, NULL);
 	}
 
-	if (no_zio_error) {
+	if (zio->io_error == 0) {
 		/* byteswap if necessary */
 		if (BP_SHOULD_BYTESWAP(zio->io_bp)) {
 			if (BP_GET_LEVEL(zio->io_bp) > 0) {
@@ -4856,7 +4866,8 @@ arc_read_done(zio_t *zio)
 	callback_list = hdr->b_l1hdr.b_acb;
 	ASSERT3P(callback_list, !=, NULL);
 
-	if (hash_lock && no_zio_error && hdr->b_l1hdr.b_state == arc_anon) {
+	if (hash_lock && zio->io_error == 0 &&
+	    hdr->b_l1hdr.b_state == arc_anon) {
 		/*
 		 * Only call arc_access on anonymous buffers.  This is because
 		 * if we've issued an I/O for an evicted buffer, we've already
@@ -4877,30 +4888,29 @@ arc_read_done(zio_t *zio)
 		if (!acb->acb_done)
 			continue;
 
-		/* This is a demand read since prefetches don't use callbacks */
 		callback_cnt++;
 
-		if (no_zio_error) {
-			int error = arc_buf_alloc_impl(hdr, acb->acb_private,
-			    acb->acb_compressed, zio->io_error == 0,
-			    &acb->acb_buf);
-			if (error != 0) {
-				/*
-				 * Decompression failed.  Set io_error
-				 * so that when we call acb_done (below),
-				 * we will indicate that the read failed.
-				 * Note that in the unusual case where one
-				 * callback is compressed and another
-				 * uncompressed, we will mark all of them
-				 * as failed, even though the uncompressed
-				 * one can't actually fail.  In this case,
-				 * the hdr will not be anonymous, because
-				 * if there are multiple callbacks, it's
-				 * because multiple threads found the same
-				 * arc buf in the hash table.
-				 */
-				zio->io_error = error;
-			}
+		if (zio->io_error != 0)
+			continue;
+
+		int error = arc_buf_alloc_impl(hdr, acb->acb_private,
+		    acb->acb_compressed, B_TRUE, &acb->acb_buf);
+		if (error != 0) {
+			/*
+			 * Decompression failed.  Set io_error
+			 * so that when we call acb_done (below),
+			 * we will indicate that the read failed.
+			 * Note that in the unusual case where one
+			 * callback is compressed and another
+			 * uncompressed, we will mark all of them
+			 * as failed, even though the uncompressed
+			 * one can't actually fail.  In this case,
+			 * the hdr will not be anonymous, because
+			 * if there are multiple callbacks, it's
+			 * because multiple threads found the same
+			 * arc buf in the hash table.
+			 */
+			zio->io_error = error;
 		}
 	}
 	/*
@@ -4923,7 +4933,7 @@ arc_read_done(zio_t *zio)
 	ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt) ||
 	    callback_list != NULL);
 
-	if (no_zio_error) {
+	if (zio->io_error == 0) {
 		arc_hdr_verify(hdr, zio->io_bp);
 	} else {
 		arc_hdr_set_flags(hdr, ARC_FLAG_IO_ERROR);
@@ -4966,7 +4976,8 @@ arc_read_done(zio_t *zio)
 				arc_buf_destroy(acb->acb_buf, acb->acb_private);
 				acb->acb_buf = NULL;
 			}
-			acb->acb_done(zio, acb->acb_buf, acb->acb_private);
+			acb->acb_done(zio, &zio->io_bookmark, zio->io_bp,
+			    acb->acb_buf, acb->acb_private);
 		}
 
 		if (acb->acb_zio_dummy != NULL) {
@@ -5001,7 +5012,7 @@ arc_read_done(zio_t *zio)
  * for readers of this block.
  */
 int
-arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp, arc_done_func_t *done,
+arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp, arc_read_done_func_t *done,
     void *private, zio_priority_t priority, int zio_flags,
     arc_flags_t *arc_flags, const zbookmark_phys_t *zb)
 {
@@ -5010,6 +5021,7 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp, arc_done_func_t *done,
 	zio_t *rzio;
 	uint64_t guid = spa_load_guid(spa);
 	boolean_t compressed_read = (zio_flags & ZIO_FLAG_RAW) != 0;
+	int rc = 0;
 
 	ASSERT(!BP_IS_EMBEDDED(bp) ||
 	    BPE_GET_ETYPE(bp) == BP_EMBEDDED_TYPE_DATA);
@@ -5028,32 +5040,20 @@ top:
 		*arc_flags |= ARC_FLAG_CACHED;
 
 		if (HDR_IO_IN_PROGRESS(hdr)) {
+			zio_t *head_zio = hdr->b_l1hdr.b_acb->acb_zio_head;
 
+			ASSERT3P(head_zio, !=, NULL);
 			if ((hdr->b_flags & ARC_FLAG_PRIO_ASYNC_READ) &&
 			    priority == ZIO_PRIORITY_SYNC_READ) {
 				/*
-				 * This sync read must wait for an
-				 * in-progress async read (e.g. a predictive
-				 * prefetch).  Async reads are queued
-				 * separately at the vdev_queue layer, so
-				 * this is a form of priority inversion.
-				 * Ideally, we would "inherit" the demand
-				 * i/o's priority by moving the i/o from
-				 * the async queue to the synchronous queue,
-				 * but there is currently no mechanism to do
-				 * so.  Track this so that we can evaluate
-				 * the magnitude of this potential performance
-				 * problem.
-				 *
-				 * Note that if the prefetch i/o is already
-				 * active (has been issued to the device),
-				 * the prefetch improved performance, because
-				 * we issued it sooner than we would have
-				 * without the prefetch.
+				 * This is a sync read that needs to wait for
+				 * an in-flight async read. Request that the
+				 * zio have its priority upgraded.
 				 */
-				DTRACE_PROBE1(arc__sync__wait__for__async,
+				zio_change_priority(head_zio, priority);
+				DTRACE_PROBE1(arc__async__upgrade__sync,
 				    arc_buf_hdr_t *, hdr);
-				ARCSTAT_BUMP(arcstat_sync_wait_for_async);
+				ARCSTAT_BUMP(arcstat_async_upgrade_sync);
 			}
 			if (hdr->b_flags & ARC_FLAG_PREDICTIVE_PREFETCH) {
 				arc_hdr_clear_flags(hdr,
@@ -5080,6 +5080,7 @@ top:
 					    spa, NULL, NULL, NULL, zio_flags);
 
 				ASSERT3P(acb->acb_done, !=, NULL);
+				acb->acb_zio_head = head_zio;
 				acb->acb_next = hdr->b_l1hdr.b_acb;
 				hdr->b_l1hdr.b_acb = acb;
 				mutex_exit(hash_lock);
@@ -5107,17 +5108,33 @@ top:
 				arc_hdr_clear_flags(hdr,
 				    ARC_FLAG_PREDICTIVE_PREFETCH);
 			}
+
+			if (hdr->b_flags & ARC_FLAG_PRESCIENT_PREFETCH) {
+				ARCSTAT_BUMP(
+				    arcstat_demand_hit_prescient_prefetch);
+				arc_hdr_clear_flags(hdr,
+				    ARC_FLAG_PRESCIENT_PREFETCH);
+			}
+
 			ASSERT(!BP_IS_EMBEDDED(bp) || !BP_IS_HOLE(bp));
 
 			/* Get a buf with the desired data in it. */
-			VERIFY0(arc_buf_alloc_impl(hdr, private,
-			    compressed_read, B_TRUE, &buf));
+			rc = arc_buf_alloc_impl(hdr, private,
+			    compressed_read, B_TRUE, &buf);
+			if (rc != 0) {
+				arc_buf_destroy(buf, private);
+				buf = NULL;
+			}
+			ASSERT((zio_flags & ZIO_FLAG_SPECULATIVE) ||
+			    rc == 0 || rc != ENOENT);
 		} else if (*arc_flags & ARC_FLAG_PREFETCH &&
 		    zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) == 0) {
 			arc_hdr_set_flags(hdr, ARC_FLAG_PREFETCH);
 		}
 		DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
 		arc_access(hdr, hash_lock);
+		if (*arc_flags & ARC_FLAG_PRESCIENT_PREFETCH)
+			arc_hdr_set_flags(hdr, ARC_FLAG_PRESCIENT_PREFETCH);
 		if (*arc_flags & ARC_FLAG_L2CACHE)
 			arc_hdr_set_flags(hdr, ARC_FLAG_L2CACHE);
 		mutex_exit(hash_lock);
@@ -5127,7 +5144,7 @@ top:
 		    data, metadata, hits);
 
 		if (done)
-			done(NULL, buf, private);
+			done(NULL, zb, bp, buf, private);
 	} else {
 		uint64_t lsize = BP_GET_LSIZE(bp);
 		uint64_t psize = BP_GET_PSIZE(bp);
@@ -5201,6 +5218,9 @@ top:
 
 		if (*arc_flags & ARC_FLAG_PREFETCH)
 			arc_hdr_set_flags(hdr, ARC_FLAG_PREFETCH);
+		if (*arc_flags & ARC_FLAG_PRESCIENT_PREFETCH)
+			arc_hdr_set_flags(hdr, ARC_FLAG_PRESCIENT_PREFETCH);
+
 		if (*arc_flags & ARC_FLAG_L2CACHE)
 			arc_hdr_set_flags(hdr, ARC_FLAG_L2CACHE);
 		if (BP_GET_LEVEL(bp) > 0)
@@ -5230,14 +5250,17 @@ top:
 				vd = NULL;
 		}
 
-		if (priority == ZIO_PRIORITY_ASYNC_READ)
+		/*
+		 * We count both async reads and scrub IOs as asynchronous so
+		 * that both can be upgraded in the event of a cache hit while
+		 * the read IO is still in-flight.
+		 */
+		if (priority == ZIO_PRIORITY_ASYNC_READ ||
+		    priority == ZIO_PRIORITY_SCRUB)
 			arc_hdr_set_flags(hdr, ARC_FLAG_PRIO_ASYNC_READ);
 		else
 			arc_hdr_clear_flags(hdr, ARC_FLAG_PRIO_ASYNC_READ);
 
-		if (hash_lock != NULL)
-			mutex_exit(hash_lock);
-
 		/*
 		 * At this point, we have a level 1 cache miss.  Try again in
 		 * L2ARC if possible.
@@ -5307,6 +5330,11 @@ top:
 				    ZIO_FLAG_CANFAIL |
 				    ZIO_FLAG_DONT_PROPAGATE |
 				    ZIO_FLAG_DONT_RETRY, B_FALSE);
+				acb->acb_zio_head = rzio;
+
+				if (hash_lock != NULL)
+					mutex_exit(hash_lock);
+
 				DTRACE_PROBE2(l2arc__read, vdev_t *, vd,
 				    zio_t *, rzio);
 				ARCSTAT_INCR(arcstat_l2_read_bytes, size);
@@ -5321,6 +5349,8 @@ top:
 					return (0);
 
 				/* l2arc read error; goto zio_read() */
+				if (hash_lock != NULL)
+					mutex_enter(hash_lock);
 			} else {
 				DTRACE_PROBE1(l2arc__miss,
 				    arc_buf_hdr_t *, hdr);
@@ -5341,6 +5371,10 @@ top:
 
 		rzio = zio_read(pio, spa, bp, hdr->b_l1hdr.b_pabd, size,
 		    arc_read_done, hdr, priority, zio_flags, zb);
+		acb->acb_zio_head = rzio;
+
+		if (hash_lock != NULL)
+			mutex_exit(hash_lock);
 
 		if (*arc_flags & ARC_FLAG_WAIT)
 			return (zio_wait(rzio));
@@ -5828,9 +5862,9 @@ arc_write_done(zio_t *zio)
 
 zio_t *
 arc_write(zio_t *pio, spa_t *spa, uint64_t txg, blkptr_t *bp, arc_buf_t *buf,
-    boolean_t l2arc, const zio_prop_t *zp, arc_done_func_t *ready,
-    arc_done_func_t *children_ready, arc_done_func_t *physdone,
-    arc_done_func_t *done, void *private, zio_priority_t priority,
+    boolean_t l2arc, const zio_prop_t *zp, arc_write_done_func_t *ready,
+    arc_write_done_func_t *children_ready, arc_write_done_func_t *physdone,
+    arc_write_done_func_t *done, void *private, zio_priority_t priority,
     int zio_flags, const zbookmark_phys_t *zb)
 {
 	arc_buf_hdr_t *hdr = buf->b_hdr;
@@ -6242,9 +6276,6 @@ arc_init(void)
 	mutex_init(&arc_adjust_lock, NULL, MUTEX_DEFAULT, NULL);
 	cv_init(&arc_adjust_waiters_cv, NULL, CV_DEFAULT, NULL);
 
-	/* Convert seconds to clock ticks */
-	arc_min_prefetch_lifespan = 1 * hz;
-
 	/* set min cache to 1/32 of all memory, or 64MB, whichever is more */
 	arc_c_min = MAX(allmem / 32, 64 << 20);
 	/* set max to 3/4 of all memory, or all but 1GB, whichever is more */
diff --git a/usr/src/uts/common/fs/zfs/dbuf.c b/usr/src/uts/common/fs/zfs/dbuf.c
index 7421ea291b..b9d6ca26fe 100644
--- a/usr/src/uts/common/fs/zfs/dbuf.c
+++ b/usr/src/uts/common/fs/zfs/dbuf.c
@@ -942,7 +942,8 @@ dbuf_whichblock(dnode_t *dn, int64_t level, uint64_t offset)
 }
 
 static void
-dbuf_read_done(zio_t *zio, arc_buf_t *buf, void *vdb)
+dbuf_read_done(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
+    arc_buf_t *buf, void *vdb)
 {
 	dmu_buf_impl_t *db = vdb;
 
@@ -961,15 +962,19 @@ dbuf_read_done(zio_t *zio, arc_buf_t *buf, void *vdb)
 		ASSERT3P(db->db_buf, ==, NULL);
 		db->db_state = DB_UNCACHED;
 	} else if (db->db_level == 0 && db->db_freed_in_flight) {
-		/* freed in flight */
+		/* we were freed in flight; disregard any error */
 		ASSERT(zio == NULL || zio->io_error == 0);
+		if (buf == NULL) {
+			buf = arc_alloc_buf(db->db_objset->os_spa,
+			    db, DBUF_GET_BUFC_TYPE(db), db->db.db_size);
+		}
 		arc_release(buf, db);
 		bzero(buf->b_data, db->db.db_size);
 		arc_buf_freeze(buf);
 		db->db_freed_in_flight = FALSE;
 		dbuf_set_data(db, buf);
 		db->db_state = DB_CACHED;
-	} else {
+	} else if (buf != NULL) {
 		/* success */
 		ASSERT(zio == NULL || zio->io_error == 0);
 		dbuf_set_data(db, buf);
@@ -2395,7 +2400,8 @@ dbuf_issue_final_prefetch(dbuf_prefetch_arg_t *dpa, blkptr_t *bp)
  * prefetch if the next block down is our target.
  */
 static void
-dbuf_prefetch_indirect_done(zio_t *zio, arc_buf_t *abuf, void *private)
+dbuf_prefetch_indirect_done(zio_t *zio, const zbookmark_phys_t *zb,
+    const blkptr_t *iobp, arc_buf_t *abuf, void *private)
 {
 	dbuf_prefetch_arg_t *dpa = private;
 
@@ -2442,11 +2448,11 @@ dbuf_prefetch_indirect_done(zio_t *zio, arc_buf_t *abuf, void *private)
 	}
 
 	dpa->dpa_curlevel--;
-
 	uint64_t nextblkid = dpa->dpa_zb.zb_blkid >>
 	    (dpa->dpa_epbs * (dpa->dpa_curlevel - dpa->dpa_zb.zb_level));
 	blkptr_t *bp = ((blkptr_t *)abuf->b_data) +
 	    P2PHASE(nextblkid, 1ULL << dpa->dpa_epbs);
+
 	if (BP_IS_HOLE(bp)) {
 		kmem_free(dpa, sizeof (*dpa));
 	} else if (dpa->dpa_curlevel == dpa->dpa_zb.zb_level) {
@@ -3854,7 +3860,7 @@ dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, dmu_tx_t *tx)
 		 * ready callback so that we can properly handle an indirect
 		 * block that only contains holes.
 		 */
-		arc_done_func_t *children_ready_cb = NULL;
+		arc_write_done_func_t *children_ready_cb = NULL;
 		if (db->db_level != 0)
 			children_ready_cb = dbuf_write_children_ready;
 
diff --git a/usr/src/uts/common/fs/zfs/ddt.c b/usr/src/uts/common/fs/zfs/ddt.c
index 963ecbd58f..1d51329511 100644
--- a/usr/src/uts/common/fs/zfs/ddt.c
+++ b/usr/src/uts/common/fs/zfs/ddt.c
@@ -1106,14 +1106,26 @@ ddt_sync_table(ddt_t *ddt, dmu_tx_t *tx, uint64_t txg)
 void
 ddt_sync(spa_t *spa, uint64_t txg)
 {
+	dsl_scan_t *scn = spa->spa_dsl_pool->dp_scan;
 	dmu_tx_t *tx;
-	zio_t *rio = zio_root(spa, NULL, NULL,
-	    ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE | ZIO_FLAG_SELF_HEAL);
+	zio_t *rio;
 
 	ASSERT(spa_syncing_txg(spa) == txg);
 
 	tx = dmu_tx_create_assigned(spa->spa_dsl_pool, txg);
 
+	rio = zio_root(spa, NULL, NULL,
+	    ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE | ZIO_FLAG_SELF_HEAL);
+
+	/*
+	 * This function may cause an immediate scan of ddt blocks (see
+	 * the comment above dsl_scan_ddt() for details). We set the
+	 * scan's root zio here so that we can wait for any scan IOs in
+	 * addition to the regular ddt IOs.
+	 */
+	ASSERT3P(scn->scn_zio_root, ==, NULL);
+	scn->scn_zio_root = rio;
+
 	for (enum zio_checksum c = 0; c < ZIO_CHECKSUM_FUNCTIONS; c++) {
 		ddt_t *ddt = spa->spa_ddt[c];
 		if (ddt == NULL)
@@ -1123,6 +1135,7 @@ ddt_sync(spa_t *spa, uint64_t txg)
 	}
 
 	(void) zio_wait(rio);
+	scn->scn_zio_root = NULL;
 
 	dmu_tx_commit(tx);
 }
diff --git a/usr/src/uts/common/fs/zfs/dmu_objset.c b/usr/src/uts/common/fs/zfs/dmu_objset.c
index d83153935f..771b803973 100644
--- a/usr/src/uts/common/fs/zfs/dmu_objset.c
+++ b/usr/src/uts/common/fs/zfs/dmu_objset.c
@@ -398,6 +398,7 @@ dmu_objset_open_impl(spa_t *spa, dsl_dataset_t *ds, blkptr_t *bp,
 
 	ASSERT(ds == NULL || MUTEX_HELD(&ds->ds_opening_lock));
 
+#if 0
 	/*
 	 * The $ORIGIN dataset (if it exists) doesn't have an associated
 	 * objset, so there's no reason to open it. The $ORIGIN dataset
@@ -408,6 +409,7 @@ dmu_objset_open_impl(spa_t *spa, dsl_dataset_t *ds, blkptr_t *bp,
 		ASSERT3P(ds->ds_dir, !=,
 		    spa_get_dsl(spa)->dp_origin_snap->ds_dir);
 	}
+#endif
 
 	os = kmem_zalloc(sizeof (objset_t), KM_SLEEP);
 	os->os_dsl_dataset = ds;
diff --git a/usr/src/uts/common/fs/zfs/dmu_traverse.c b/usr/src/uts/common/fs/zfs/dmu_traverse.c
index a95bf44fef..f57e510530 100644
--- a/usr/src/uts/common/fs/zfs/dmu_traverse.c
+++ b/usr/src/uts/common/fs/zfs/dmu_traverse.c
@@ -492,7 +492,8 @@ traverse_prefetcher(spa_t *spa, zilog_t *zilog, const blkptr_t *bp,
     const zbookmark_phys_t *zb, const dnode_phys_t *dnp, void *arg)
 {
 	prefetch_data_t *pfd = arg;
-	arc_flags_t aflags = ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH;
+	arc_flags_t aflags = ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH |
+	    ARC_FLAG_PRESCIENT_PREFETCH;
 
 	ASSERT(pfd->pd_bytes_fetched >= 0);
 	if (bp == NULL)
diff --git a/usr/src/uts/common/fs/zfs/dsl_scan.c b/usr/src/uts/common/fs/zfs/dsl_scan.c
index c19e43bd9f..00bd1498a2 100644
--- a/usr/src/uts/common/fs/zfs/dsl_scan.c
+++ b/usr/src/uts/common/fs/zfs/dsl_scan.c
@@ -51,34 +51,157 @@
 #include <sys/sa_impl.h>
 #include <sys/zfeature.h>
 #include <sys/abd.h>
+#include <sys/range_tree.h>
 #ifdef _KERNEL
 #include <sys/zfs_vfsops.h>
 #endif
 
+/*
+ * Grand theory statement on scan queue sorting
+ *
+ * Scanning is implemented by recursively traversing all indirection levels
+ * in an object and reading all blocks referenced from said objects. This
+ * results in us approximately traversing the object from lowest logical
+ * offset to the highest. For best performance, we would want the logical
+ * blocks to be physically contiguous. However, this is frequently not the
+ * case with pools given the allocation patterns of copy-on-write filesystems.
+ * So instead, we put the I/Os into a reordering queue and issue them in a
+ * way that will most benefit physical disks (LBA-order).
+ *
+ * Queue management:
+ *
+ * Ideally, we would want to scan all metadata and queue up all block I/O
+ * prior to starting to issue it, because that allows us to do an optimal
+ * sorting job. This can however consume large amounts of memory. Therefore
+ * we continuously monitor the size of the queues and constrain them to 5%
+ * (zfs_scan_mem_lim_fact) of physmem. If the queues grow larger than this
+ * limit, we clear out a few of the largest extents at the head of the queues
+ * to make room for more scanning. Hopefully, these extents will be fairly
+ * large and contiguous, allowing us to approach sequential I/O throughput
+ * even without a fully sorted tree.
+ *
+ * Metadata scanning takes place in dsl_scan_visit(), which is called from
+ * dsl_scan_sync() every spa_sync(). If we have either fully scanned all
+ * metadata on the pool, or we need to make room in memory because our
+ * queues are too large, dsl_scan_visit() is postponed and
+ * scan_io_queues_run() is called from dsl_scan_sync() instead. This implies
+ * that metadata scanning and queued I/O issuing are mutually exclusive. This
+ * allows us to provide maximum sequential I/O throughput for the majority of
+ * I/O's issued since sequential I/O performance is significantly negatively
+ * impacted if it is interleaved with random I/O.
+ *
+ * Implementation Notes
+ *
+ * One side effect of the queued scanning algorithm is that the scanning code
+ * needs to be notified whenever a block is freed. This is needed to allow
+ * the scanning code to remove these I/Os from the issuing queue. Additionally,
+ * we do not attempt to queue gang blocks to be issued sequentially since this
+ * is very hard to do and would have an extremely limited performance benefit.
+ * Instead, we simply issue gang I/Os as soon as we find them using the legacy
+ * algorithm.
+ *
+ * Backwards compatibility
+ *
+ * This new algorithm is backwards compatible with the legacy on-disk data
+ * structures (and therefore does not require a new feature flag).
+ * Periodically during scanning (see zfs_scan_checkpoint_intval), the scan
+ * will stop scanning metadata (in logical order) and wait for all outstanding
+ * sorted I/O to complete. Once this is done, we write out a checkpoint
+ * bookmark, indicating that we have scanned everything logically before it.
+ * If the pool is imported on a machine without the new sorting algorithm,
+ * the scan simply resumes from the last checkpoint using the legacy algorithm.
+ */
+
 typedef int (scan_cb_t)(dsl_pool_t *, const blkptr_t *,
     const zbookmark_phys_t *);
 
 static scan_cb_t dsl_scan_scrub_cb;
-static void dsl_scan_cancel_sync(void *, dmu_tx_t *);
-static void dsl_scan_sync_state(dsl_scan_t *, dmu_tx_t *);
-static boolean_t dsl_scan_restarting(dsl_scan_t *, dmu_tx_t *);
 
+static int scan_ds_queue_compare(const void *a, const void *b);
+static int scan_prefetch_queue_compare(const void *a, const void *b);
+static void scan_ds_queue_clear(dsl_scan_t *scn);
+static boolean_t scan_ds_queue_contains(dsl_scan_t *scn, uint64_t dsobj,
+    uint64_t *txg);
+static void scan_ds_queue_insert(dsl_scan_t *scn, uint64_t dsobj, uint64_t txg);
+static void scan_ds_queue_remove(dsl_scan_t *scn, uint64_t dsobj);
+static void scan_ds_queue_sync(dsl_scan_t *scn, dmu_tx_t *tx);
+
+extern int zfs_vdev_async_write_active_min_dirty_percent;
+
+/*
+ * By default zfs will check to ensure it is not over the hard memory
+ * limit before each txg. If finer-grained control of this is needed
+ * this value can be set to 1 to enable checking before scanning each
+ * block.
+ */
+int zfs_scan_strict_mem_lim = B_FALSE;
+
+/*
+ * Maximum number of parallelly executing I/Os per top-level vdev.
+ * Tune with care. Very high settings (hundreds) are known to trigger
+ * some firmware bugs and resets on certain SSDs.
+ */
 int zfs_top_maxinflight = 32;		/* maximum I/Os per top-level */
-int zfs_resilver_delay = 2;		/* number of ticks to delay resilver */
-int zfs_scrub_delay = 4;		/* number of ticks to delay scrub */
-int zfs_scan_idle = 50;			/* idle window in clock ticks */
-
-int zfs_scan_min_time_ms = 1000; /* min millisecs to scrub per txg */
-int zfs_free_min_time_ms = 1000; /* min millisecs to free per txg */
-int zfs_obsolete_min_time_ms = 500; /* min millisecs to obsolete per txg */
-int zfs_resilver_min_time_ms = 3000; /* min millisecs to resilver per txg */
+unsigned int zfs_resilver_delay = 2;	/* number of ticks to delay resilver */
+unsigned int zfs_scrub_delay = 4;	/* number of ticks to delay scrub */
+unsigned int zfs_scan_idle = 50;	/* idle window in clock ticks */
+
+/*
+ * Maximum number of parallelly executed bytes per leaf vdev. We attempt
+ * to strike a balance here between keeping the vdev queues full of I/Os
+ * at all times and not overflowing the queues to cause long latency,
+ * which would cause long txg sync times. No matter what, we will not
+ * overload the drives with I/O, since that is protected by
+ * zfs_vdev_scrub_max_active.
+ */
+unsigned long zfs_scan_vdev_limit = 4 << 20;
+
+int zfs_scan_issue_strategy = 0;
+int zfs_scan_legacy = B_FALSE;	/* don't queue & sort zios, go direct */
+uint64_t zfs_scan_max_ext_gap = 2 << 20;	/* in bytes */
+
+unsigned int zfs_scan_checkpoint_intval = 7200;	/* seconds */
+#define	ZFS_SCAN_CHECKPOINT_INTVAL	SEC_TO_TICK(zfs_scan_checkpoint_intval)
+
+/*
+ * fill_weight is non-tunable at runtime, so we copy it at module init from
+ * zfs_scan_fill_weight. Runtime adjustments to zfs_scan_fill_weight would
+ * break queue sorting.
+ */
+uint64_t zfs_scan_fill_weight = 3;
+static uint64_t fill_weight;
+
+/* See dsl_scan_should_clear() for details on the memory limit tunables */
+uint64_t zfs_scan_mem_lim_min = 16 << 20;	/* bytes */
+uint64_t zfs_scan_mem_lim_soft_max = 128 << 20;	/* bytes */
+int zfs_scan_mem_lim_fact = 20;		/* fraction of physmem */
+int zfs_scan_mem_lim_soft_fact = 20;	/* fraction of mem lim above */
+
+unsigned int zfs_scrub_min_time_ms = 1000; /* min millisecs to scrub per txg */
+unsigned int zfs_free_min_time_ms = 1000; /* min millisecs to free per txg */
+/* min millisecs to obsolete per txg */
+unsigned int zfs_obsolete_min_time_ms = 500;
+/* min millisecs to resilver per txg */
+unsigned int zfs_resilver_min_time_ms = 3000;
 boolean_t zfs_no_scrub_io = B_FALSE; /* set to disable scrub i/o */
 boolean_t zfs_no_scrub_prefetch = B_FALSE; /* set to disable scrub prefetch */
 enum ddt_class zfs_scrub_ddt_class_max = DDT_CLASS_DUPLICATE;
-int dsl_scan_delay_completion = B_FALSE; /* set to delay scan completion */
 /* max number of blocks to free in a single TXG */
 uint64_t zfs_async_block_max_blocks = UINT64_MAX;
 
+/*
+ * We wait a few txgs after importing a pool to begin scanning so that
+ * the import / mounting code isn't held up by scrub / resilver IO.
+ * Unfortunately, it is a bit difficult to determine exactly how long
+ * this will take since userspace will trigger fs mounts asynchronously
+ * and the kernel will create zvol minors asynchronously. As a result,
+ * the value provided here is a bit arbitrary, but represents a
+ * reasonable estimate of how many txgs it will take to finish fully
+ * importing a pool
+ */
+#define	SCAN_IMPORT_WAIT_TXGS		5
+
+
 #define	DSL_SCAN_IS_SCRUB_RESILVER(scn) \
 	((scn)->scn_phys.scn_func == POOL_SCAN_SCRUB || \
 	(scn)->scn_phys.scn_func == POOL_SCAN_RESILVER)
@@ -97,6 +220,163 @@ static scan_cb_t *scan_funcs[POOL_SCAN_FUNCS] = {
 	dsl_scan_scrub_cb,	/* POOL_SCAN_RESILVER */
 };
 
+/* In core node for the scn->scn_queue. Represents a dataset to be scanned */
+typedef struct {
+	uint64_t	sds_dsobj;
+	uint64_t	sds_txg;
+	avl_node_t	sds_node;
+} scan_ds_t;
+
+/*
+ * This controls what conditions are placed on dsl_scan_sync_state():
+ * SYNC_OPTIONAL) write out scn_phys iff scn_bytes_pending == 0
+ * SYNC_MANDATORY) write out scn_phys always. scn_bytes_pending must be 0.
+ * SYNC_CACHED) if scn_bytes_pending == 0, write out scn_phys. Otherwise
+ *	write out the scn_phys_cached version.
+ * See dsl_scan_sync_state for details.
+ */
+typedef enum {
+	SYNC_OPTIONAL,
+	SYNC_MANDATORY,
+	SYNC_CACHED
+} state_sync_type_t;
+
+/*
+ * This struct represents the minimum information needed to reconstruct a
+ * zio for sequential scanning. This is useful because many of these will
+ * accumulate in the sequential IO queues before being issued, so saving
+ * memory matters here.
+ */
+typedef struct scan_io {
+	/* fields from blkptr_t */
+	uint64_t		sio_offset;
+	uint64_t		sio_blk_prop;
+	uint64_t		sio_phys_birth;
+	uint64_t		sio_birth;
+	zio_cksum_t		sio_cksum;
+	uint32_t		sio_asize;
+
+	/* fields from zio_t */
+	int			sio_flags;
+	zbookmark_phys_t	sio_zb;
+
+	/* members for queue sorting */
+	union {
+		avl_node_t	sio_addr_node; /* link into issueing queue */
+		list_node_t	sio_list_node; /* link for issuing to disk */
+	} sio_nodes;
+} scan_io_t;
+
+struct dsl_scan_io_queue {
+	dsl_scan_t	*q_scn; /* associated dsl_scan_t */
+	vdev_t		*q_vd; /* top-level vdev that this queue represents */
+
+	/* trees used for sorting I/Os and extents of I/Os */
+	range_tree_t	*q_exts_by_addr;
+	avl_tree_t	q_exts_by_size;
+	avl_tree_t	q_sios_by_addr;
+
+	/* members for zio rate limiting */
+	uint64_t	q_maxinflight_bytes;
+	uint64_t	q_inflight_bytes;
+	kcondvar_t	q_zio_cv; /* used under vd->vdev_scan_io_queue_lock */
+
+	/* per txg statistics */
+	uint64_t	q_total_seg_size_this_txg;
+	uint64_t	q_segs_this_txg;
+	uint64_t	q_total_zio_size_this_txg;
+	uint64_t	q_zios_this_txg;
+};
+
+/* private data for dsl_scan_prefetch_cb() */
+typedef struct scan_prefetch_ctx {
+	zfs_refcount_t spc_refcnt;	/* refcount for memory management */
+	dsl_scan_t *spc_scn;		/* dsl_scan_t for the pool */
+	boolean_t spc_root;		/* is this prefetch for an objset? */
+	uint8_t spc_indblkshift;	/* dn_indblkshift of current dnode */
+	uint16_t spc_datablkszsec;	/* dn_idatablkszsec of current dnode */
+} scan_prefetch_ctx_t;
+
+/* private data for dsl_scan_prefetch() */
+typedef struct scan_prefetch_issue_ctx {
+	avl_node_t spic_avl_node;	/* link into scn->scn_prefetch_queue */
+	scan_prefetch_ctx_t *spic_spc;	/* spc for the callback */
+	blkptr_t spic_bp;		/* bp to prefetch */
+	zbookmark_phys_t spic_zb;	/* bookmark to prefetch */
+} scan_prefetch_issue_ctx_t;
+
+static void scan_exec_io(dsl_pool_t *dp, const blkptr_t *bp, int zio_flags,
+    const zbookmark_phys_t *zb, dsl_scan_io_queue_t *queue);
+static void scan_io_queue_insert_impl(dsl_scan_io_queue_t *queue,
+    scan_io_t *sio);
+
+static dsl_scan_io_queue_t *scan_io_queue_create(vdev_t *vd);
+static void scan_io_queues_destroy(dsl_scan_t *scn);
+
+static kmem_cache_t *sio_cache;
+
+void
+scan_init(void)
+{
+	/*
+	 * This is used in ext_size_compare() to weight segments
+	 * based on how sparse they are. This cannot be changed
+	 * mid-scan and the tree comparison functions don't currently
+	 * have a mechansim for passing additional context to the
+	 * compare functions. Thus we store this value globally and
+	 * we only allow it to be set at module intiailization time
+	 */
+	fill_weight = zfs_scan_fill_weight;
+
+	sio_cache = kmem_cache_create("sio_cache",
+	    sizeof (scan_io_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
+}
+
+void
+scan_fini(void)
+{
+	kmem_cache_destroy(sio_cache);
+}
+
+static inline boolean_t
+dsl_scan_is_running(const dsl_scan_t *scn)
+{
+	return (scn->scn_phys.scn_state == DSS_SCANNING);
+}
+
+boolean_t
+dsl_scan_resilvering(dsl_pool_t *dp)
+{
+	return (dsl_scan_is_running(dp->dp_scan) &&
+	    dp->dp_scan->scn_phys.scn_func == POOL_SCAN_RESILVER);
+}
+
+static inline void
+sio2bp(const scan_io_t *sio, blkptr_t *bp, uint64_t vdev_id)
+{
+	bzero(bp, sizeof (*bp));
+	DVA_SET_ASIZE(&bp->blk_dva[0], sio->sio_asize);
+	DVA_SET_VDEV(&bp->blk_dva[0], vdev_id);
+	DVA_SET_OFFSET(&bp->blk_dva[0], sio->sio_offset);
+	bp->blk_prop = sio->sio_blk_prop;
+	bp->blk_phys_birth = sio->sio_phys_birth;
+	bp->blk_birth = sio->sio_birth;
+	bp->blk_fill = 1;	/* we always only work with data pointers */
+	bp->blk_cksum = sio->sio_cksum;
+}
+
+static inline void
+bp2sio(const blkptr_t *bp, scan_io_t *sio, int dva_i)
+{
+	/* we discard the vdev id, since we can deduce it from the queue */
+	sio->sio_offset = DVA_GET_OFFSET(&bp->blk_dva[dva_i]);
+	sio->sio_asize = DVA_GET_ASIZE(&bp->blk_dva[dva_i]);
+	sio->sio_blk_prop = bp->blk_prop;
+	sio->sio_phys_birth = bp->blk_phys_birth;
+	sio->sio_birth = bp->blk_birth;
+	sio->sio_cksum = bp->blk_cksum;
+}
+
 int
 dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
 {
@@ -117,6 +397,13 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
 	scn->scn_async_destroying = spa_feature_is_active(dp->dp_spa,
 	    SPA_FEATURE_ASYNC_DESTROY);
 
+	bcopy(&scn->scn_phys, &scn->scn_phys_cached, sizeof (scn->scn_phys));
+	avl_create(&scn->scn_queue, scan_ds_queue_compare, sizeof (scan_ds_t),
+	    offsetof(scan_ds_t, sds_node));
+	avl_create(&scn->scn_prefetch_queue, scan_prefetch_queue_compare,
+	    sizeof (scan_prefetch_issue_ctx_t),
+	    offsetof(scan_prefetch_issue_ctx_t, spic_avl_node));
+
 	err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
 	    "scrub_func", sizeof (uint64_t), 1, &f);
 	if (err == 0) {
@@ -127,7 +414,7 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
 		scn->scn_restart_txg = txg;
 		zfs_dbgmsg("old-style scrub was in progress; "
 		    "restarting new-style scrub in txg %llu",
-		    scn->scn_restart_txg);
+		    (longlong_t)scn->scn_restart_txg);
 
 		/*
 		 * Load the queue obj from the old location so that it
@@ -145,7 +432,14 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
 		else if (err)
 			return (err);
 
-		if (scn->scn_phys.scn_state == DSS_SCANNING &&
+		/*
+		 * We might be restarting after a reboot, so jump the issued
+		 * counter to how far we've scanned. We know we're consistent
+		 * up to here.
+		 */
+		scn->scn_issued_before_pass = scn->scn_phys.scn_examined;
+
+		if (dsl_scan_is_running(scn) &&
 		    spa_prev_software_version(dp->dp_spa) < SPA_VERSION_SCAN) {
 			/*
 			 * A new-type scrub was in progress on an old
@@ -157,8 +451,24 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
 			scn->scn_restart_txg = txg;
 			zfs_dbgmsg("new-style scrub was modified "
 			    "by old software; restarting in txg %llu",
-			    scn->scn_restart_txg);
+			    (longlong_t)scn->scn_restart_txg);
+		}
+	}
+
+	/* reload the queue into the in-core state */
+	if (scn->scn_phys.scn_queue_obj != 0) {
+		zap_cursor_t zc;
+		zap_attribute_t za;
+
+		for (zap_cursor_init(&zc, dp->dp_meta_objset,
+		    scn->scn_phys.scn_queue_obj);
+		    zap_cursor_retrieve(&zc, &za) == 0;
+		    (void) zap_cursor_advance(&zc)) {
+			scan_ds_queue_insert(scn,
+			    zfs_strtonum(za.za_name, NULL),
+			    za.za_first_integer);
 		}
+		zap_cursor_fini(&zc);
 	}
 
 	spa_scan_stat_init(spa);
@@ -168,19 +478,116 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
 void
 dsl_scan_fini(dsl_pool_t *dp)
 {
-	if (dp->dp_scan) {
+	if (dp->dp_scan != NULL) {
+		dsl_scan_t *scn = dp->dp_scan;
+
+		if (scn->scn_taskq != NULL)
+			taskq_destroy(scn->scn_taskq);
+		scan_ds_queue_clear(scn);
+		avl_destroy(&scn->scn_queue);
+		avl_destroy(&scn->scn_prefetch_queue);
+
 		kmem_free(dp->dp_scan, sizeof (dsl_scan_t));
 		dp->dp_scan = NULL;
 	}
 }
 
+static boolean_t
+dsl_scan_restarting(dsl_scan_t *scn, dmu_tx_t *tx)
+{
+	return (scn->scn_restart_txg != 0 &&
+	    scn->scn_restart_txg <= tx->tx_txg);
+}
+
+boolean_t
+dsl_scan_scrubbing(const dsl_pool_t *dp)
+{
+	dsl_scan_phys_t *scn_phys = &dp->dp_scan->scn_phys;
+
+	return (scn_phys->scn_state == DSS_SCANNING &&
+	    scn_phys->scn_func == POOL_SCAN_SCRUB);
+}
+
+boolean_t
+dsl_scan_is_paused_scrub(const dsl_scan_t *scn)
+{
+	return (dsl_scan_scrubbing(scn->scn_dp) &&
+	    scn->scn_phys.scn_flags & DSF_SCRUB_PAUSED);
+}
+
+/*
+ * Writes out a persistent dsl_scan_phys_t record to the pool directory.
+ * Because we can be running in the block sorting algorithm, we do not always
+ * want to write out the record, only when it is "safe" to do so. This safety
+ * condition is achieved by making sure that the sorting queues are empty
+ * (scn_bytes_pending == 0). When this condition is not true, the sync'd state
+ * is inconsistent with how much actual scanning progress has been made. The
+ * kind of sync to be performed is specified by the sync_type argument. If the
+ * sync is optional, we only sync if the queues are empty. If the sync is
+ * mandatory, we do a hard ASSERT to make sure that the queues are empty. The
+ * third possible state is a "cached" sync. This is done in response to:
+ * 1) The dataset that was in the last sync'd dsl_scan_phys_t having been
+ *	destroyed, so we wouldn't be able to restart scanning from it.
+ * 2) The snapshot that was in the last sync'd dsl_scan_phys_t having been
+ *	superseded by a newer snapshot.
+ * 3) The dataset that was in the last sync'd dsl_scan_phys_t having been
+ *	swapped with its clone.
+ * In all cases, a cached sync simply rewrites the last record we've written,
+ * just slightly modified. For the modifications that are performed to the
+ * last written dsl_scan_phys_t, see dsl_scan_ds_destroyed,
+ * dsl_scan_ds_snapshotted and dsl_scan_ds_clone_swapped.
+ */
+static void
+dsl_scan_sync_state(dsl_scan_t *scn, dmu_tx_t *tx, state_sync_type_t sync_type)
+{
+	int i;
+	spa_t *spa = scn->scn_dp->dp_spa;
+
+	ASSERT(sync_type != SYNC_MANDATORY || scn->scn_bytes_pending == 0);
+	if (scn->scn_bytes_pending == 0) {
+		for (i = 0; i < spa->spa_root_vdev->vdev_children; i++) {
+			vdev_t *vd = spa->spa_root_vdev->vdev_child[i];
+			dsl_scan_io_queue_t *q = vd->vdev_scan_io_queue;
+
+			if (q == NULL)
+				continue;
+
+			mutex_enter(&vd->vdev_scan_io_queue_lock);
+			ASSERT3P(avl_first(&q->q_sios_by_addr), ==, NULL);
+			ASSERT3P(avl_first(&q->q_exts_by_size), ==, NULL);
+			ASSERT3P(range_tree_first(q->q_exts_by_addr), ==, NULL);
+			mutex_exit(&vd->vdev_scan_io_queue_lock);
+		}
+
+		if (scn->scn_phys.scn_queue_obj != 0)
+			scan_ds_queue_sync(scn, tx);
+		VERIFY0(zap_update(scn->scn_dp->dp_meta_objset,
+		    DMU_POOL_DIRECTORY_OBJECT,
+		    DMU_POOL_SCAN, sizeof (uint64_t), SCAN_PHYS_NUMINTS,
+		    &scn->scn_phys, tx));
+		bcopy(&scn->scn_phys, &scn->scn_phys_cached,
+		    sizeof (scn->scn_phys));
+
+		if (scn->scn_checkpointing)
+			zfs_dbgmsg("finish scan checkpoint");
+
+		scn->scn_checkpointing = B_FALSE;
+		scn->scn_last_checkpoint = ddi_get_lbolt();
+	} else if (sync_type == SYNC_CACHED) {
+		VERIFY0(zap_update(scn->scn_dp->dp_meta_objset,
+		    DMU_POOL_DIRECTORY_OBJECT,
+		    DMU_POOL_SCAN, sizeof (uint64_t), SCAN_PHYS_NUMINTS,
+		    &scn->scn_phys_cached, tx));
+	}
+}
+
 /* ARGSUSED */
 static int
 dsl_scan_setup_check(void *arg, dmu_tx_t *tx)
 {
 	dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;
 
-	if (scn->scn_phys.scn_state == DSS_SCANNING)
+	if (dsl_scan_is_running(scn))
 		return (SET_ERROR(EBUSY));
 
 	return (0);
@@ -195,7 +602,7 @@ dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
 	dsl_pool_t *dp = scn->scn_dp;
 	spa_t *spa = dp->dp_spa;
 
-	ASSERT(scn->scn_phys.scn_state != DSS_SCANNING);
+	ASSERT(!dsl_scan_is_running(scn));
 	ASSERT(*funcp > POOL_SCAN_NONE && *funcp < POOL_SCAN_FUNCS);
 	bzero(&scn->scn_phys, sizeof (scn->scn_phys));
 	scn->scn_phys.scn_func = *funcp;
@@ -206,8 +613,11 @@ dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
 	scn->scn_phys.scn_start_time = gethrestime_sec();
 	scn->scn_phys.scn_errors = 0;
 	scn->scn_phys.scn_to_examine = spa->spa_root_vdev->vdev_stat.vs_alloc;
+	scn->scn_issued_before_pass = 0;
 	scn->scn_restart_txg = 0;
 	scn->scn_done_txg = 0;
+	scn->scn_last_checkpoint = 0;
+	scn->scn_checkpointing = B_FALSE;
 	spa_scan_stat_init(spa);
 
 	if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
@@ -240,8 +650,10 @@ dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
 	if (dp->dp_blkstats == NULL) {
 		dp->dp_blkstats =
 		    kmem_alloc(sizeof (zfs_all_blkstats_t), KM_SLEEP);
+		mutex_init(&dp->dp_blkstats->zab_lock, NULL,
+		    MUTEX_DEFAULT, NULL);
 	}
-	bzero(dp->dp_blkstats, sizeof (zfs_all_blkstats_t));
+	bzero(&dp->dp_blkstats->zab_type, sizeof (dp->dp_blkstats->zab_type));
 
 	if (spa_version(spa) < SPA_VERSION_DSL_SCRUB)
 		ot = DMU_OT_ZAP_OTHER;
@@ -249,13 +661,53 @@ dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
 	scn->scn_phys.scn_queue_obj = zap_create(dp->dp_meta_objset,
 	    ot ? ot : DMU_OT_SCAN_QUEUE, DMU_OT_NONE, 0, tx);
 
-	dsl_scan_sync_state(scn, tx);
+	bcopy(&scn->scn_phys, &scn->scn_phys_cached, sizeof (scn->scn_phys));
+
+	dsl_scan_sync_state(scn, tx, SYNC_MANDATORY);
 
 	spa_history_log_internal(spa, "scan setup", tx,
 	    "func=%u mintxg=%llu maxtxg=%llu",
 	    *funcp, scn->scn_phys.scn_min_txg, scn->scn_phys.scn_max_txg);
 }
 
+/*
+ * Called by the ZFS_IOC_POOL_SCAN ioctl to start a scrub or resilver.
+ * Can also be called to resume a paused scrub.
+ */
+int
+dsl_scan(dsl_pool_t *dp, pool_scan_func_t func)
+{
+	spa_t *spa = dp->dp_spa;
+	dsl_scan_t *scn = dp->dp_scan;
+
+	/*
+	 * Purge all vdev caches and probe all devices.  We do this here
+	 * rather than in sync context because this requires a writer lock
+	 * on the spa_config lock, which we can't do from sync context.  The
+	 * spa_scrub_reopen flag indicates that vdev_open() should not
+	 * attempt to start another scrub.
+	 */
+	spa_vdev_state_enter(spa, SCL_NONE);
+	spa->spa_scrub_reopen = B_TRUE;
+	vdev_reopen(spa->spa_root_vdev);
+	spa->spa_scrub_reopen = B_FALSE;
+	(void) spa_vdev_state_exit(spa, NULL, 0);
+
+	if (func == POOL_SCAN_SCRUB && dsl_scan_is_paused_scrub(scn)) {
+		/* got scrub start cmd, resume paused scrub */
+		int err = dsl_scrub_set_pause_resume(scn->scn_dp,
+		    POOL_SCRUB_NORMAL);
+		if (err == 0) {
+			spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_RESUME);
+			return (ECANCELED);
+		}
+		return (SET_ERROR(err));
+	}
+
+	return (dsl_sync_task(spa_name(spa), dsl_scan_setup_check,
+	    dsl_scan_setup_sync, &func, 0, ZFS_SPACE_CHECK_EXTRA_RESERVED));
+}
+
 /* ARGSUSED */
 static void
 dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
@@ -283,10 +735,11 @@ dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
 	}
 
 	if (scn->scn_phys.scn_queue_obj != 0) {
-		VERIFY(0 == dmu_object_free(dp->dp_meta_objset,
+		VERIFY0(dmu_object_free(dp->dp_meta_objset,
 		    scn->scn_phys.scn_queue_obj, tx));
 		scn->scn_phys.scn_queue_obj = 0;
 	}
+	scan_ds_queue_clear(scn);
 
 	scn->scn_phys.scn_flags &= ~DSF_SCRUB_PAUSED;
 
@@ -294,13 +747,22 @@ dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
 	 * If we were "restarted" from a stopped state, don't bother
 	 * with anything else.
 	 */
-	if (scn->scn_phys.scn_state != DSS_SCANNING)
+	if (!dsl_scan_is_running(scn)) {
+		ASSERT(!scn->scn_is_sorted);
 		return;
+	}
 
-	if (complete)
-		scn->scn_phys.scn_state = DSS_FINISHED;
-	else
-		scn->scn_phys.scn_state = DSS_CANCELED;
+	if (scn->scn_is_sorted) {
+		scan_io_queues_destroy(scn);
+		scn->scn_is_sorted = B_FALSE;
+
+		if (scn->scn_taskq != NULL) {
+			taskq_destroy(scn->scn_taskq);
+			scn->scn_taskq = NULL;
+		}
+	}
+
+	scn->scn_phys.scn_state = complete ? DSS_FINISHED : DSS_CANCELED;
 
 	if (dsl_scan_restarting(scn, tx))
 		spa_history_log_internal(spa, "scan aborted, restarting", tx,
@@ -313,12 +775,6 @@ dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
 		    "errors=%llu", spa_get_errlog_size(spa));
 
 	if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
-		mutex_enter(&spa->spa_scrub_lock);
-		while (spa->spa_scrub_inflight > 0) {
-			cv_wait(&spa->spa_scrub_io_cv,
-			    &spa->spa_scrub_lock);
-		}
-		mutex_exit(&spa->spa_scrub_lock);
 		spa->spa_scrub_started = B_FALSE;
 		spa->spa_scrub_active = B_FALSE;
 
@@ -354,6 +810,8 @@ dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
 	}
 
 	scn->scn_phys.scn_end_time = gethrestime_sec();
+
+	ASSERT(!dsl_scan_is_running(scn));
 }
 
 /* ARGSUSED */
@@ -362,7 +820,7 @@ dsl_scan_cancel_check(void *arg, dmu_tx_t *tx)
 {
 	dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;
 
-	if (scn->scn_phys.scn_state != DSS_SCANNING)
+	if (!dsl_scan_is_running(scn))
 		return (SET_ERROR(ENOENT));
 	return (0);
 }
@@ -374,7 +832,7 @@ dsl_scan_cancel_sync(void *arg, dmu_tx_t *tx)
 	dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;
 
 	dsl_scan_done(scn, B_FALSE, tx);
-	dsl_scan_sync_state(scn, tx);
+	dsl_scan_sync_state(scn, tx, SYNC_MANDATORY);
 	spa_event_notify(scn->scn_dp->dp_spa, NULL, NULL, ESC_ZFS_SCRUB_ABORT);
 }
 
@@ -385,16 +843,6 @@ dsl_scan_cancel(dsl_pool_t *dp)
 	    dsl_scan_cancel_sync, NULL, 3, ZFS_SPACE_CHECK_RESERVED));
 }
 
-boolean_t
-dsl_scan_is_paused_scrub(const dsl_scan_t *scn)
-{
-	if (dsl_scan_scrubbing(scn->scn_dp) &&
-	    scn->scn_phys.scn_flags & DSF_SCRUB_PAUSED)
-		return (B_TRUE);
-
-	return (B_FALSE);
-}
-
 static int
 dsl_scrub_pause_resume_check(void *arg, dmu_tx_t *tx)
 {
@@ -429,7 +877,7 @@ dsl_scrub_pause_resume_sync(void *arg, dmu_tx_t *tx)
 		/* can't pause a scrub when there is no in-progress scrub */
 		spa->spa_scan_pass_scrub_pause = gethrestime_sec();
 		scn->scn_phys.scn_flags |= DSF_SCRUB_PAUSED;
-		dsl_scan_sync_state(scn, tx);
+		dsl_scan_sync_state(scn, tx, SYNC_CACHED);
 		spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_PAUSED);
 	} else {
 		ASSERT3U(*cmd, ==, POOL_SCRUB_NORMAL);
@@ -443,7 +891,7 @@ dsl_scrub_pause_resume_sync(void *arg, dmu_tx_t *tx)
 			    gethrestime_sec() - spa->spa_scan_pass_scrub_pause;
 			spa->spa_scan_pass_scrub_pause = 0;
 			scn->scn_phys.scn_flags &= ~DSF_SCRUB_PAUSED;
-			dsl_scan_sync_state(scn, tx);
+			dsl_scan_sync_state(scn, tx, SYNC_CACHED);
 		}
 	}
 }
@@ -459,25 +907,25 @@ dsl_scrub_set_pause_resume(const dsl_pool_t *dp, pool_scrub_cmd_t cmd)
 	    ZFS_SPACE_CHECK_RESERVED));
 }
 
-boolean_t
-dsl_scan_scrubbing(const dsl_pool_t *dp)
-{
-	dsl_scan_t *scn = dp->dp_scan;
 
-	if (scn->scn_phys.scn_state == DSS_SCANNING &&
-	    scn->scn_phys.scn_func == POOL_SCAN_SCRUB)
-		return (B_TRUE);
+/* start a new scan, or restart an existing one. */
+void
+dsl_resilver_restart(dsl_pool_t *dp, uint64_t txg)
+{
+	if (txg == 0) {
+		dmu_tx_t *tx;
+		tx = dmu_tx_create_dd(dp->dp_mos_dir);
+		VERIFY(0 == dmu_tx_assign(tx, TXG_WAIT));
 
-	return (B_FALSE);
+		txg = dmu_tx_get_txg(tx);
+		dp->dp_scan->scn_restart_txg = txg;
+		dmu_tx_commit(tx);
+	} else {
+		dp->dp_scan->scn_restart_txg = txg;
+	}
+	zfs_dbgmsg("restarting resilver txg=%llu", txg);
 }
 
-static void dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
-    dnode_phys_t *dnp, dsl_dataset_t *ds, dsl_scan_t *scn,
-    dmu_objset_type_t ostype, dmu_tx_t *tx);
-static void dsl_scan_visitdnode(dsl_scan_t *, dsl_dataset_t *ds,
-    dmu_objset_type_t ostype,
-    dnode_phys_t *dnp, uint64_t object, dmu_tx_t *tx);
-
 void
 dsl_free(dsl_pool_t *dp, uint64_t txg, const blkptr_t *bp)
 {
@@ -491,25 +939,169 @@ dsl_free_sync(zio_t *pio, dsl_pool_t *dp, uint64_t txg, const blkptr_t *bpp)
 	zio_nowait(zio_free_sync(pio, dp->dp_spa, txg, bpp, pio->io_flags));
 }
 
-static uint64_t
-dsl_scan_ds_maxtxg(dsl_dataset_t *ds)
+static int
+scan_ds_queue_compare(const void *a, const void *b)
 {
-	uint64_t smt = ds->ds_dir->dd_pool->dp_scan->scn_phys.scn_max_txg;
-	if (ds->ds_is_snapshot)
-		return (MIN(smt, dsl_dataset_phys(ds)->ds_creation_txg));
-	return (smt);
+	const scan_ds_t *sds_a = a, *sds_b = b;
+
+	if (sds_a->sds_dsobj < sds_b->sds_dsobj)
+		return (-1);
+	if (sds_a->sds_dsobj == sds_b->sds_dsobj)
+		return (0);
+	return (1);
 }
 
 static void
-dsl_scan_sync_state(dsl_scan_t *scn, dmu_tx_t *tx)
+scan_ds_queue_clear(dsl_scan_t *scn)
 {
-	VERIFY0(zap_update(scn->scn_dp->dp_meta_objset,
-	    DMU_POOL_DIRECTORY_OBJECT,
-	    DMU_POOL_SCAN, sizeof (uint64_t), SCAN_PHYS_NUMINTS,
-	    &scn->scn_phys, tx));
+	void *cookie = NULL;
+	scan_ds_t *sds;
+	while ((sds = avl_destroy_nodes(&scn->scn_queue, &cookie)) != NULL) {
+		kmem_free(sds, sizeof (*sds));
+	}
 }
 
-extern int zfs_vdev_async_write_active_min_dirty_percent;
+static boolean_t
+scan_ds_queue_contains(dsl_scan_t *scn, uint64_t dsobj, uint64_t *txg)
+{
+	scan_ds_t srch, *sds;
+
+	srch.sds_dsobj = dsobj;
+	sds = avl_find(&scn->scn_queue, &srch, NULL);
+	if (sds != NULL && txg != NULL)
+		*txg = sds->sds_txg;
+	return (sds != NULL);
+}
+
+static void
+scan_ds_queue_insert(dsl_scan_t *scn, uint64_t dsobj, uint64_t txg)
+{
+	scan_ds_t *sds;
+	avl_index_t where;
+
+	sds = kmem_zalloc(sizeof (*sds), KM_SLEEP);
+	sds->sds_dsobj = dsobj;
+	sds->sds_txg = txg;
+
+	VERIFY3P(avl_find(&scn->scn_queue, sds, &where), ==, NULL);
+	avl_insert(&scn->scn_queue, sds, where);
+}
+
+static void
+scan_ds_queue_remove(dsl_scan_t *scn, uint64_t dsobj)
+{
+	scan_ds_t srch, *sds;
+
+	srch.sds_dsobj = dsobj;
+
+	sds = avl_find(&scn->scn_queue, &srch, NULL);
+	VERIFY(sds != NULL);
+	avl_remove(&scn->scn_queue, sds);
+	kmem_free(sds, sizeof (*sds));
+}
+
+static void
+scan_ds_queue_sync(dsl_scan_t *scn, dmu_tx_t *tx)
+{
+	dsl_pool_t *dp = scn->scn_dp;
+	spa_t *spa = dp->dp_spa;
+	dmu_object_type_t ot = (spa_version(spa) >= SPA_VERSION_DSL_SCRUB) ?
+	    DMU_OT_SCAN_QUEUE : DMU_OT_ZAP_OTHER;
+
+	ASSERT0(scn->scn_bytes_pending);
+	ASSERT(scn->scn_phys.scn_queue_obj != 0);
+
+	VERIFY0(dmu_object_free(dp->dp_meta_objset,
+	    scn->scn_phys.scn_queue_obj, tx));
+	scn->scn_phys.scn_queue_obj = zap_create(dp->dp_meta_objset, ot,
+	    DMU_OT_NONE, 0, tx);
+	for (scan_ds_t *sds = avl_first(&scn->scn_queue);
+	    sds != NULL; sds = AVL_NEXT(&scn->scn_queue, sds)) {
+		VERIFY0(zap_add_int_key(dp->dp_meta_objset,
+		    scn->scn_phys.scn_queue_obj, sds->sds_dsobj,
+		    sds->sds_txg, tx));
+	}
+}
+
+/*
+ * Computes the memory limit state that we're currently in. A sorted scan
+ * needs quite a bit of memory to hold the sorting queue, so we need to
+ * reasonably constrain the size so it doesn't impact overall system
+ * performance. We compute two limits:
+ * 1) Hard memory limit: if the amount of memory used by the sorting
+ *	queues on a pool gets above this value, we stop the metadata
+ *	scanning portion and start issuing the queued up and sorted
+ *	I/Os to reduce memory usage.
+ *	This limit is calculated as a fraction of physmem (by default 5%).
+ *	We constrain the lower bound of the hard limit to an absolute
+ *	minimum of zfs_scan_mem_lim_min (default: 16 MiB). We also constrain
+ *	the upper bound to 5% of the total pool size - no chance we'll
+ *	ever need that much memory, but just to keep the value in check.
+ * 2) Soft memory limit: once we hit the hard memory limit, we start
+ *	issuing I/O to reduce queue memory usage, but we don't want to
+ *	completely empty out the queues, since we might be able to find I/Os
+ *	that will fill in the gaps of our non-sequential IOs at some point
+ *	in the future. So we stop the issuing of I/Os once the amount of
+ *	memory used drops below the soft limit (at which point we stop issuing
+ *	I/O and start scanning metadata again).
+ *
+ *	This limit is calculated by subtracting a fraction of the hard
+ *	limit from the hard limit. By default this fraction is 5%, so
+ *	the soft limit is 95% of the hard limit. We cap the size of the
+ *	difference between the hard and soft limits at an absolute
+ *	maximum of zfs_scan_mem_lim_soft_max (default: 128 MiB) - this is
+ *	sufficient to not cause too frequent switching between the
+ *	metadata scan and I/O issue (even at 2k recordsize, 128 MiB's
+ *	worth of queues is about 1.2 GiB of on-pool data, so scanning
+ *	that should take at least a decent fraction of a second).
+ */
+static boolean_t
+dsl_scan_should_clear(dsl_scan_t *scn)
+{
+	vdev_t *rvd = scn->scn_dp->dp_spa->spa_root_vdev;
+	uint64_t mlim_hard, mlim_soft, mused;
+	uint64_t alloc = metaslab_class_get_alloc(spa_normal_class(
+	    scn->scn_dp->dp_spa));
+
+	mlim_hard = MAX((physmem / zfs_scan_mem_lim_fact) * PAGESIZE,
+	    zfs_scan_mem_lim_min);
+	mlim_hard = MIN(mlim_hard, alloc / 20);
+	mlim_soft = mlim_hard - MIN(mlim_hard / zfs_scan_mem_lim_soft_fact,
+	    zfs_scan_mem_lim_soft_max);
+	mused = 0;
+	for (uint64_t i = 0; i < rvd->vdev_children; i++) {
+		vdev_t *tvd = rvd->vdev_child[i];
+		dsl_scan_io_queue_t *queue;
+
+		mutex_enter(&tvd->vdev_scan_io_queue_lock);
+		queue = tvd->vdev_scan_io_queue;
+		if (queue != NULL) {
+			/* #extents in exts_by_size = # in exts_by_addr */
+			mused += avl_numnodes(&queue->q_exts_by_size) *
+			    sizeof (range_seg_t) +
+			    avl_numnodes(&queue->q_sios_by_addr) *
+			    sizeof (scan_io_t);
+		}
+		mutex_exit(&tvd->vdev_scan_io_queue_lock);
+	}
+
+	dprintf("current scan memory usage: %llu bytes\n", (longlong_t)mused);
+
+	if (mused == 0)
+		ASSERT0(scn->scn_bytes_pending);
+
+	/*
+	 * If we are above our hard limit, we need to clear out memory.
+	 * If we are below our soft limit, we need to accumulate sequential IOs.
+	 * Otherwise, we should keep doing whatever we are currently doing.
+	 */
+	if (mused >= mlim_hard)
+		return (B_TRUE);
+	else if (mused < mlim_soft)
+		return (B_FALSE);
+	else
+		return (scn->scn_clearing);
+}
 
 static boolean_t
 dsl_scan_check_suspend(dsl_scan_t *scn, const zbookmark_phys_t *zb)
@@ -530,9 +1122,6 @@ dsl_scan_check_suspend(dsl_scan_t *scn, const zbookmark_phys_t *zb)
 
 	/*
 	 * We suspend if:
-	 *  - we have scanned for the maximum time: an entire txg
-	 *    timeout (default 5 sec)
-	 *  or
 	 *  - we have scanned for at least the minimum time (default 1 sec
 	 *    for scrub, 3 sec for resilver), and either we have sufficient
 	 *    dirty data that we are starting to write more quickly
@@ -541,16 +1130,24 @@ dsl_scan_check_suspend(dsl_scan_t *scn, const zbookmark_phys_t *zb)
 	 *  or
 	 *  - the spa is shutting down because this pool is being exported
 	 *    or the machine is rebooting.
+	 *  or
+	 *  - the scan queue has reached its memory use limit
 	 */
-	int mintime = (scn->scn_phys.scn_func == POOL_SCAN_RESILVER) ?
-	    zfs_resilver_min_time_ms : zfs_scan_min_time_ms;
-	uint64_t elapsed_nanosecs = gethrtime() - scn->scn_sync_start_time;
+	hrtime_t curr_time_ns = gethrtime();
+	uint64_t scan_time_ns = curr_time_ns - scn->scn_sync_start_time;
+	uint64_t sync_time_ns = curr_time_ns -
+	    scn->scn_dp->dp_spa->spa_sync_starttime;
+
 	int dirty_pct = scn->scn_dp->dp_dirty_total * 100 / zfs_dirty_data_max;
-	if (elapsed_nanosecs / NANOSEC >= zfs_txg_timeout ||
-	    (NSEC2MSEC(elapsed_nanosecs) > mintime &&
-	    (txg_sync_waiting(scn->scn_dp) ||
-	    dirty_pct >= zfs_vdev_async_write_active_min_dirty_percent)) ||
-	    spa_shutting_down(scn->scn_dp->dp_spa)) {
+	int mintime = (scn->scn_phys.scn_func == POOL_SCAN_RESILVER) ?
+	    zfs_resilver_min_time_ms : zfs_scrub_min_time_ms;
+
+	if ((NSEC2MSEC(scan_time_ns) > mintime &&
+	    (dirty_pct >= zfs_vdev_async_write_active_min_dirty_percent ||
+	    txg_sync_waiting(scn->scn_dp) ||
+	    NSEC2SEC(sync_time_ns) >= zfs_txg_timeout)) ||
+	    spa_shutting_down(scn->scn_dp->dp_spa) ||
+	    (zfs_scan_strict_mem_lim && dsl_scan_should_clear(scn))) {
 		if (zb) {
 			dprintf("suspending at bookmark %llx/%llx/%llx/%llx\n",
 			    (longlong_t)zb->zb_objset,
@@ -558,12 +1155,16 @@ dsl_scan_check_suspend(dsl_scan_t *scn, const zbookmark_phys_t *zb)
 			    (longlong_t)zb->zb_level,
 			    (longlong_t)zb->zb_blkid);
 			scn->scn_phys.scn_bookmark = *zb;
+		} else {
+			dsl_scan_phys_t *scnp = &scn->scn_phys;
+
+			dprintf("suspending at DDT bookmark "
+			    "%llx/%llx/%llx/%llx\n",
+			    (longlong_t)scnp->scn_ddt_bookmark.ddb_class,
+			    (longlong_t)scnp->scn_ddt_bookmark.ddb_type,
+			    (longlong_t)scnp->scn_ddt_bookmark.ddb_checksum,
+			    (longlong_t)scnp->scn_ddt_bookmark.ddb_cursor);
 		}
-		dprintf("suspending at DDT bookmark %llx/%llx/%llx/%llx\n",
-		    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_class,
-		    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_type,
-		    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_checksum,
-		    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_cursor);
 		scn->scn_suspending = B_TRUE;
 		return (B_TRUE);
 	}
@@ -662,26 +1263,278 @@ dsl_scan_zil(dsl_pool_t *dp, zil_header_t *zh)
 	zil_free(zilog);
 }
 
-/* ARGSUSED */
+/*
+ * We compare scan_prefetch_issue_ctx_t's based on their bookmarks. The idea
+ * here is to sort the AVL tree by the order each block will be needed.
+ */
+static int
+scan_prefetch_queue_compare(const void *a, const void *b)
+{
+	const scan_prefetch_issue_ctx_t *spic_a = a, *spic_b = b;
+	const scan_prefetch_ctx_t *spc_a = spic_a->spic_spc;
+	const scan_prefetch_ctx_t *spc_b = spic_b->spic_spc;
+
+	return (zbookmark_compare(spc_a->spc_datablkszsec,
+	    spc_a->spc_indblkshift, spc_b->spc_datablkszsec,
+	    spc_b->spc_indblkshift, &spic_a->spic_zb, &spic_b->spic_zb));
+}
+
 static void
-dsl_scan_prefetch(dsl_scan_t *scn, arc_buf_t *buf, blkptr_t *bp,
-    uint64_t objset, uint64_t object, uint64_t blkid)
+scan_prefetch_ctx_rele(scan_prefetch_ctx_t *spc, void *tag)
+{
+	if (zfs_refcount_remove(&spc->spc_refcnt, tag) == 0) {
+		zfs_refcount_destroy(&spc->spc_refcnt);
+		kmem_free(spc, sizeof (scan_prefetch_ctx_t));
+	}
+}
+
+static scan_prefetch_ctx_t *
+scan_prefetch_ctx_create(dsl_scan_t *scn, dnode_phys_t *dnp, void *tag)
 {
-	zbookmark_phys_t czb;
-	arc_flags_t flags = ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH;
+	scan_prefetch_ctx_t *spc;
+
+	spc = kmem_alloc(sizeof (scan_prefetch_ctx_t), KM_SLEEP);
+	zfs_refcount_create(&spc->spc_refcnt);
+	zfs_refcount_add(&spc->spc_refcnt, tag);
+	spc->spc_scn = scn;
+	if (dnp != NULL) {
+		spc->spc_datablkszsec = dnp->dn_datablkszsec;
+		spc->spc_indblkshift = dnp->dn_indblkshift;
+		spc->spc_root = B_FALSE;
+	} else {
+		spc->spc_datablkszsec = 0;
+		spc->spc_indblkshift = 0;
+		spc->spc_root = B_TRUE;
+	}
+
+	return (spc);
+}
+
+static void
+scan_prefetch_ctx_add_ref(scan_prefetch_ctx_t *spc, void *tag)
+{
+	zfs_refcount_add(&spc->spc_refcnt, tag);
+}
+
+static boolean_t
+dsl_scan_check_prefetch_resume(scan_prefetch_ctx_t *spc,
+    const zbookmark_phys_t *zb)
+{
+	zbookmark_phys_t *last_zb = &spc->spc_scn->scn_prefetch_bookmark;
+	dnode_phys_t tmp_dnp;
+	dnode_phys_t *dnp = (spc->spc_root) ? NULL : &tmp_dnp;
+
+	if (zb->zb_objset != last_zb->zb_objset)
+		return (B_TRUE);
+	if ((int64_t)zb->zb_object < 0)
+		return (B_FALSE);
+
+	tmp_dnp.dn_datablkszsec = spc->spc_datablkszsec;
+	tmp_dnp.dn_indblkshift = spc->spc_indblkshift;
+
+	if (zbookmark_subtree_completed(dnp, zb, last_zb))
+		return (B_TRUE);
+
+	return (B_FALSE);
+}
+
+static void
+dsl_scan_prefetch(scan_prefetch_ctx_t *spc, blkptr_t *bp, zbookmark_phys_t *zb)
+{
+	avl_index_t idx;
+	dsl_scan_t *scn = spc->spc_scn;
+	spa_t *spa = scn->scn_dp->dp_spa;
+	scan_prefetch_issue_ctx_t *spic;
 
 	if (zfs_no_scrub_prefetch)
 		return;
 
-	if (BP_IS_HOLE(bp) || bp->blk_birth <= scn->scn_phys.scn_min_txg ||
-	    (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_DNODE))
+	if (BP_IS_HOLE(bp) || bp->blk_birth <= scn->scn_phys.scn_cur_min_txg ||
+	    (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_DNODE &&
+	    BP_GET_TYPE(bp) != DMU_OT_OBJSET))
+		return;
+
+	if (dsl_scan_check_prefetch_resume(spc, zb))
 		return;
 
-	SET_BOOKMARK(&czb, objset, object, BP_GET_LEVEL(bp), blkid);
+	scan_prefetch_ctx_add_ref(spc, scn);
+	spic = kmem_alloc(sizeof (scan_prefetch_issue_ctx_t), KM_SLEEP);
+	spic->spic_spc = spc;
+	spic->spic_bp = *bp;
+	spic->spic_zb = *zb;
+
+	/*
+	 * Add the IO to the queue of blocks to prefetch. This allows us to
+	 * prioritize blocks that we will need first for the main traversal
+	 * thread.
+	 */
+	mutex_enter(&spa->spa_scrub_lock);
+	if (avl_find(&scn->scn_prefetch_queue, spic, &idx) != NULL) {
+		/* this block is already queued for prefetch */
+		kmem_free(spic, sizeof (scan_prefetch_issue_ctx_t));
+		scan_prefetch_ctx_rele(spc, scn);
+		mutex_exit(&spa->spa_scrub_lock);
+		return;
+	}
+
+	avl_insert(&scn->scn_prefetch_queue, spic, idx);
+	cv_broadcast(&spa->spa_scrub_io_cv);
+	mutex_exit(&spa->spa_scrub_lock);
+}
+
+static void
+dsl_scan_prefetch_dnode(dsl_scan_t *scn, dnode_phys_t *dnp,
+    uint64_t objset, uint64_t object)
+{
+	int i;
+	zbookmark_phys_t zb;
+	scan_prefetch_ctx_t *spc;
+
+	if (dnp->dn_nblkptr == 0 && !(dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR))
+		return;
+
+	SET_BOOKMARK(&zb, objset, object, 0, 0);
+
+	spc = scan_prefetch_ctx_create(scn, dnp, FTAG);
+
+	for (i = 0; i < dnp->dn_nblkptr; i++) {
+		zb.zb_level = BP_GET_LEVEL(&dnp->dn_blkptr[i]);
+		zb.zb_blkid = i;
+		dsl_scan_prefetch(spc, &dnp->dn_blkptr[i], &zb);
+	}
+
+	if (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR) {
+		zb.zb_level = 0;
+		zb.zb_blkid = DMU_SPILL_BLKID;
+		dsl_scan_prefetch(spc, &dnp->dn_spill, &zb);
+	}
+
+	scan_prefetch_ctx_rele(spc, FTAG);
+}
+
+void
+dsl_scan_prefetch_cb(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
+    arc_buf_t *buf, void *private)
+{
+	scan_prefetch_ctx_t *spc = private;
+	dsl_scan_t *scn = spc->spc_scn;
+	spa_t *spa = scn->scn_dp->dp_spa;
+
+	/* broadcast that the IO has completed for rate limitting purposes */
+	mutex_enter(&spa->spa_scrub_lock);
+	ASSERT3U(spa->spa_scrub_inflight, >=, BP_GET_PSIZE(bp));
+	spa->spa_scrub_inflight -= BP_GET_PSIZE(bp);
+	cv_broadcast(&spa->spa_scrub_io_cv);
+	mutex_exit(&spa->spa_scrub_lock);
+
+	/* if there was an error or we are done prefetching, just cleanup */
+	if (buf == NULL || scn->scn_suspending)
+		goto out;
+
+	if (BP_GET_LEVEL(bp) > 0) {
+		int i;
+		blkptr_t *cbp;
+		int epb = BP_GET_LSIZE(bp) >> SPA_BLKPTRSHIFT;
+		zbookmark_phys_t czb;
+
+		for (i = 0, cbp = buf->b_data; i < epb; i++, cbp++) {
+			SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
+			    zb->zb_level - 1, zb->zb_blkid * epb + i);
+			dsl_scan_prefetch(spc, cbp, &czb);
+		}
+	} else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
+		dnode_phys_t *cdnp = buf->b_data;
+		int i;
+		int epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;
+
+		for (i = 0, cdnp = buf->b_data; i < epb;
+		    i += cdnp->dn_extra_slots + 1,
+		    cdnp += cdnp->dn_extra_slots + 1) {
+			dsl_scan_prefetch_dnode(scn, cdnp,
+			    zb->zb_objset, zb->zb_blkid * epb + i);
+		}
+	} else if (BP_GET_TYPE(bp) == DMU_OT_OBJSET) {
+		objset_phys_t *osp = buf->b_data;
+
+		dsl_scan_prefetch_dnode(scn, &osp->os_meta_dnode,
+		    zb->zb_objset, DMU_META_DNODE_OBJECT);
+
+		if (OBJSET_BUF_HAS_USERUSED(buf)) {
+			dsl_scan_prefetch_dnode(scn,
+			    &osp->os_groupused_dnode, zb->zb_objset,
+			    DMU_GROUPUSED_OBJECT);
+			dsl_scan_prefetch_dnode(scn,
+			    &osp->os_userused_dnode, zb->zb_objset,
+			    DMU_USERUSED_OBJECT);
+		}
+	}
+
+out:
+	if (buf != NULL)
+		arc_buf_destroy(buf, private);
+	scan_prefetch_ctx_rele(spc, scn);
+}
+
+/* ARGSUSED */
+static void
+dsl_scan_prefetch_thread(void *arg)
+{
+	dsl_scan_t *scn = arg;
+	spa_t *spa = scn->scn_dp->dp_spa;
+	vdev_t *rvd = spa->spa_root_vdev;
+	uint64_t maxinflight = rvd->vdev_children * zfs_top_maxinflight;
+	scan_prefetch_issue_ctx_t *spic;
+
+	/* loop until we are told to stop */
+	while (!scn->scn_prefetch_stop) {
+		arc_flags_t flags = ARC_FLAG_NOWAIT |
+		    ARC_FLAG_PRESCIENT_PREFETCH | ARC_FLAG_PREFETCH;
+		int zio_flags = ZIO_FLAG_CANFAIL | ZIO_FLAG_SCAN_THREAD;
 
-	(void) arc_read(scn->scn_zio_root, scn->scn_dp->dp_spa, bp,
-	    NULL, NULL, ZIO_PRIORITY_ASYNC_READ,
-	    ZIO_FLAG_CANFAIL | ZIO_FLAG_SCAN_THREAD, &flags, &czb);
+		mutex_enter(&spa->spa_scrub_lock);
+
+		/*
+		 * Wait until we have an IO to issue and are not above our
+		 * maximum in flight limit.
+		 */
+		while (!scn->scn_prefetch_stop &&
+		    (avl_numnodes(&scn->scn_prefetch_queue) == 0 ||
+		    spa->spa_scrub_inflight >= scn->scn_maxinflight_bytes)) {
+			cv_wait(&spa->spa_scrub_io_cv, &spa->spa_scrub_lock);
+		}
+
+		/* recheck if we should stop since we waited for the cv */
+		if (scn->scn_prefetch_stop) {
+			mutex_exit(&spa->spa_scrub_lock);
+			break;
+		}
+
+		/* remove the prefetch IO from the tree */
+		spic = avl_first(&scn->scn_prefetch_queue);
+		spa->spa_scrub_inflight += BP_GET_PSIZE(&spic->spic_bp);
+		avl_remove(&scn->scn_prefetch_queue, spic);
+
+		mutex_exit(&spa->spa_scrub_lock);
+
+		/* issue the prefetch asynchronously */
+		(void) arc_read(scn->scn_zio_root, scn->scn_dp->dp_spa,
+		    &spic->spic_bp, dsl_scan_prefetch_cb, spic->spic_spc,
+		    ZIO_PRIORITY_SCRUB, zio_flags, &flags, &spic->spic_zb);
+
+		kmem_free(spic, sizeof (scan_prefetch_issue_ctx_t));
+	}
+
+	ASSERT(scn->scn_prefetch_stop);
+
+	/* free any prefetches we didn't get to complete */
+	mutex_enter(&spa->spa_scrub_lock);
+	while ((spic = avl_first(&scn->scn_prefetch_queue)) != NULL) {
+		avl_remove(&scn->scn_prefetch_queue, spic);
+		scan_prefetch_ctx_rele(spic->spic_spc, scn);
+		kmem_free(spic, sizeof (scan_prefetch_issue_ctx_t));
+	}
+	ASSERT0(avl_numnodes(&scn->scn_prefetch_queue));
+	mutex_exit(&spa->spa_scrub_lock);
 }
 
 static boolean_t
@@ -720,6 +1573,13 @@ dsl_scan_check_resume(dsl_scan_t *scn, const dnode_phys_t *dnp,
 	return (B_FALSE);
 }
 
+static void dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
+    dnode_phys_t *dnp, dsl_dataset_t *ds, dsl_scan_t *scn,
+    dmu_objset_type_t ostype, dmu_tx_t *tx);
+static void dsl_scan_visitdnode(
+    dsl_scan_t *, dsl_dataset_t *ds, dmu_objset_type_t ostype,
+    dnode_phys_t *dnp, uint64_t object, dmu_tx_t *tx);
+
 /*
  * Return nonzero on i/o error.
  * Return new buf to write out in *bufp.
@@ -741,16 +1601,12 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
 		arc_buf_t *buf;
 
 		err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
-		    ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
+		    ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
 		if (err) {
 			scn->scn_phys.scn_errors++;
 			return (err);
 		}
 		for (i = 0, cbp = buf->b_data; i < epb; i++, cbp++) {
-			dsl_scan_prefetch(scn, buf, cbp, zb->zb_objset,
-			    zb->zb_object, zb->zb_blkid * epb + i);
-		}
-		for (i = 0, cbp = buf->b_data; i < epb; i++, cbp++) {
 			zbookmark_phys_t czb;
 
 			SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
@@ -763,12 +1619,12 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
 	} else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
 		arc_flags_t flags = ARC_FLAG_WAIT;
 		dnode_phys_t *cdnp;
-		int i, j;
+		int i;
 		int epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;
 		arc_buf_t *buf;
 
 		err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
-		    ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
+		    ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
 		if (err) {
 			scn->scn_phys.scn_errors++;
 			return (err);
@@ -776,15 +1632,6 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
 		for (i = 0, cdnp = buf->b_data; i < epb;
 		    i += cdnp->dn_extra_slots + 1,
 		    cdnp += cdnp->dn_extra_slots + 1) {
-			for (j = 0; j < cdnp->dn_nblkptr; j++) {
-				blkptr_t *cbp = &cdnp->dn_blkptr[j];
-				dsl_scan_prefetch(scn, buf, cbp,
-				    zb->zb_objset, zb->zb_blkid * epb + i, j);
-			}
-		}
-		for (i = 0, cdnp = buf->b_data; i < epb;
-		    i += cdnp->dn_extra_slots + 1,
-		    cdnp += cdnp->dn_extra_slots + 1) {
 			dsl_scan_visitdnode(scn, ds, ostype,
 			    cdnp, zb->zb_blkid * epb + i, tx);
 		}
@@ -796,7 +1643,7 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
 		arc_buf_t *buf;
 
 		err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
-		    ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
+		    ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
 		if (err) {
 			scn->scn_phys.scn_errors++;
 			return (err);
@@ -862,10 +1709,7 @@ dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
     dmu_objset_type_t ostype, dmu_tx_t *tx)
 {
 	dsl_pool_t *dp = scn->scn_dp;
-	arc_buf_t *buf = NULL;
-	blkptr_t bp_toread = *bp;
-
-	/* ASSERT(pbuf == NULL || arc_released(pbuf)); */
+	blkptr_t *bp_toread = NULL;
 
 	if (dsl_scan_check_suspend(scn, zb))
 		return;
@@ -873,22 +1717,36 @@ dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
 	if (dsl_scan_check_resume(scn, dnp, zb))
 		return;
 
-	if (BP_IS_HOLE(bp))
-		return;
-
 	scn->scn_visited_this_txg++;
 
-	dprintf_bp(bp,
-	    "visiting ds=%p/%llu zb=%llx/%llx/%llx/%llx bp=%p",
-	    ds, ds ? ds->ds_object : 0,
-	    zb->zb_objset, zb->zb_object, zb->zb_level, zb->zb_blkid,
-	    bp);
+	/*
+	 * This debugging is commented out to conserve stack space.  This
+	 * function is called recursively and the debugging addes several
+	 * bytes to the stack for each call.  It can be commented back in
+	 * if required to debug an issue in dsl_scan_visitbp().
+	 *
+	 * dprintf_bp(bp,
+	 *	"visiting ds=%p/%llu zb=%llx/%llx/%llx/%llx bp=%p",
+	 *	ds, ds ? ds->ds_object : 0,
+	 *	zb->zb_objset, zb->zb_object, zb->zb_level, zb->zb_blkid,
+	 *	bp);
+	 */
 
-	if (bp->blk_birth <= scn->scn_phys.scn_cur_min_txg)
+	if (BP_IS_HOLE(bp)) {
+		scn->scn_holes_this_txg++;
 		return;
+	}
 
-	if (dsl_scan_recurse(scn, ds, ostype, dnp, &bp_toread, zb, tx) != 0)
+	if (bp->blk_birth <= scn->scn_phys.scn_cur_min_txg) {
+		scn->scn_lt_min_this_txg++;
 		return;
+	}
+
+	bp_toread = kmem_alloc(sizeof (blkptr_t), KM_SLEEP);
+	*bp_toread = *bp;
+
+	if (dsl_scan_recurse(scn, ds, ostype, dnp, bp_toread, zb, tx) != 0)
+		goto out;
 
 	/*
 	 * If dsl_scan_ddt() has already visited this block, it will have
@@ -897,8 +1755,8 @@ dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
 	 */
 	if (ddt_class_contains(dp->dp_spa,
 	    scn->scn_phys.scn_ddt_class_max, bp)) {
-		ASSERT(buf == NULL);
-		return;
+		scn->scn_ddt_contained_this_txg++;
+		goto out;
 	}
 
 	/*
@@ -908,9 +1766,15 @@ dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
 	 * Don't scan it now unless we need to because something
 	 * under it was modified.
 	 */
-	if (BP_PHYSICAL_BIRTH(bp) <= scn->scn_phys.scn_cur_max_txg) {
-		scan_funcs[scn->scn_phys.scn_func](dp, bp, zb);
+	if (BP_PHYSICAL_BIRTH(bp) > scn->scn_phys.scn_cur_max_txg) {
+		scn->scn_gt_max_this_txg++;
+		goto out;
 	}
+
+	scan_funcs[scn->scn_phys.scn_func](dp, bp, zb);
+
+out:
+	kmem_free(bp_toread, sizeof (blkptr_t));
 }
 
 static void
@@ -918,26 +1782,33 @@ dsl_scan_visit_rootbp(dsl_scan_t *scn, dsl_dataset_t *ds, blkptr_t *bp,
     dmu_tx_t *tx)
 {
 	zbookmark_phys_t zb;
+	scan_prefetch_ctx_t *spc;
 
 	SET_BOOKMARK(&zb, ds ? ds->ds_object : DMU_META_OBJSET,
 	    ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);
-	dsl_scan_visitbp(bp, &zb, NULL,
-	    ds, scn, DMU_OST_NONE, tx);
+
+	if (ZB_IS_ZERO(&scn->scn_phys.scn_bookmark)) {
+		SET_BOOKMARK(&scn->scn_prefetch_bookmark,
+		    zb.zb_objset, 0, 0, 0);
+	} else {
+		scn->scn_prefetch_bookmark = scn->scn_phys.scn_bookmark;
+	}
+
+	scn->scn_objsets_visited_this_txg++;
+
+	spc = scan_prefetch_ctx_create(scn, NULL, FTAG);
+	dsl_scan_prefetch(spc, bp, &zb);
+	scan_prefetch_ctx_rele(spc, FTAG);
+
+	dsl_scan_visitbp(bp, &zb, NULL, ds, scn, DMU_OST_NONE, tx);
 
 	dprintf_ds(ds, "finished scan%s", "");
 }
 
-void
-dsl_scan_ds_destroyed(dsl_dataset_t *ds, dmu_tx_t *tx)
+static void
+ds_destroyed_scn_phys(dsl_dataset_t *ds, dsl_scan_phys_t *scn_phys)
 {
-	dsl_pool_t *dp = ds->ds_dir->dd_pool;
-	dsl_scan_t *scn = dp->dp_scan;
-	uint64_t mintxg;
-
-	if (scn->scn_phys.scn_state != DSS_SCANNING)
-		return;
-
-	if (scn->scn_phys.scn_bookmark.zb_objset == ds->ds_object) {
+	if (scn_phys->scn_bookmark.zb_objset == ds->ds_object) {
 		if (ds->ds_is_snapshot) {
 			/*
 			 * Note:
@@ -949,23 +1820,57 @@ dsl_scan_ds_destroyed(dsl_dataset_t *ds, dmu_tx_t *tx)
 			 *    ignore it when we retraverse it in
 			 *    dsl_scan_visitds().
 			 */
-			scn->scn_phys.scn_bookmark.zb_objset =
+			scn_phys->scn_bookmark.zb_objset =
 			    dsl_dataset_phys(ds)->ds_next_snap_obj;
 			zfs_dbgmsg("destroying ds %llu; currently traversing; "
 			    "reset zb_objset to %llu",
 			    (u_longlong_t)ds->ds_object,
 			    (u_longlong_t)dsl_dataset_phys(ds)->
 			    ds_next_snap_obj);
-			scn->scn_phys.scn_flags |= DSF_VISIT_DS_AGAIN;
+			scn_phys->scn_flags |= DSF_VISIT_DS_AGAIN;
 		} else {
-			SET_BOOKMARK(&scn->scn_phys.scn_bookmark,
+			SET_BOOKMARK(&scn_phys->scn_bookmark,
 			    ZB_DESTROYED_OBJSET, 0, 0, 0);
 			zfs_dbgmsg("destroying ds %llu; currently traversing; "
 			    "reset bookmark to -1,0,0,0",
 			    (u_longlong_t)ds->ds_object);
 		}
-	} else if (zap_lookup_int_key(dp->dp_meta_objset,
-	    scn->scn_phys.scn_queue_obj, ds->ds_object, &mintxg) == 0) {
+	}
+}
+
+/*
+ * Invoked when a dataset is destroyed. We need to make sure that:
+ *
+ * 1) If it is the dataset that was currently being scanned, we write
+ *	a new dsl_scan_phys_t and marking the objset reference in it
+ *	as destroyed.
+ * 2) Remove it from the work queue, if it was present.
+ *
+ * If the dataset was actually a snapshot, instead of marking the dataset
+ * as destroyed, we instead substitute the next snapshot in line.
+ */
+void
+dsl_scan_ds_destroyed(dsl_dataset_t *ds, dmu_tx_t *tx)
+{
+	dsl_pool_t *dp = ds->ds_dir->dd_pool;
+	dsl_scan_t *scn = dp->dp_scan;
+	uint64_t mintxg;
+
+	if (!dsl_scan_is_running(scn))
+		return;
+
+	ds_destroyed_scn_phys(ds, &scn->scn_phys);
+	ds_destroyed_scn_phys(ds, &scn->scn_phys_cached);
+
+	if (scan_ds_queue_contains(scn, ds->ds_object, &mintxg)) {
+		scan_ds_queue_remove(scn, ds->ds_object);
+		if (ds->ds_is_snapshot)
+			scan_ds_queue_insert(scn,
+			    dsl_dataset_phys(ds)->ds_next_snap_obj, mintxg);
+	}
+
+	if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
+	    ds->ds_object, &mintxg) == 0) {
 		ASSERT3U(dsl_dataset_phys(ds)->ds_num_children, <=, 1);
 		VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
 		    scn->scn_phys.scn_queue_obj, ds->ds_object, tx));
@@ -994,9 +1899,28 @@ dsl_scan_ds_destroyed(dsl_dataset_t *ds, dmu_tx_t *tx)
 	 * dsl_scan_sync() should be called after this, and should sync
 	 * out our changed state, but just to be safe, do it here.
 	 */
-	dsl_scan_sync_state(scn, tx);
+	dsl_scan_sync_state(scn, tx, SYNC_CACHED);
+}
+
+static void
+ds_snapshotted_bookmark(dsl_dataset_t *ds, zbookmark_phys_t *scn_bookmark)
+{
+	if (scn_bookmark->zb_objset == ds->ds_object) {
+		scn_bookmark->zb_objset =
+		    dsl_dataset_phys(ds)->ds_prev_snap_obj;
+		zfs_dbgmsg("snapshotting ds %llu; currently traversing; "
+		    "reset zb_objset to %llu",
+		    (u_longlong_t)ds->ds_object,
+		    (u_longlong_t)dsl_dataset_phys(ds)->ds_prev_snap_obj);
+	}
 }
 
+/*
+ * Called when a dataset is snapshotted. If we were currently traversing
+ * this snapshot, we reset our bookmark to point at the newly created
+ * snapshot. We also modify our work queue to remove the old snapshot and
+ * replace with the new one.
+ */
 void
 dsl_scan_ds_snapshotted(dsl_dataset_t *ds, dmu_tx_t *tx)
 {
@@ -1004,20 +1928,22 @@ dsl_scan_ds_snapshotted(dsl_dataset_t *ds, dmu_tx_t *tx)
 	dsl_scan_t *scn = dp->dp_scan;
 	uint64_t mintxg;
 
-	if (scn->scn_phys.scn_state != DSS_SCANNING)
+	if (!dsl_scan_is_running(scn))
 		return;
 
 	ASSERT(dsl_dataset_phys(ds)->ds_prev_snap_obj != 0);
 
-	if (scn->scn_phys.scn_bookmark.zb_objset == ds->ds_object) {
-		scn->scn_phys.scn_bookmark.zb_objset =
-		    dsl_dataset_phys(ds)->ds_prev_snap_obj;
-		zfs_dbgmsg("snapshotting ds %llu; currently traversing; "
-		    "reset zb_objset to %llu",
-		    (u_longlong_t)ds->ds_object,
-		    (u_longlong_t)dsl_dataset_phys(ds)->ds_prev_snap_obj);
-	} else if (zap_lookup_int_key(dp->dp_meta_objset,
-	    scn->scn_phys.scn_queue_obj, ds->ds_object, &mintxg) == 0) {
+	ds_snapshotted_bookmark(ds, &scn->scn_phys.scn_bookmark);
+	ds_snapshotted_bookmark(ds, &scn->scn_phys_cached.scn_bookmark);
+
+	if (scan_ds_queue_contains(scn, ds->ds_object, &mintxg)) {
+		scan_ds_queue_remove(scn, ds->ds_object);
+		scan_ds_queue_insert(scn,
+		    dsl_dataset_phys(ds)->ds_prev_snap_obj, mintxg);
+	}
+
+	if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
+	    ds->ds_object, &mintxg) == 0) {
 		VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
 		    scn->scn_phys.scn_queue_obj, ds->ds_object, tx));
 		VERIFY(zap_add_int_key(dp->dp_meta_objset,
@@ -1028,37 +1954,59 @@ dsl_scan_ds_snapshotted(dsl_dataset_t *ds, dmu_tx_t *tx)
 		    (u_longlong_t)ds->ds_object,
 		    (u_longlong_t)dsl_dataset_phys(ds)->ds_prev_snap_obj);
 	}
-	dsl_scan_sync_state(scn, tx);
+
+	dsl_scan_sync_state(scn, tx, SYNC_CACHED);
 }
 
-void
-dsl_scan_ds_clone_swapped(dsl_dataset_t *ds1, dsl_dataset_t *ds2, dmu_tx_t *tx)
+static void
+ds_clone_swapped_bookmark(dsl_dataset_t *ds1, dsl_dataset_t *ds2,
+    zbookmark_phys_t *scn_bookmark)
 {
-	dsl_pool_t *dp = ds1->ds_dir->dd_pool;
-	dsl_scan_t *scn = dp->dp_scan;
-	uint64_t mintxg;
-
-	if (scn->scn_phys.scn_state != DSS_SCANNING)
-		return;
-
-	if (scn->scn_phys.scn_bookmark.zb_objset == ds1->ds_object) {
-		scn->scn_phys.scn_bookmark.zb_objset = ds2->ds_object;
+	if (scn_bookmark->zb_objset == ds1->ds_object) {
+		scn_bookmark->zb_objset = ds2->ds_object;
 		zfs_dbgmsg("clone_swap ds %llu; currently traversing; "
 		    "reset zb_objset to %llu",
 		    (u_longlong_t)ds1->ds_object,
 		    (u_longlong_t)ds2->ds_object);
-	} else if (scn->scn_phys.scn_bookmark.zb_objset == ds2->ds_object) {
-		scn->scn_phys.scn_bookmark.zb_objset = ds1->ds_object;
+	} else if (scn_bookmark->zb_objset == ds2->ds_object) {
+		scn_bookmark->zb_objset = ds1->ds_object;
 		zfs_dbgmsg("clone_swap ds %llu; currently traversing; "
 		    "reset zb_objset to %llu",
 		    (u_longlong_t)ds2->ds_object,
 		    (u_longlong_t)ds1->ds_object);
 	}
+}
+
+/*
+ * Called when a parent dataset and its clone are swapped. If we were
+ * currently traversing the dataset, we need to switch to traversing the
+ * newly promoted parent.
+ */
+void
+dsl_scan_ds_clone_swapped(dsl_dataset_t *ds1, dsl_dataset_t *ds2, dmu_tx_t *tx)
+{
+	dsl_pool_t *dp = ds1->ds_dir->dd_pool;
+	dsl_scan_t *scn = dp->dp_scan;
+	uint64_t mintxg;
+
+	if (!dsl_scan_is_running(scn))
+		return;
+
+	ds_clone_swapped_bookmark(ds1, ds2, &scn->scn_phys.scn_bookmark);
+	ds_clone_swapped_bookmark(ds1, ds2, &scn->scn_phys_cached.scn_bookmark);
+
+	if (scan_ds_queue_contains(scn, ds1->ds_object, &mintxg)) {
+		scan_ds_queue_remove(scn, ds1->ds_object);
+		scan_ds_queue_insert(scn, ds2->ds_object, mintxg);
+	}
+	if (scan_ds_queue_contains(scn, ds2->ds_object, &mintxg)) {
+		scan_ds_queue_remove(scn, ds2->ds_object);
+		scan_ds_queue_insert(scn, ds1->ds_object, mintxg);
+	}
 
 	if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
 	    ds1->ds_object, &mintxg) == 0) {
 		int err;
-
 		ASSERT3U(mintxg, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
 		ASSERT3U(mintxg, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
 		VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
@@ -1076,8 +2024,9 @@ dsl_scan_ds_clone_swapped(dsl_dataset_t *ds1, dsl_dataset_t *ds2, dmu_tx_t *tx)
 		    "replacing with %llu",
 		    (u_longlong_t)ds1->ds_object,
 		    (u_longlong_t)ds2->ds_object);
-	} else if (zap_lookup_int_key(dp->dp_meta_objset,
-	    scn->scn_phys.scn_queue_obj, ds2->ds_object, &mintxg) == 0) {
+	}
+	if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
+	    ds2->ds_object, &mintxg) == 0) {
 		ASSERT3U(mintxg, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
 		ASSERT3U(mintxg, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
 		VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
@@ -1090,31 +2039,26 @@ dsl_scan_ds_clone_swapped(dsl_dataset_t *ds1, dsl_dataset_t *ds2, dmu_tx_t *tx)
 		    (u_longlong_t)ds1->ds_object);
 	}
 
-	dsl_scan_sync_state(scn, tx);
+	dsl_scan_sync_state(scn, tx, SYNC_CACHED);
 }
 
-struct enqueue_clones_arg {
-	dmu_tx_t *tx;
-	uint64_t originobj;
-};
-
 /* ARGSUSED */
 static int
 enqueue_clones_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
 {
-	struct enqueue_clones_arg *eca = arg;
+	uint64_t originobj = *(uint64_t *)arg;
 	dsl_dataset_t *ds;
 	int err;
 	dsl_scan_t *scn = dp->dp_scan;
 
-	if (dsl_dir_phys(hds->ds_dir)->dd_origin_obj != eca->originobj)
+	if (dsl_dir_phys(hds->ds_dir)->dd_origin_obj != originobj)
 		return (0);
 
 	err = dsl_dataset_hold_obj(dp, hds->ds_object, FTAG, &ds);
 	if (err)
 		return (err);
 
-	while (dsl_dataset_phys(ds)->ds_prev_snap_obj != eca->originobj) {
+	while (dsl_dataset_phys(ds)->ds_prev_snap_obj != originobj) {
 		dsl_dataset_t *prev;
 		err = dsl_dataset_hold_obj(dp,
 		    dsl_dataset_phys(ds)->ds_prev_snap_obj, FTAG, &prev);
@@ -1124,9 +2068,8 @@ enqueue_clones_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
 			return (err);
 		ds = prev;
 	}
-	VERIFY(zap_add_int_key(dp->dp_meta_objset,
-	    scn->scn_phys.scn_queue_obj, ds->ds_object,
-	    dsl_dataset_phys(ds)->ds_prev_snap_txg, eca->tx) == 0);
+	scan_ds_queue_insert(scn, ds->ds_object,
+	    dsl_dataset_phys(ds)->ds_prev_snap_txg);
 	dsl_dataset_rele(ds, FTAG);
 	return (0);
 }
@@ -1171,9 +2114,9 @@ dsl_scan_visitds(dsl_scan_t *scn, uint64_t dsobj, dmu_tx_t *tx)
 		dsl_dataset_name(ds, dsname);
 		zfs_dbgmsg("scanning dataset %llu (%s) is unnecessary because "
 		    "cur_min_txg (%llu) >= max_txg (%llu)",
-		    dsobj, dsname,
-		    scn->scn_phys.scn_cur_min_txg,
-		    scn->scn_phys.scn_max_txg);
+		    (longlong_t)dsobj, dsname,
+		    (longlong_t)scn->scn_phys.scn_cur_min_txg,
+		    (longlong_t)scn->scn_phys.scn_max_txg);
 		kmem_free(dsname, MAXNAMELEN);
 
 		goto out;
@@ -1229,9 +2172,8 @@ dsl_scan_visitds(dsl_scan_t *scn, uint64_t dsobj, dmu_tx_t *tx)
 	if (scn->scn_phys.scn_flags & DSF_VISIT_DS_AGAIN) {
 		zfs_dbgmsg("incomplete pass; visiting again");
 		scn->scn_phys.scn_flags &= ~DSF_VISIT_DS_AGAIN;
-		VERIFY(zap_add_int_key(dp->dp_meta_objset,
-		    scn->scn_phys.scn_queue_obj, ds->ds_object,
-		    scn->scn_phys.scn_cur_max_txg, tx) == 0);
+		scan_ds_queue_insert(scn, ds->ds_object,
+		    scn->scn_phys.scn_cur_max_txg);
 		goto out;
 	}
 
@@ -1239,10 +2181,9 @@ dsl_scan_visitds(dsl_scan_t *scn, uint64_t dsobj, dmu_tx_t *tx)
 	 * Add descendent datasets to work queue.
 	 */
 	if (dsl_dataset_phys(ds)->ds_next_snap_obj != 0) {
-		VERIFY(zap_add_int_key(dp->dp_meta_objset,
-		    scn->scn_phys.scn_queue_obj,
+		scan_ds_queue_insert(scn,
 		    dsl_dataset_phys(ds)->ds_next_snap_obj,
-		    dsl_dataset_phys(ds)->ds_creation_txg, tx) == 0);
+		    dsl_dataset_phys(ds)->ds_creation_txg);
 	}
 	if (dsl_dataset_phys(ds)->ds_num_children > 1) {
 		boolean_t usenext = B_FALSE;
@@ -1263,17 +2204,21 @@ dsl_scan_visitds(dsl_scan_t *scn, uint64_t dsobj, dmu_tx_t *tx)
 		}
 
 		if (usenext) {
-			VERIFY0(zap_join_key(dp->dp_meta_objset,
-			    dsl_dataset_phys(ds)->ds_next_clones_obj,
-			    scn->scn_phys.scn_queue_obj,
-			    dsl_dataset_phys(ds)->ds_creation_txg, tx));
+			zap_cursor_t zc;
+			zap_attribute_t za;
+			for (zap_cursor_init(&zc, dp->dp_meta_objset,
+			    dsl_dataset_phys(ds)->ds_next_clones_obj);
+			    zap_cursor_retrieve(&zc, &za) == 0;
+			    (void) zap_cursor_advance(&zc)) {
+				scan_ds_queue_insert(scn,
+				    zfs_strtonum(za.za_name, NULL),
+				    dsl_dataset_phys(ds)->ds_creation_txg);
+			}
+			zap_cursor_fini(&zc);
 		} else {
-			struct enqueue_clones_arg eca;
-			eca.tx = tx;
-			eca.originobj = ds->ds_object;
-
 			VERIFY0(dmu_objset_find_dp(dp, dp->dp_root_dir_obj,
-			    enqueue_clones_cb, &eca, DS_FIND_CHILDREN));
+			    enqueue_clones_cb, &ds->ds_object,
+			    DS_FIND_CHILDREN));
 		}
 	}
 
@@ -1285,7 +2230,6 @@ out:
 static int
 enqueue_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
 {
-	dmu_tx_t *tx = arg;
 	dsl_dataset_t *ds;
 	int err;
 	dsl_scan_t *scn = dp->dp_scan;
@@ -1315,12 +2259,37 @@ enqueue_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
 		ds = prev;
 	}
 
-	VERIFY(zap_add_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
-	    ds->ds_object, dsl_dataset_phys(ds)->ds_prev_snap_txg, tx) == 0);
+	scan_ds_queue_insert(scn, ds->ds_object,
+	    dsl_dataset_phys(ds)->ds_prev_snap_txg);
 	dsl_dataset_rele(ds, FTAG);
 	return (0);
 }
 
+/* ARGSUSED */
+void
+dsl_scan_ddt_entry(dsl_scan_t *scn, enum zio_checksum checksum,
+    ddt_entry_t *dde, dmu_tx_t *tx)
+{
+	const ddt_key_t *ddk = &dde->dde_key;
+	ddt_phys_t *ddp = dde->dde_phys;
+	blkptr_t bp;
+	zbookmark_phys_t zb = { 0 };
+	int p;
+
+	if (scn->scn_phys.scn_state != DSS_SCANNING)
+		return;
+
+	for (p = 0; p < DDT_PHYS_TYPES; p++, ddp++) {
+		if (ddp->ddp_phys_birth == 0 ||
+		    ddp->ddp_phys_birth > scn->scn_phys.scn_max_txg)
+			continue;
+		ddt_bp_create(checksum, ddk, ddp, &bp);
+
+		scn->scn_visited_this_txg++;
+		scan_funcs[scn->scn_phys.scn_func](scn->scn_dp, &bp, &zb);
+	}
+}
+
 /*
  * Scrub/dedup interaction.
  *
@@ -1393,36 +2362,20 @@ dsl_scan_ddt(dsl_scan_t *scn, dmu_tx_t *tx)
 	    ddb->ddb_class > scn->scn_phys.scn_ddt_class_max);
 }
 
-/* ARGSUSED */
-void
-dsl_scan_ddt_entry(dsl_scan_t *scn, enum zio_checksum checksum,
-    ddt_entry_t *dde, dmu_tx_t *tx)
+static uint64_t
+dsl_scan_ds_maxtxg(dsl_dataset_t *ds)
 {
-	const ddt_key_t *ddk = &dde->dde_key;
-	ddt_phys_t *ddp = dde->dde_phys;
-	blkptr_t bp;
-	zbookmark_phys_t zb = { 0 };
-
-	if (scn->scn_phys.scn_state != DSS_SCANNING)
-		return;
-
-	for (int p = 0; p < DDT_PHYS_TYPES; p++, ddp++) {
-		if (ddp->ddp_phys_birth == 0 ||
-		    ddp->ddp_phys_birth > scn->scn_phys.scn_max_txg)
-			continue;
-		ddt_bp_create(checksum, ddk, ddp, &bp);
-
-		scn->scn_visited_this_txg++;
-		scan_funcs[scn->scn_phys.scn_func](scn->scn_dp, &bp, &zb);
-	}
+	uint64_t smt = ds->ds_dir->dd_pool->dp_scan->scn_phys.scn_max_txg;
+	if (ds->ds_is_snapshot)
+		return (MIN(smt, dsl_dataset_phys(ds)->ds_creation_txg));
+	return (smt);
 }
 
 static void
 dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
 {
+	scan_ds_t *sds;
 	dsl_pool_t *dp = scn->scn_dp;
-	zap_cursor_t zc;
-	zap_attribute_t za;
 
 	if (scn->scn_phys.scn_ddt_bookmark.ddb_class <=
 	    scn->scn_phys.scn_ddt_class_max) {
@@ -1446,7 +2399,7 @@ dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
 
 		if (spa_version(dp->dp_spa) < SPA_VERSION_DSL_SCRUB) {
 			VERIFY0(dmu_objset_find_dp(dp, dp->dp_root_dir_obj,
-			    enqueue_cb, tx, DS_FIND_CHILDREN));
+			    enqueue_cb, NULL, DS_FIND_CHILDREN));
 		} else {
 			dsl_scan_visitds(scn,
 			    dp->dp_origin_snap->ds_object, tx);
@@ -1454,40 +2407,42 @@ dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
 		ASSERT(!scn->scn_suspending);
 	} else if (scn->scn_phys.scn_bookmark.zb_objset !=
 	    ZB_DESTROYED_OBJSET) {
+		uint64_t dsobj = scn->scn_phys.scn_bookmark.zb_objset;
 		/*
-		 * If we were suspended, continue from here.  Note if the
+		 * If we were suspended, continue from here. Note if the
 		 * ds we were suspended on was deleted, the zb_objset may
 		 * be -1, so we will skip this and find a new objset
 		 * below.
 		 */
-		dsl_scan_visitds(scn, scn->scn_phys.scn_bookmark.zb_objset, tx);
+		dsl_scan_visitds(scn, dsobj, tx);
 		if (scn->scn_suspending)
 			return;
 	}
 
 	/*
-	 * In case we were suspended right at the end of the ds, zero the
+	 * In case we suspended right at the end of the ds, zero the
 	 * bookmark so we don't think that we're still trying to resume.
 	 */
 	bzero(&scn->scn_phys.scn_bookmark, sizeof (zbookmark_phys_t));
 
-	/* keep pulling things out of the zap-object-as-queue */
-	while (zap_cursor_init(&zc, dp->dp_meta_objset,
-	    scn->scn_phys.scn_queue_obj),
-	    zap_cursor_retrieve(&zc, &za) == 0) {
+	/*
+	 * Keep pulling things out of the dataset avl queue. Updates to the
+	 * persistent zap-object-as-queue happen only at checkpoints.
+	 */
+	while ((sds = avl_first(&scn->scn_queue)) != NULL) {
 		dsl_dataset_t *ds;
-		uint64_t dsobj;
+		uint64_t dsobj = sds->sds_dsobj;
+		uint64_t txg = sds->sds_txg;
 
-		dsobj = zfs_strtonum(za.za_name, NULL);
-		VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
-		    scn->scn_phys.scn_queue_obj, dsobj, tx));
+		/* dequeue and free the ds from the queue */
+		scan_ds_queue_remove(scn, dsobj);
+		sds = NULL;	/* must not be touched after removal */
 
-		/* Set up min/max txg */
+		/* Set up min / max txg */
 		VERIFY3U(0, ==, dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds));
-		if (za.za_first_integer != 0) {
+		if (txg != 0) {
 			scn->scn_phys.scn_cur_min_txg =
-			    MAX(scn->scn_phys.scn_min_txg,
-			    za.za_first_integer);
+			    MAX(scn->scn_phys.scn_min_txg, txg);
 		} else {
 			scn->scn_phys.scn_cur_min_txg =
 			    MAX(scn->scn_phys.scn_min_txg,
@@ -1497,11 +2452,365 @@ dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
 		dsl_dataset_rele(ds, FTAG);
 
 		dsl_scan_visitds(scn, dsobj, tx);
-		zap_cursor_fini(&zc);
 		if (scn->scn_suspending)
 			return;
 	}
-	zap_cursor_fini(&zc);
+	/* No more objsets to fetch, we're done */
+	scn->scn_phys.scn_bookmark.zb_objset = ZB_DESTROYED_OBJSET;
+	ASSERT0(scn->scn_suspending);
+}
+
+static uint64_t
+dsl_scan_count_leaves(vdev_t *vd)
+{
+	uint64_t i, leaves = 0;
+
+	/* we only count leaves that belong to the main pool and are readable */
+	if (vd->vdev_islog || vd->vdev_isspare ||
+	    vd->vdev_isl2cache || !vdev_readable(vd))
+		return (0);
+
+	if (vd->vdev_ops->vdev_op_leaf)
+		return (1);
+
+	for (i = 0; i < vd->vdev_children; i++) {
+		leaves += dsl_scan_count_leaves(vd->vdev_child[i]);
+	}
+
+	return (leaves);
+}
+
+
+static void
+scan_io_queues_update_zio_stats(dsl_scan_io_queue_t *q, const blkptr_t *bp)
+{
+	int i;
+	uint64_t cur_size = 0;
+
+	for (i = 0; i < BP_GET_NDVAS(bp); i++) {
+		cur_size += DVA_GET_ASIZE(&bp->blk_dva[i]);
+	}
+
+	q->q_total_zio_size_this_txg += cur_size;
+	q->q_zios_this_txg++;
+}
+
+static void
+scan_io_queues_update_seg_stats(dsl_scan_io_queue_t *q, uint64_t start,
+    uint64_t end)
+{
+	q->q_total_seg_size_this_txg += end - start;
+	q->q_segs_this_txg++;
+}
+
+static boolean_t
+scan_io_queue_check_suspend(dsl_scan_t *scn)
+{
+	/* See comment in dsl_scan_check_suspend() */
+	uint64_t curr_time_ns = gethrtime();
+	uint64_t scan_time_ns = curr_time_ns - scn->scn_sync_start_time;
+	uint64_t sync_time_ns = curr_time_ns -
+	    scn->scn_dp->dp_spa->spa_sync_starttime;
+	int dirty_pct = scn->scn_dp->dp_dirty_total * 100 / zfs_dirty_data_max;
+	int mintime = (scn->scn_phys.scn_func == POOL_SCAN_RESILVER) ?
+	    zfs_resilver_min_time_ms : zfs_scrub_min_time_ms;
+
+	return ((NSEC2MSEC(scan_time_ns) > mintime &&
+	    (dirty_pct >= zfs_vdev_async_write_active_min_dirty_percent ||
+	    txg_sync_waiting(scn->scn_dp) ||
+	    NSEC2SEC(sync_time_ns) >= zfs_txg_timeout)) ||
+	    spa_shutting_down(scn->scn_dp->dp_spa));
+}
+
+/*
+ * Given a list of scan_io_t's in io_list, this issues the io's out to
+ * disk. This consumes the io_list and frees the scan_io_t's. This is
+ * called when emptying queues, either when we're up against the memory
+ * limit or when we have finished scanning. Returns B_TRUE if we stopped
+ * processing the list before we finished. Any zios that were not issued
+ * will remain in the io_list.
+ */
+static boolean_t
+scan_io_queue_issue(dsl_scan_io_queue_t *queue, list_t *io_list)
+{
+	dsl_scan_t *scn = queue->q_scn;
+	scan_io_t *sio;
+	int64_t bytes_issued = 0;
+	boolean_t suspended = B_FALSE;
+
+	while ((sio = list_head(io_list)) != NULL) {
+		blkptr_t bp;
+
+		if (scan_io_queue_check_suspend(scn)) {
+			suspended = B_TRUE;
+			break;
+		}
+
+		sio2bp(sio, &bp, queue->q_vd->vdev_id);
+		bytes_issued += sio->sio_asize;
+		scan_exec_io(scn->scn_dp, &bp, sio->sio_flags,
+		    &sio->sio_zb, queue);
+		(void) list_remove_head(io_list);
+		scan_io_queues_update_zio_stats(queue, &bp);
+		kmem_free(sio, sizeof (*sio));
+	}
+
+	atomic_add_64(&scn->scn_bytes_pending, -bytes_issued);
+
+	return (suspended);
+}
+
+/*
+ * Given a range_seg_t (extent) and a list, this function passes over a
+ * scan queue and gathers up the appropriate ios which fit into that
+ * scan seg (starting from lowest LBA). At the end, we remove the segment
+ * from the q_exts_by_addr range tree.
+ */
+static boolean_t
+scan_io_queue_gather(dsl_scan_io_queue_t *queue, range_seg_t *rs, list_t *list)
+{
+	scan_io_t srch_sio, *sio, *next_sio;
+	avl_index_t idx;
+	uint_t num_sios = 0;
+	int64_t bytes_issued = 0;
+
+	ASSERT(rs != NULL);
+	ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));
+
+	srch_sio.sio_offset = rs->rs_start;
+
+	/*
+	 * The exact start of the extent might not contain any matching zios,
+	 * so if that's the case, examine the next one in the tree.
+	 */
+	sio = avl_find(&queue->q_sios_by_addr, &srch_sio, &idx);
+	if (sio == NULL)
+		sio = avl_nearest(&queue->q_sios_by_addr, idx, AVL_AFTER);
+
+	while (sio != NULL && sio->sio_offset < rs->rs_end && num_sios <= 32) {
+		ASSERT3U(sio->sio_offset, >=, rs->rs_start);
+		ASSERT3U(sio->sio_offset + sio->sio_asize, <=, rs->rs_end);
+
+		next_sio = AVL_NEXT(&queue->q_sios_by_addr, sio);
+		avl_remove(&queue->q_sios_by_addr, sio);
+
+		bytes_issued += sio->sio_asize;
+		num_sios++;
+		list_insert_tail(list, sio);
+		sio = next_sio;
+	}
+
+	/*
+	 * We limit the number of sios we process at once to 32 to avoid
+	 * biting off more than we can chew. If we didn't take everything
+	 * in the segment we update it to reflect the work we were able to
+	 * complete. Otherwise, we remove it from the range tree entirely.
+	 */
+	if (sio != NULL && sio->sio_offset < rs->rs_end) {
+		range_tree_adjust_fill(queue->q_exts_by_addr, rs,
+		    -bytes_issued);
+		range_tree_resize_segment(queue->q_exts_by_addr, rs,
+		    sio->sio_offset, rs->rs_end - sio->sio_offset);
+
+		return (B_TRUE);
+	} else {
+		range_tree_remove(queue->q_exts_by_addr, rs->rs_start,
+		    rs->rs_end - rs->rs_start);
+		return (B_FALSE);
+	}
+}
+
+
+/*
+ * This is called from the queue emptying thread and selects the next
+ * extent from which we are to issue io's. The behavior of this function
+ * depends on the state of the scan, the current memory consumption and
+ * whether or not we are performing a scan shutdown.
+ * 1) We select extents in an elevator algorithm (LBA-order) if the scan
+ *	needs to perform a checkpoint
+ * 2) We select the largest available extent if we are up against the
+ *	memory limit.
+ * 3) Otherwise we don't select any extents.
+ */
+static const range_seg_t *
+scan_io_queue_fetch_ext(dsl_scan_io_queue_t *queue)
+{
+	dsl_scan_t *scn = queue->q_scn;
+
+	ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));
+	ASSERT(scn->scn_is_sorted);
+
+	/* handle tunable overrides */
+	if (scn->scn_checkpointing || scn->scn_clearing) {
+		if (zfs_scan_issue_strategy == 1) {
+			return (range_tree_first(queue->q_exts_by_addr));
+		} else if (zfs_scan_issue_strategy == 2) {
+			return (avl_first(&queue->q_exts_by_size));
+		}
+	}
+
+	/*
+	 * During normal clearing, we want to issue our largest segments
+	 * first, keeping IO as sequential as possible, and leaving the
+	 * smaller extents for later with the hope that they might eventually
+	 * grow to larger sequential segments. However, when the scan is
+	 * checkpointing, no new extents will be added to the sorting queue,
+	 * so the way we are sorted now is as good as it will ever get.
+	 * In this case, we instead switch to issuing extents in LBA order.
+	 */
+	if (scn->scn_checkpointing) {
+		return (range_tree_first(queue->q_exts_by_addr));
+	} else if (scn->scn_clearing) {
+		return (avl_first(&queue->q_exts_by_size));
+	} else {
+		return (NULL);
+	}
+}
+
+static void
+scan_io_queues_run_one(void *arg)
+{
+	dsl_scan_io_queue_t *queue = arg;
+	kmutex_t *q_lock = &queue->q_vd->vdev_scan_io_queue_lock;
+	boolean_t suspended = B_FALSE;
+	range_seg_t *rs = NULL;
+	scan_io_t *sio = NULL;
+	list_t sio_list;
+	uint64_t bytes_per_leaf = zfs_scan_vdev_limit;
+	uint64_t nr_leaves = dsl_scan_count_leaves(queue->q_vd);
+
+	ASSERT(queue->q_scn->scn_is_sorted);
+
+	list_create(&sio_list, sizeof (scan_io_t),
+	    offsetof(scan_io_t, sio_nodes.sio_list_node));
+	mutex_enter(q_lock);
+
+	/* calculate maximum in-flight bytes for this txg (min 1MB) */
+	queue->q_maxinflight_bytes =
+	    MAX(nr_leaves * bytes_per_leaf, 1ULL << 20);
+
+	/* reset per-queue scan statistics for this txg */
+	queue->q_total_seg_size_this_txg = 0;
+	queue->q_segs_this_txg = 0;
+	queue->q_total_zio_size_this_txg = 0;
+	queue->q_zios_this_txg = 0;
+
+	/* loop until we have run out of time or sios */
+	while ((rs = (range_seg_t *)scan_io_queue_fetch_ext(queue)) != NULL) {
+		uint64_t seg_start = 0, seg_end = 0;
+		boolean_t more_left = B_TRUE;
+
+		ASSERT(list_is_empty(&sio_list));
+
+		/* loop while we still have sios left to process in this rs */
+		while (more_left) {
+			scan_io_t *first_sio, *last_sio;
+
+			/*
+			 * We have selected which extent needs to be
+			 * processed next. Gather up the corresponding sios.
+			 */
+			more_left = scan_io_queue_gather(queue, rs, &sio_list);
+			ASSERT(!list_is_empty(&sio_list));
+			first_sio = list_head(&sio_list);
+			last_sio = list_tail(&sio_list);
+
+			seg_end = last_sio->sio_offset + last_sio->sio_asize;
+			if (seg_start == 0)
+				seg_start = first_sio->sio_offset;
+
+			/*
+			 * Issuing sios can take a long time so drop the
+			 * queue lock. The sio queue won't be updated by
+			 * other threads since we're in syncing context so
+			 * we can be sure that our trees will remain exactly
+			 * as we left them.
+			 */
+			mutex_exit(q_lock);
+			suspended = scan_io_queue_issue(queue, &sio_list);
+			mutex_enter(q_lock);
+
+			if (suspended)
+				break;
+		}
+		/* update statistics for debugging purposes */
+		scan_io_queues_update_seg_stats(queue, seg_start, seg_end);
+
+		if (suspended)
+			break;
+	}
+
+
+	/*
+	 * If we were suspended in the middle of processing,
+	 * requeue any unfinished sios and exit.
+	 */
+	while ((sio = list_head(&sio_list)) != NULL) {
+		list_remove(&sio_list, sio);
+		scan_io_queue_insert_impl(queue, sio);
+	}
+
+	mutex_exit(q_lock);
+	list_destroy(&sio_list);
+}
+
+/*
+ * Performs an emptying run on all scan queues in the pool. This just
+ * punches out one thread per top-level vdev, each of which processes
+ * only that vdev's scan queue. We can parallelize the I/O here because
+ * we know that each queue's io's only affect its own top-level vdev.
+ *
+ * This function waits for the queue runs to complete, and must be
+ * called from dsl_scan_sync (or in general, syncing context).
+ */
+static void
+scan_io_queues_run(dsl_scan_t *scn)
+{
+	spa_t *spa = scn->scn_dp->dp_spa;
+
+	ASSERT(scn->scn_is_sorted);
+	ASSERT(spa_config_held(spa, SCL_CONFIG, RW_READER));
+
+	if (scn->scn_bytes_pending == 0)
+		return;
+
+	if (scn->scn_taskq == NULL) {
+		char *tq_name = kmem_zalloc(ZFS_MAX_DATASET_NAME_LEN + 16,
+		    KM_SLEEP);
+		int nthreads = spa->spa_root_vdev->vdev_children;
+
+		/*
+		 * We need to make this taskq *always* execute as many
+		 * threads in parallel as we have top-level vdevs and no
+		 * less, otherwise strange serialization of the calls to
+		 * scan_io_queues_run_one can occur during spa_sync runs
+		 * and that significantly impacts performance.
+		 */
+		(void) snprintf(tq_name, ZFS_MAX_DATASET_NAME_LEN + 16,
+		    "dsl_scan_tq_%s", spa->spa_name);
+		scn->scn_taskq = taskq_create(tq_name, nthreads, minclsyspri,
+		    nthreads, nthreads, TASKQ_PREPOPULATE);
+		kmem_free(tq_name, ZFS_MAX_DATASET_NAME_LEN + 16);
+	}
+
+	for (uint64_t i = 0; i < spa->spa_root_vdev->vdev_children; i++) {
+		vdev_t *vd = spa->spa_root_vdev->vdev_child[i];
+
+		mutex_enter(&vd->vdev_scan_io_queue_lock);
+		if (vd->vdev_scan_io_queue != NULL) {
+			VERIFY(taskq_dispatch(scn->scn_taskq,
+			    scan_io_queues_run_one, vd->vdev_scan_io_queue,
+			    TQ_SLEEP) != TASKQID_INVALID);
+		}
+		mutex_exit(&vd->vdev_scan_io_queue_lock);
+	}
+
+	/*
+	 * Wait for the queues to finish issuing thir IOs for this run
+	 * before we return. There may still be IOs in flight at this
+	 * point.
+	 */
+	taskq_wait(scn->scn_taskq);
 }
 
 static boolean_t
@@ -1542,6 +2851,41 @@ dsl_scan_free_block_cb(void *arg, const blkptr_t *bp, dmu_tx_t *tx)
 	return (0);
 }
 
+static void
+dsl_scan_update_stats(dsl_scan_t *scn)
+{
+	spa_t *spa = scn->scn_dp->dp_spa;
+	uint64_t i;
+	uint64_t seg_size_total = 0, zio_size_total = 0;
+	uint64_t seg_count_total = 0, zio_count_total = 0;
+
+	for (i = 0; i < spa->spa_root_vdev->vdev_children; i++) {
+		vdev_t *vd = spa->spa_root_vdev->vdev_child[i];
+		dsl_scan_io_queue_t *queue = vd->vdev_scan_io_queue;
+
+		if (queue == NULL)
+			continue;
+
+		seg_size_total += queue->q_total_seg_size_this_txg;
+		zio_size_total += queue->q_total_zio_size_this_txg;
+		seg_count_total += queue->q_segs_this_txg;
+		zio_count_total += queue->q_zios_this_txg;
+	}
+
+	if (seg_count_total == 0 || zio_count_total == 0) {
+		scn->scn_avg_seg_size_this_txg = 0;
+		scn->scn_avg_zio_size_this_txg = 0;
+		scn->scn_segs_this_txg = 0;
+		scn->scn_zios_this_txg = 0;
+		return;
+	}
+
+	scn->scn_avg_seg_size_this_txg = seg_size_total / seg_count_total;
+	scn->scn_avg_zio_size_this_txg = zio_size_total / zio_count_total;
+	scn->scn_segs_this_txg = seg_count_total;
+	scn->scn_zios_this_txg = zio_count_total;
+}
+
 static int
 dsl_scan_obsolete_block_cb(void *arg, const blkptr_t *bp, dmu_tx_t *tx)
 {
@@ -1568,8 +2912,7 @@ dsl_scan_active(dsl_scan_t *scn)
 		return (B_FALSE);
 	if (spa_shutting_down(spa))
 		return (B_FALSE);
-	if ((scn->scn_phys.scn_state == DSS_SCANNING &&
-	    !dsl_scan_is_paused_scrub(scn)) ||
+	if ((dsl_scan_is_running(scn) && !dsl_scan_is_paused_scrub(scn)) ||
 	    (scn->scn_async_destroying && !scn->scn_async_stalled))
 		return (B_TRUE);
 
@@ -1580,25 +2923,76 @@ dsl_scan_active(dsl_scan_t *scn)
 	return (used != 0);
 }
 
+static boolean_t
+dsl_scan_need_resilver(spa_t *spa, const dva_t *dva, size_t psize,
+    uint64_t phys_birth)
+{
+	vdev_t *vd;
+
+	vd = vdev_lookup_top(spa, DVA_GET_VDEV(dva));
+
+	if (vd->vdev_ops == &vdev_indirect_ops) {
+		/*
+		 * The indirect vdev can point to multiple
+		 * vdevs.  For simplicity, always create
+		 * the resilver zio_t. zio_vdev_io_start()
+		 * will bypass the child resilver i/o's if
+		 * they are on vdevs that don't have DTL's.
+		 */
+		return (B_TRUE);
+	}
+
+	if (DVA_GET_GANG(dva)) {
+		/*
+		 * Gang members may be spread across multiple
+		 * vdevs, so the best estimate we have is the
+		 * scrub range, which has already been checked.
+		 * XXX -- it would be better to change our
+		 * allocation policy to ensure that all
+		 * gang members reside on the same vdev.
+		 */
+		return (B_TRUE);
+	}
+
+	/*
+	 * Check if the txg falls within the range which must be
+	 * resilvered.  DVAs outside this range can always be skipped.
+	 */
+	if (!vdev_dtl_contains(vd, DTL_PARTIAL, phys_birth, 1))
+		return (B_FALSE);
+
+	/*
+	 * Check if the top-level vdev must resilver this offset.
+	 * When the offset does not intersect with a dirty leaf DTL
+	 * then it may be possible to skip the resilver IO.  The psize
+	 * is provided instead of asize to simplify the check for RAIDZ.
+	 */
+	if (!vdev_dtl_need_resilver(vd, DVA_GET_OFFSET(dva), psize))
+		return (B_FALSE);
+
+	return (B_TRUE);
+}
+
 static int
 dsl_process_async_destroys(dsl_pool_t *dp, dmu_tx_t *tx)
 {
+	int err = 0;
 	dsl_scan_t *scn = dp->dp_scan;
 	spa_t *spa = dp->dp_spa;
-	int err = 0;
 
 	if (spa_suspend_async_destroy(spa))
 		return (0);
 
 	if (zfs_free_bpobj_enabled &&
-	    spa_version(dp->dp_spa) >= SPA_VERSION_DEADLISTS) {
+	    spa_version(spa) >= SPA_VERSION_DEADLISTS) {
 		scn->scn_is_bptree = B_FALSE;
 		scn->scn_async_block_min_time_ms = zfs_free_min_time_ms;
-		scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
+		scn->scn_zio_root = zio_root(spa, NULL,
 		    NULL, ZIO_FLAG_MUSTSUCCEED);
 		err = bpobj_iterate(&dp->dp_free_bpobj,
 		    dsl_scan_free_block_cb, scn, tx);
-		VERIFY3U(0, ==, zio_wait(scn->scn_zio_root));
+		VERIFY0(zio_wait(scn->scn_zio_root));
+		scn->scn_zio_root = NULL;
 
 		if (err != 0 && err != ERESTART)
 			zfs_panic_recover("error %u from bpobj_iterate()", err);
@@ -1607,11 +3001,12 @@ dsl_process_async_destroys(dsl_pool_t *dp, dmu_tx_t *tx)
 	if (err == 0 && spa_feature_is_active(spa, SPA_FEATURE_ASYNC_DESTROY)) {
 		ASSERT(scn->scn_async_destroying);
 		scn->scn_is_bptree = B_TRUE;
-		scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
+		scn->scn_zio_root = zio_root(spa, NULL,
 		    NULL, ZIO_FLAG_MUSTSUCCEED);
 		err = bptree_iterate(dp->dp_meta_objset,
 		    dp->dp_bptree_obj, B_TRUE, dsl_scan_free_block_cb, scn, tx);
 		VERIFY0(zio_wait(scn->scn_zio_root));
+		scn->scn_zio_root = NULL;
 
 		if (err == EIO || err == ECKSUM) {
 			err = 0;
@@ -1648,7 +3043,7 @@ dsl_process_async_destroys(dsl_pool_t *dp, dmu_tx_t *tx)
 	}
 	if (scn->scn_visited_this_txg) {
 		zfs_dbgmsg("freed %llu blocks in %llums from "
-		    "free_bpobj/bptree txg %llu; err=%u",
+		    "free_bpobj/bptree txg %llu; err=%d",
 		    (longlong_t)scn->scn_visited_this_txg,
 		    (longlong_t)
 		    NSEC2MSEC(gethrtime() - scn->scn_sync_start_time),
@@ -1720,12 +3115,21 @@ dsl_process_async_destroys(dsl_pool_t *dp, dmu_tx_t *tx)
 	return (0);
 }
 
+/*
+ * This is the primary entry point for scans that is called from syncing
+ * context. Scans must happen entirely during syncing context so that we
+ * cna guarantee that blocks we are currently scanning will not change out
+ * from under us. While a scan is active, this funciton controls how quickly
+ * transaction groups proceed, instead of the normal handling provided by
+ * txg_sync_thread().
+ */
 void
 dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
 {
 	dsl_scan_t *scn = dp->dp_scan;
 	spa_t *spa = dp->dp_spa;
 	int err = 0;
+	state_sync_type_t sync_type = SYNC_OPTIONAL;
 
 	/*
 	 * Check for scn_restart_txg before checking spa_load_state, so
@@ -1738,7 +3142,7 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
 		if (vdev_resilver_needed(spa->spa_root_vdev, NULL, NULL))
 			func = POOL_SCAN_RESILVER;
 		zfs_dbgmsg("restarting scan func=%u txg=%llu",
-		    func, tx->tx_txg);
+		    func, (longlong_t)tx->tx_txg);
 		dsl_scan_setup_sync(&func, tx);
 	}
 
@@ -1762,7 +3166,17 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
 	if (!scn->scn_async_stalled && !dsl_scan_active(scn))
 		return;
 
+	/* reset scan statistics */
 	scn->scn_visited_this_txg = 0;
+	scn->scn_holes_this_txg = 0;
+	scn->scn_lt_min_this_txg = 0;
+	scn->scn_gt_max_this_txg = 0;
+	scn->scn_ddt_contained_this_txg = 0;
+	scn->scn_objsets_visited_this_txg = 0;
+	scn->scn_avg_seg_size_this_txg = 0;
+	scn->scn_segs_this_txg = 0;
+	scn->scn_avg_zio_size_this_txg = 0;
+	scn->scn_zios_this_txg = 0;
 	scn->scn_suspending = B_FALSE;
 	scn->scn_sync_start_time = gethrtime();
 	spa->spa_scrub_active = B_TRUE;
@@ -1778,110 +3192,189 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
 	if (err != 0)
 		return;
 
-	if (scn->scn_phys.scn_state != DSS_SCANNING)
+	if (!dsl_scan_is_running(scn) || dsl_scan_is_paused_scrub(scn))
 		return;
 
-	if (scn->scn_done_txg == tx->tx_txg) {
-		ASSERT(!scn->scn_suspending);
-		/* finished with scan. */
-		zfs_dbgmsg("txg %llu scan complete", tx->tx_txg);
-		dsl_scan_done(scn, B_TRUE, tx);
-		ASSERT3U(spa->spa_scrub_inflight, ==, 0);
-		dsl_scan_sync_state(scn, tx);
+	/*
+	 * Wait a few txgs after importing to begin scanning so that
+	 * we can get the pool imported quickly.
+	 */
+	if (spa->spa_syncing_txg < spa->spa_first_txg + SCAN_IMPORT_WAIT_TXGS)
 		return;
-	}
 
-	if (dsl_scan_is_paused_scrub(scn))
-		return;
+	/*
+	 * It is possible to switch from unsorted to sorted at any time,
+	 * but afterwards the scan will remain sorted unless reloaded from
+	 * a checkpoint after a reboot.
+	 */
+	if (!zfs_scan_legacy) {
+		scn->scn_is_sorted = B_TRUE;
+		if (scn->scn_last_checkpoint == 0)
+			scn->scn_last_checkpoint = ddi_get_lbolt();
+	}
 
-	if (scn->scn_phys.scn_ddt_bookmark.ddb_class <=
-	    scn->scn_phys.scn_ddt_class_max) {
-		zfs_dbgmsg("doing scan sync txg %llu; "
-		    "ddt bm=%llu/%llu/%llu/%llx",
-		    (longlong_t)tx->tx_txg,
-		    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_class,
-		    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_type,
-		    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_checksum,
-		    (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_cursor);
-		ASSERT(scn->scn_phys.scn_bookmark.zb_objset == 0);
-		ASSERT(scn->scn_phys.scn_bookmark.zb_object == 0);
-		ASSERT(scn->scn_phys.scn_bookmark.zb_level == 0);
-		ASSERT(scn->scn_phys.scn_bookmark.zb_blkid == 0);
+	/*
+	 * For sorted scans, determine what kind of work we will be doing
+	 * this txg based on our memory limitations and whether or not we
+	 * need to perform a checkpoint.
+	 */
+	if (scn->scn_is_sorted) {
+		/*
+		 * If we are over our checkpoint interval, set scn_clearing
+		 * so that we can begin checkpointing immediately. The
+		 * checkpoint allows us to save a consisent bookmark
+		 * representing how much data we have scrubbed so far.
+		 * Otherwise, use the memory limit to determine if we should
+		 * scan for metadata or start issue scrub IOs. We accumulate
+		 * metadata until we hit our hard memory limit at which point
+		 * we issue scrub IOs until we are at our soft memory limit.
+		 */
+		if (scn->scn_checkpointing ||
+		    ddi_get_lbolt() - scn->scn_last_checkpoint >
+		    SEC_TO_TICK(zfs_scan_checkpoint_intval)) {
+			if (!scn->scn_checkpointing)
+				zfs_dbgmsg("begin scan checkpoint");
+
+			scn->scn_checkpointing = B_TRUE;
+			scn->scn_clearing = B_TRUE;
+		} else {
+			boolean_t should_clear = dsl_scan_should_clear(scn);
+			if (should_clear && !scn->scn_clearing) {
+				zfs_dbgmsg("begin scan clearing");
+				scn->scn_clearing = B_TRUE;
+			} else if (!should_clear && scn->scn_clearing) {
+				zfs_dbgmsg("finish scan clearing");
+				scn->scn_clearing = B_FALSE;
+			}
+		}
 	} else {
-		zfs_dbgmsg("doing scan sync txg %llu; bm=%llu/%llu/%llu/%llu",
-		    (longlong_t)tx->tx_txg,
-		    (longlong_t)scn->scn_phys.scn_bookmark.zb_objset,
-		    (longlong_t)scn->scn_phys.scn_bookmark.zb_object,
-		    (longlong_t)scn->scn_phys.scn_bookmark.zb_level,
-		    (longlong_t)scn->scn_phys.scn_bookmark.zb_blkid);
+		ASSERT0(scn->scn_checkpointing);
+		ASSERT0(scn->scn_clearing);
 	}
 
-	scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
-	    NULL, ZIO_FLAG_CANFAIL);
-	dsl_pool_config_enter(dp, FTAG);
-	dsl_scan_visit(scn, tx);
-	dsl_pool_config_exit(dp, FTAG);
-	(void) zio_wait(scn->scn_zio_root);
-	scn->scn_zio_root = NULL;
+	if (!scn->scn_clearing && scn->scn_done_txg == 0) {
+		/* Need to scan metadata for more blocks to scrub */
+		dsl_scan_phys_t *scnp = &scn->scn_phys;
+		taskqid_t prefetch_tqid;
+		uint64_t bytes_per_leaf = zfs_scan_vdev_limit;
+		uint64_t nr_leaves = dsl_scan_count_leaves(spa->spa_root_vdev);
 
-	zfs_dbgmsg("visited %llu blocks in %llums",
-	    (longlong_t)scn->scn_visited_this_txg,
-	    (longlong_t)NSEC2MSEC(gethrtime() - scn->scn_sync_start_time));
+		/*
+		 * Calculate the max number of in-flight bytes for pool-wide
+		 * scanning operations (minimum 1MB). Limits for the issuing
+		 * phase are done per top-level vdev and are handled separately.
+		 */
+		scn->scn_maxinflight_bytes =
+		    MAX(nr_leaves * bytes_per_leaf, 1ULL << 20);
+
+		if (scnp->scn_ddt_bookmark.ddb_class <=
+		    scnp->scn_ddt_class_max) {
+			ASSERT(ZB_IS_ZERO(&scnp->scn_bookmark));
+			zfs_dbgmsg("doing scan sync txg %llu; "
+			    "ddt bm=%llu/%llu/%llu/%llx",
+			    (longlong_t)tx->tx_txg,
+			    (longlong_t)scnp->scn_ddt_bookmark.ddb_class,
+			    (longlong_t)scnp->scn_ddt_bookmark.ddb_type,
+			    (longlong_t)scnp->scn_ddt_bookmark.ddb_checksum,
+			    (longlong_t)scnp->scn_ddt_bookmark.ddb_cursor);
+		} else {
+			zfs_dbgmsg("doing scan sync txg %llu; "
+			    "bm=%llu/%llu/%llu/%llu",
+			    (longlong_t)tx->tx_txg,
+			    (longlong_t)scnp->scn_bookmark.zb_objset,
+			    (longlong_t)scnp->scn_bookmark.zb_object,
+			    (longlong_t)scnp->scn_bookmark.zb_level,
+			    (longlong_t)scnp->scn_bookmark.zb_blkid);
+		}
 
-	if (!scn->scn_suspending) {
-		scn->scn_done_txg = tx->tx_txg + 1;
-		zfs_dbgmsg("txg %llu traversal complete, waiting till txg %llu",
-		    tx->tx_txg, scn->scn_done_txg);
-	}
+		scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
+		    NULL, ZIO_FLAG_CANFAIL);
 
-	if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
-		mutex_enter(&spa->spa_scrub_lock);
-		while (spa->spa_scrub_inflight > 0) {
-			cv_wait(&spa->spa_scrub_io_cv,
-			    &spa->spa_scrub_lock);
-		}
-		mutex_exit(&spa->spa_scrub_lock);
-	}
+		scn->scn_prefetch_stop = B_FALSE;
+		prefetch_tqid = taskq_dispatch(dp->dp_sync_taskq,
+		    dsl_scan_prefetch_thread, scn, TQ_SLEEP);
+		ASSERT(prefetch_tqid != TASKQID_INVALID);
 
-	dsl_scan_sync_state(scn, tx);
-}
+		dsl_pool_config_enter(dp, FTAG);
+		dsl_scan_visit(scn, tx);
+		dsl_pool_config_exit(dp, FTAG);
 
-/*
- * This will start a new scan, or restart an existing one.
- */
-void
-dsl_resilver_restart(dsl_pool_t *dp, uint64_t txg)
-{
-	if (txg == 0) {
-		dmu_tx_t *tx;
-		tx = dmu_tx_create_dd(dp->dp_mos_dir);
-		VERIFY(0 == dmu_tx_assign(tx, TXG_WAIT));
+		mutex_enter(&dp->dp_spa->spa_scrub_lock);
+		scn->scn_prefetch_stop = B_TRUE;
+		cv_broadcast(&spa->spa_scrub_io_cv);
+		mutex_exit(&dp->dp_spa->spa_scrub_lock);
 
-		txg = dmu_tx_get_txg(tx);
-		dp->dp_scan->scn_restart_txg = txg;
-		dmu_tx_commit(tx);
-	} else {
-		dp->dp_scan->scn_restart_txg = txg;
+		taskq_wait_id(dp->dp_sync_taskq, prefetch_tqid);
+		(void) zio_wait(scn->scn_zio_root);
+		scn->scn_zio_root = NULL;
+
+		zfs_dbgmsg("scan visited %llu blocks in %llums "
+		    "(%llu os's, %llu holes, %llu < mintxg, "
+		    "%llu in ddt, %llu > maxtxg)",
+		    (longlong_t)scn->scn_visited_this_txg,
+		    (longlong_t)NSEC2MSEC(gethrtime() -
+		    scn->scn_sync_start_time),
+		    (longlong_t)scn->scn_objsets_visited_this_txg,
+		    (longlong_t)scn->scn_holes_this_txg,
+		    (longlong_t)scn->scn_lt_min_this_txg,
+		    (longlong_t)scn->scn_ddt_contained_this_txg,
+		    (longlong_t)scn->scn_gt_max_this_txg);
+
+		if (!scn->scn_suspending) {
+			ASSERT0(avl_numnodes(&scn->scn_queue));
+			scn->scn_done_txg = tx->tx_txg + 1;
+			if (scn->scn_is_sorted) {
+				scn->scn_checkpointing = B_TRUE;
+				scn->scn_clearing = B_TRUE;
+			}
+			zfs_dbgmsg("scan complete txg %llu",
+			    (longlong_t)tx->tx_txg);
+		}
+	} else if (scn->scn_is_sorted && scn->scn_bytes_pending != 0) {
+		/* need to issue scrubbing IOs from per-vdev queues */
+		scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
+		    NULL, ZIO_FLAG_CANFAIL);
+		scan_io_queues_run(scn);
+		(void) zio_wait(scn->scn_zio_root);
+		scn->scn_zio_root = NULL;
+
+		/* calculate and dprintf the current memory usage */
+		(void) dsl_scan_should_clear(scn);
+		dsl_scan_update_stats(scn);
+
+		zfs_dbgmsg("scrubbed %llu blocks (%llu segs) in %llums "
+		    "(avg_block_size = %llu, avg_seg_size = %llu)",
+		    (longlong_t)scn->scn_zios_this_txg,
+		    (longlong_t)scn->scn_segs_this_txg,
+		    (longlong_t)NSEC2MSEC(gethrtime() -
+		    scn->scn_sync_start_time),
+		    (longlong_t)scn->scn_avg_zio_size_this_txg,
+		    (longlong_t)scn->scn_avg_seg_size_this_txg);
+	} else if (scn->scn_done_txg != 0 && scn->scn_done_txg <= tx->tx_txg) {
+		/* Finished with everything. Mark the scrub as complete */
+		zfs_dbgmsg("scan issuing complete txg %llu",
+		    (longlong_t)tx->tx_txg);
+		ASSERT3U(scn->scn_done_txg, !=, 0);
+		ASSERT0(spa->spa_scrub_inflight);
+		ASSERT0(scn->scn_bytes_pending);
+		dsl_scan_done(scn, B_TRUE, tx);
+		sync_type = SYNC_MANDATORY;
 	}
-	zfs_dbgmsg("restarting resilver txg=%llu", txg);
-}
 
-boolean_t
-dsl_scan_resilvering(dsl_pool_t *dp)
-{
-	return (dp->dp_scan->scn_phys.scn_state == DSS_SCANNING &&
-	    dp->dp_scan->scn_phys.scn_func == POOL_SCAN_RESILVER);
+	dsl_scan_sync_state(scn, tx, sync_type);
 }
 
-/*
- * scrub consumers
- */
-
 static void
-count_block(zfs_all_blkstats_t *zab, const blkptr_t *bp)
+count_block(dsl_scan_t *scn, zfs_all_blkstats_t *zab, const blkptr_t *bp)
 {
 	int i;
 
+	/* update the spa's stats on how many bytes we have issued */
+	for (i = 0; i < BP_GET_NDVAS(bp); i++) {
+		atomic_add_64(&scn->scn_dp->dp_spa->spa_scan_pass_issued,
+		    DVA_GET_ASIZE(&bp->blk_dva[i]));
+	}
+
 	/*
 	 * If we resume after a reboot, zab will be NULL; don't record
 	 * incomplete stats in that case.
@@ -1889,6 +3382,8 @@ count_block(zfs_all_blkstats_t *zab, const blkptr_t *bp)
 	if (zab == NULL)
 		return;
 
+	mutex_enter(&zab->zab_lock);
+
 	for (i = 0; i < 4; i++) {
 		int l = (i < 2) ? BP_GET_LEVEL(bp) : DN_MAX_LEVELS;
 		int t = (i & 1) ? BP_GET_TYPE(bp) : DMU_OT_TOTAL;
@@ -1923,24 +3418,96 @@ count_block(zfs_all_blkstats_t *zab, const blkptr_t *bp)
 			break;
 		}
 	}
+
+	mutex_exit(&zab->zab_lock);
 }
 
 static void
-dsl_scan_scrub_done(zio_t *zio)
+scan_io_queue_insert_impl(dsl_scan_io_queue_t *queue, scan_io_t *sio)
 {
-	spa_t *spa = zio->io_spa;
+	avl_index_t idx;
+	int64_t asize = sio->sio_asize;
+	dsl_scan_t *scn = queue->q_scn;
 
-	abd_free(zio->io_abd);
+	ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));
 
-	mutex_enter(&spa->spa_scrub_lock);
-	spa->spa_scrub_inflight--;
-	cv_broadcast(&spa->spa_scrub_io_cv);
+	if (avl_find(&queue->q_sios_by_addr, sio, &idx) != NULL) {
+		/* block is already scheduled for reading */
+		atomic_add_64(&scn->scn_bytes_pending, -asize);
+		kmem_free(sio, sizeof (*sio));
+		return;
+	}
+	avl_insert(&queue->q_sios_by_addr, sio, idx);
+	range_tree_add(queue->q_exts_by_addr, sio->sio_offset, asize);
+}
 
-	if (zio->io_error && (zio->io_error != ECKSUM ||
-	    !(zio->io_flags & ZIO_FLAG_SPECULATIVE))) {
-		spa->spa_dsl_pool->dp_scan->scn_phys.scn_errors++;
+/*
+ * Given all the info we got from our metadata scanning process, we
+ * construct a scan_io_t and insert it into the scan sorting queue. The
+ * I/O must already be suitable for us to process. This is controlled
+ * by dsl_scan_enqueue().
+ */
+static void
+scan_io_queue_insert(dsl_scan_io_queue_t *queue, const blkptr_t *bp, int dva_i,
+    int zio_flags, const zbookmark_phys_t *zb)
+{
+	dsl_scan_t *scn = queue->q_scn;
+	scan_io_t *sio = kmem_zalloc(sizeof (*sio), KM_SLEEP);
+
+	ASSERT0(BP_IS_GANG(bp));
+	ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));
+
+	bp2sio(bp, sio, dva_i);
+	sio->sio_flags = zio_flags;
+	sio->sio_zb = *zb;
+
+	/*
+	 * Increment the bytes pending counter now so that we can't
+	 * get an integer underflow in case the worker processes the
+	 * zio before we get to incrementing this counter.
+	 */
+	atomic_add_64(&scn->scn_bytes_pending, sio->sio_asize);
+
+	scan_io_queue_insert_impl(queue, sio);
+}
+
+/*
+ * Given a set of I/O parameters as discovered by the metadata traversal
+ * process, attempts to place the I/O into the sorted queues (if allowed),
+ * or immediately executes the I/O.
+ */
+static void
+dsl_scan_enqueue(dsl_pool_t *dp, const blkptr_t *bp, int zio_flags,
+    const zbookmark_phys_t *zb)
+{
+	spa_t *spa = dp->dp_spa;
+
+	ASSERT(!BP_IS_EMBEDDED(bp));
+
+	/*
+	 * Gang blocks are hard to issue sequentially, so we just issue them
+	 * here immediately instead of queuing them.
+	 */
+	if (!dp->dp_scan->scn_is_sorted || BP_IS_GANG(bp)) {
+		scan_exec_io(dp, bp, zio_flags, zb, NULL);
+		return;
+	}
+	for (int i = 0; i < BP_GET_NDVAS(bp); i++) {
+		dva_t dva;
+		vdev_t *vdev;
+
+		dva = bp->blk_dva[i];
+		vdev = vdev_lookup_top(spa, DVA_GET_VDEV(&dva));
+		ASSERT(vdev != NULL);
+
+		mutex_enter(&vdev->vdev_scan_io_queue_lock);
+		if (vdev->vdev_scan_io_queue == NULL)
+			vdev->vdev_scan_io_queue = scan_io_queue_create(vdev);
+		ASSERT(dp->dp_scan != NULL);
+		scan_io_queue_insert(vdev->vdev_scan_io_queue, bp,
+		    i, zio_flags, zb);
+		mutex_exit(&vdev->vdev_scan_io_queue_lock);
 	}
-	mutex_exit(&spa->spa_scrub_lock);
 }
 
 static int
@@ -1948,18 +3515,18 @@ dsl_scan_scrub_cb(dsl_pool_t *dp,
     const blkptr_t *bp, const zbookmark_phys_t *zb)
 {
 	dsl_scan_t *scn = dp->dp_scan;
-	size_t size = BP_GET_PSIZE(bp);
 	spa_t *spa = dp->dp_spa;
 	uint64_t phys_birth = BP_PHYSICAL_BIRTH(bp);
+	size_t psize = BP_GET_PSIZE(bp);
 	boolean_t needs_io;
 	int zio_flags = ZIO_FLAG_SCAN_THREAD | ZIO_FLAG_RAW | ZIO_FLAG_CANFAIL;
-	int scan_delay = 0;
-
-	count_block(dp->dp_blkstats, bp);
+	int d;
 
 	if (phys_birth <= scn->scn_phys.scn_min_txg ||
-	    phys_birth >= scn->scn_phys.scn_max_txg)
+	    phys_birth >= scn->scn_phys.scn_max_txg) {
+		count_block(scn, dp->dp_blkstats, bp);
 		return (0);
+	}
 
 	/* Embedded BP's have phys_birth==0, so we reject them above. */
 	ASSERT(!BP_IS_EMBEDDED(bp));
@@ -1968,125 +3535,359 @@ dsl_scan_scrub_cb(dsl_pool_t *dp,
 	if (scn->scn_phys.scn_func == POOL_SCAN_SCRUB) {
 		zio_flags |= ZIO_FLAG_SCRUB;
 		needs_io = B_TRUE;
-		scan_delay = zfs_scrub_delay;
 	} else {
 		ASSERT3U(scn->scn_phys.scn_func, ==, POOL_SCAN_RESILVER);
 		zio_flags |= ZIO_FLAG_RESILVER;
 		needs_io = B_FALSE;
-		scan_delay = zfs_resilver_delay;
 	}
 
 	/* If it's an intent log block, failure is expected. */
 	if (zb->zb_level == ZB_ZIL_LEVEL)
 		zio_flags |= ZIO_FLAG_SPECULATIVE;
 
-	for (int d = 0; d < BP_GET_NDVAS(bp); d++) {
-		vdev_t *vd = vdev_lookup_top(spa,
-		    DVA_GET_VDEV(&bp->blk_dva[d]));
+	for (d = 0; d < BP_GET_NDVAS(bp); d++) {
+		const dva_t *dva = &bp->blk_dva[d];
 
 		/*
 		 * Keep track of how much data we've examined so that
 		 * zpool(1M) status can make useful progress reports.
 		 */
-		scn->scn_phys.scn_examined += DVA_GET_ASIZE(&bp->blk_dva[d]);
-		spa->spa_scan_pass_exam += DVA_GET_ASIZE(&bp->blk_dva[d]);
+		scn->scn_phys.scn_examined += DVA_GET_ASIZE(dva);
+		spa->spa_scan_pass_exam += DVA_GET_ASIZE(dva);
 
 		/* if it's a resilver, this may not be in the target range */
-		if (!needs_io) {
-			if (vd->vdev_ops == &vdev_indirect_ops) {
-				/*
-				 * The indirect vdev can point to multiple
-				 * vdevs.  For simplicity, always create
-				 * the resilver zio_t. zio_vdev_io_start()
-				 * will bypass the child resilver i/o's if
-				 * they are on vdevs that don't have DTL's.
-				 */
-				needs_io = B_TRUE;
-			} else if (DVA_GET_GANG(&bp->blk_dva[d])) {
-				/*
-				 * Gang members may be spread across multiple
-				 * vdevs, so the best estimate we have is the
-				 * scrub range, which has already been checked.
-				 * XXX -- it would be better to change our
-				 * allocation policy to ensure that all
-				 * gang members reside on the same vdev.
-				 */
-				needs_io = B_TRUE;
-			} else {
-				needs_io = vdev_dtl_contains(vd, DTL_PARTIAL,
-				    phys_birth, 1);
-			}
-		}
+		if (!needs_io)
+			needs_io = dsl_scan_need_resilver(spa, dva, psize,
+			    phys_birth);
 	}
 
 	if (needs_io && !zfs_no_scrub_io) {
-		vdev_t *rvd = spa->spa_root_vdev;
-		uint64_t maxinflight = rvd->vdev_children * zfs_top_maxinflight;
+		dsl_scan_enqueue(dp, bp, zio_flags, zb);
+	} else {
+		count_block(scn, dp->dp_blkstats, bp);
+	}
+
+	/* do not relocate this block */
+	return (0);
+}
 
+static void
+dsl_scan_scrub_done(zio_t *zio)
+{
+	spa_t *spa = zio->io_spa;
+	blkptr_t *bp = zio->io_bp;
+	dsl_scan_io_queue_t *queue = zio->io_private;
+
+	abd_free(zio->io_abd);
+
+	if (queue == NULL) {
 		mutex_enter(&spa->spa_scrub_lock);
-		while (spa->spa_scrub_inflight >= maxinflight)
+		ASSERT3U(spa->spa_scrub_inflight, >=, BP_GET_PSIZE(bp));
+		spa->spa_scrub_inflight -= BP_GET_PSIZE(bp);
+		cv_broadcast(&spa->spa_scrub_io_cv);
+		mutex_exit(&spa->spa_scrub_lock);
+	} else {
+		mutex_enter(&queue->q_vd->vdev_scan_io_queue_lock);
+		ASSERT3U(queue->q_inflight_bytes, >=, BP_GET_PSIZE(bp));
+		queue->q_inflight_bytes -= BP_GET_PSIZE(bp);
+		cv_broadcast(&queue->q_zio_cv);
+		mutex_exit(&queue->q_vd->vdev_scan_io_queue_lock);
+	}
+
+	if (zio->io_error && (zio->io_error != ECKSUM ||
+	    !(zio->io_flags & ZIO_FLAG_SPECULATIVE))) {
+		atomic_inc_64(&spa->spa_dsl_pool->dp_scan->scn_phys.scn_errors);
+	}
+}
+
+/*
+ * Given a scanning zio's information, executes the zio. The zio need
+ * not necessarily be only sortable, this function simply executes the
+ * zio, no matter what it is. The optional queue argument allows the
+ * caller to specify that they want per top level vdev IO rate limiting
+ * instead of the legacy global limiting.
+ */
+static void
+scan_exec_io(dsl_pool_t *dp, const blkptr_t *bp, int zio_flags,
+    const zbookmark_phys_t *zb, dsl_scan_io_queue_t *queue)
+{
+	spa_t *spa = dp->dp_spa;
+	dsl_scan_t *scn = dp->dp_scan;
+	size_t size = BP_GET_PSIZE(bp);
+	abd_t *data = abd_alloc_for_io(size, B_FALSE);
+
+	if (queue == NULL) {
+		mutex_enter(&spa->spa_scrub_lock);
+		while (spa->spa_scrub_inflight >= scn->scn_maxinflight_bytes)
 			cv_wait(&spa->spa_scrub_io_cv, &spa->spa_scrub_lock);
-		spa->spa_scrub_inflight++;
+		spa->spa_scrub_inflight += BP_GET_PSIZE(bp);
 		mutex_exit(&spa->spa_scrub_lock);
+	} else {
+		kmutex_t *q_lock = &queue->q_vd->vdev_scan_io_queue_lock;
 
-		/*
-		 * If we're seeing recent (zfs_scan_idle) "important" I/Os
-		 * then throttle our workload to limit the impact of a scan.
-		 */
-		if (ddi_get_lbolt64() - spa->spa_last_io <= zfs_scan_idle)
-			delay(scan_delay);
+		mutex_enter(q_lock);
+		while (queue->q_inflight_bytes >= queue->q_maxinflight_bytes)
+			cv_wait(&queue->q_zio_cv, q_lock);
+		queue->q_inflight_bytes += BP_GET_PSIZE(bp);
+		mutex_exit(q_lock);
+	}
+
+	count_block(dp->dp_scan, dp->dp_blkstats, bp);
+	zio_nowait(zio_read(dp->dp_scan->scn_zio_root, spa, bp, data, size,
+	    dsl_scan_scrub_done, queue, ZIO_PRIORITY_SCRUB, zio_flags, zb));
+}
 
-		zio_nowait(zio_read(NULL, spa, bp,
-		    abd_alloc_for_io(size, B_FALSE), size, dsl_scan_scrub_done,
-		    NULL, ZIO_PRIORITY_SCRUB, zio_flags, zb));
+/*
+ * This is the primary extent sorting algorithm. We balance two parameters:
+ * 1) how many bytes of I/O are in an extent
+ * 2) how well the extent is filled with I/O (as a fraction of its total size)
+ * Since we allow extents to have gaps between their constituent I/Os, it's
+ * possible to have a fairly large extent that contains the same amount of
+ * I/O bytes than a much smaller extent, which just packs the I/O more tightly.
+ * The algorithm sorts based on a score calculated from the extent's size,
+ * the relative fill volume (in %) and a "fill weight" parameter that controls
+ * the split between whether we prefer larger extents or more well populated
+ * extents:
+ *
+ * SCORE = FILL_IN_BYTES + (FILL_IN_PERCENT * FILL_IN_BYTES * FILL_WEIGHT)
+ *
+ * Example:
+ * 1) assume extsz = 64 MiB
+ * 2) assume fill = 32 MiB (extent is half full)
+ * 3) assume fill_weight = 3
+ * 4)	SCORE = 32M + (((32M * 100) / 64M) * 3 * 32M) / 100
+ *	SCORE = 32M + (50 * 3 * 32M) / 100
+ *	SCORE = 32M + (4800M / 100)
+ *	SCORE = 32M + 48M
+ *		^	^
+ *		|	+--- final total relative fill-based score
+ *		+--------- final total fill-based score
+ *	SCORE = 80M
+ *
+ * As can be seen, at fill_ratio=3, the algorithm is slightly biased towards
+ * extents that are more completely filled (in a 3:2 ratio) vs just larger.
+ * Note that as an optimization, we replace multiplication and division by
+ * 100 with bitshifting by 7 (which effecitvely multiplies and divides by 128).
+ */
+static int
+ext_size_compare(const void *x, const void *y)
+{
+	const range_seg_t *rsa = x, *rsb = y;
+	uint64_t sa = rsa->rs_end - rsa->rs_start,
+	    sb = rsb->rs_end - rsb->rs_start;
+	uint64_t score_a, score_b;
+
+	score_a = rsa->rs_fill + ((((rsa->rs_fill << 7) / sa) *
+	    fill_weight * rsa->rs_fill) >> 7);
+	score_b = rsb->rs_fill + ((((rsb->rs_fill << 7) / sb) *
+	    fill_weight * rsb->rs_fill) >> 7);
+
+	if (score_a > score_b)
+		return (-1);
+	if (score_a == score_b) {
+		if (rsa->rs_start < rsb->rs_start)
+			return (-1);
+		if (rsa->rs_start == rsb->rs_start)
+			return (0);
+		return (1);
 	}
+	return (1);
+}
 
-	/* do not relocate this block */
-	return (0);
+/*
+ * Comparator for the q_sios_by_addr tree. Sorting is simply performed
+ * based on LBA-order (from lowest to highest).
+ */
+static int
+io_addr_compare(const void *x, const void *y)
+{
+	const scan_io_t *a = x, *b = y;
+
+	if (a->sio_offset < b->sio_offset)
+		return (-1);
+	if (a->sio_offset == b->sio_offset)
+		return (0);
+	return (1);
+}
+
+/* IO queues are created on demand when they are needed. */
+static dsl_scan_io_queue_t *
+scan_io_queue_create(vdev_t *vd)
+{
+	dsl_scan_t *scn = vd->vdev_spa->spa_dsl_pool->dp_scan;
+	dsl_scan_io_queue_t *q = kmem_zalloc(sizeof (*q), KM_SLEEP);
+
+	q->q_scn = scn;
+	q->q_vd = vd;
+	cv_init(&q->q_zio_cv, NULL, CV_DEFAULT, NULL);
+	q->q_exts_by_addr = range_tree_create_impl(&rt_avl_ops,
+	    &q->q_exts_by_size, ext_size_compare, zfs_scan_max_ext_gap);
+	avl_create(&q->q_sios_by_addr, io_addr_compare,
+	    sizeof (scan_io_t), offsetof(scan_io_t, sio_nodes.sio_addr_node));
+
+	return (q);
 }
 
 /*
- * Called by the ZFS_IOC_POOL_SCAN ioctl to start a scrub or resilver.
- * Can also be called to resume a paused scrub.
+ * Destroys a scan queue and all segments and scan_io_t's contained in it.
+ * No further execution of I/O occurs, anything pending in the queue is
+ * simply freed without being executed.
  */
-int
-dsl_scan(dsl_pool_t *dp, pool_scan_func_t func)
+void
+dsl_scan_io_queue_destroy(dsl_scan_io_queue_t *queue)
 {
-	spa_t *spa = dp->dp_spa;
+	dsl_scan_t *scn = queue->q_scn;
+	scan_io_t *sio;
+	void *cookie = NULL;
+	int64_t bytes_dequeued = 0;
+
+	ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));
+
+	while ((sio = avl_destroy_nodes(&queue->q_sios_by_addr, &cookie)) !=
+	    NULL) {
+		ASSERT(range_tree_contains(queue->q_exts_by_addr,
+		    sio->sio_offset, sio->sio_asize));
+		bytes_dequeued += sio->sio_asize;
+		kmem_free(sio, sizeof (*sio));
+	}
+
+	atomic_add_64(&scn->scn_bytes_pending, -bytes_dequeued);
+	range_tree_vacate(queue->q_exts_by_addr, NULL, queue);
+	range_tree_destroy(queue->q_exts_by_addr);
+	avl_destroy(&queue->q_sios_by_addr);
+	cv_destroy(&queue->q_zio_cv);
+
+	kmem_free(queue, sizeof (*queue));
+}
+
+/*
+ * Properly transfers a dsl_scan_queue_t from `svd' to `tvd'. This is
+ * called on behalf of vdev_top_transfer when creating or destroying
+ * a mirror vdev due to zpool attach/detach.
+ */
+void
+dsl_scan_io_queue_vdev_xfer(vdev_t *svd, vdev_t *tvd)
+{
+	mutex_enter(&svd->vdev_scan_io_queue_lock);
+	mutex_enter(&tvd->vdev_scan_io_queue_lock);
+
+	VERIFY3P(tvd->vdev_scan_io_queue, ==, NULL);
+	tvd->vdev_scan_io_queue = svd->vdev_scan_io_queue;
+	svd->vdev_scan_io_queue = NULL;
+	if (tvd->vdev_scan_io_queue != NULL)
+		tvd->vdev_scan_io_queue->q_vd = tvd;
+
+	mutex_exit(&tvd->vdev_scan_io_queue_lock);
+	mutex_exit(&svd->vdev_scan_io_queue_lock);
+}
+
+static void
+scan_io_queues_destroy(dsl_scan_t *scn)
+{
+	vdev_t *rvd = scn->scn_dp->dp_spa->spa_root_vdev;
+
+	for (uint64_t i = 0; i < rvd->vdev_children; i++) {
+		vdev_t *tvd = rvd->vdev_child[i];
+
+		mutex_enter(&tvd->vdev_scan_io_queue_lock);
+		if (tvd->vdev_scan_io_queue != NULL)
+			dsl_scan_io_queue_destroy(tvd->vdev_scan_io_queue);
+		tvd->vdev_scan_io_queue = NULL;
+		mutex_exit(&tvd->vdev_scan_io_queue_lock);
+	}
+}
+
+static void
+dsl_scan_freed_dva(spa_t *spa, const blkptr_t *bp, int dva_i)
+{
+	dsl_pool_t *dp = spa->spa_dsl_pool;
 	dsl_scan_t *scn = dp->dp_scan;
+	vdev_t *vdev;
+	kmutex_t *q_lock;
+	dsl_scan_io_queue_t *queue;
+	scan_io_t srch, *sio;
+	avl_index_t idx;
+	uint64_t start, size;
+
+	vdev = vdev_lookup_top(spa, DVA_GET_VDEV(&bp->blk_dva[dva_i]));
+	ASSERT(vdev != NULL);
+	q_lock = &vdev->vdev_scan_io_queue_lock;
+	queue = vdev->vdev_scan_io_queue;
+
+	mutex_enter(q_lock);
+	if (queue == NULL) {
+		mutex_exit(q_lock);
+		return;
+	}
+
+	bp2sio(bp, &srch, dva_i);
+	start = srch.sio_offset;
+	size = srch.sio_asize;
 
 	/*
-	 * Purge all vdev caches and probe all devices.  We do this here
-	 * rather than in sync context because this requires a writer lock
-	 * on the spa_config lock, which we can't do from sync context.  The
-	 * spa_scrub_reopen flag indicates that vdev_open() should not
-	 * attempt to start another scrub.
+	 * We can find the zio in two states:
+	 * 1) Cold, just sitting in the queue of zio's to be issued at
+	 *	some point in the future. In this case, all we do is
+	 *	remove the zio from the q_sios_by_addr tree, decrement
+	 *	its data volume from the containing range_seg_t and
+	 *	resort the q_exts_by_size tree to reflect that the
+	 *	range_seg_t has lost some of its 'fill'. We don't shorten
+	 *	the range_seg_t - this is usually rare enough not to be
+	 *	worth the extra hassle of trying keep track of precise
+	 *	extent boundaries.
+	 * 2) Hot, where the zio is currently in-flight in
+	 *	dsl_scan_issue_ios. In this case, we can't simply
+	 *	reach in and stop the in-flight zio's, so we instead
+	 *	block the caller. Eventually, dsl_scan_issue_ios will
+	 *	be done with issuing the zio's it gathered and will
+	 *	signal us.
 	 */
-	spa_vdev_state_enter(spa, SCL_NONE);
-	spa->spa_scrub_reopen = B_TRUE;
-	vdev_reopen(spa->spa_root_vdev);
-	spa->spa_scrub_reopen = B_FALSE;
-	(void) spa_vdev_state_exit(spa, NULL, 0);
+	sio = avl_find(&queue->q_sios_by_addr, &srch, &idx);
+	if (sio != NULL) {
+		int64_t asize = sio->sio_asize;
+		blkptr_t tmpbp;
 
-	if (func == POOL_SCAN_SCRUB && dsl_scan_is_paused_scrub(scn)) {
-		/* got scrub start cmd, resume paused scrub */
-		int err = dsl_scrub_set_pause_resume(scn->scn_dp,
-		    POOL_SCRUB_NORMAL);
-		if (err == 0) {
-			spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_RESUME);
-			return (ECANCELED);
-		}
+		/* Got it while it was cold in the queue */
+		ASSERT3U(start, ==, sio->sio_offset);
+		ASSERT3U(size, ==, asize);
+		avl_remove(&queue->q_sios_by_addr, sio);
 
-		return (SET_ERROR(err));
-	}
+		ASSERT(range_tree_contains(queue->q_exts_by_addr, start, size));
+		range_tree_remove_fill(queue->q_exts_by_addr, start, size);
 
-	return (dsl_sync_task(spa_name(spa), dsl_scan_setup_check,
-	    dsl_scan_setup_sync, &func, 0, ZFS_SPACE_CHECK_EXTRA_RESERVED));
+		/*
+		 * We only update scn_bytes_pending in the cold path,
+		 * otherwise it will already have been accounted for as
+		 * part of the zio's execution.
+		 */
+		atomic_add_64(&scn->scn_bytes_pending, -asize);
+
+		/* count the block as though we issued it */
+		sio2bp(sio, &tmpbp, dva_i);
+		count_block(scn, dp->dp_blkstats, &tmpbp);
+
+		kmem_free(sio, sizeof (*sio));
+	}
+	mutex_exit(q_lock);
 }
 
-static boolean_t
-dsl_scan_restarting(dsl_scan_t *scn, dmu_tx_t *tx)
+/*
+ * Callback invoked when a zio_free() zio is executing. This needs to be
+ * intercepted to prevent the zio from deallocating a particular portion
+ * of disk space and it then getting reallocated and written to, while we
+ * still have it queued up for processing.
+ */
+void
+dsl_scan_freed(spa_t *spa, const blkptr_t *bp)
 {
-	return (scn->scn_restart_txg != 0 &&
-	    scn->scn_restart_txg <= tx->tx_txg);
+	dsl_pool_t *dp = spa->spa_dsl_pool;
+	dsl_scan_t *scn = dp->dp_scan;
+
+	ASSERT(!BP_IS_EMBEDDED(bp));
+	ASSERT(scn != NULL);
+	if (!dsl_scan_is_running(scn))
+		return;
+
+	for (int i = 0; i < BP_GET_NDVAS(bp); i++)
+		dsl_scan_freed_dva(spa, bp, i);
 }
diff --git a/usr/src/uts/common/fs/zfs/metaslab.c b/usr/src/uts/common/fs/zfs/metaslab.c
index 0044f37964..1c004f87f3 100644
--- a/usr/src/uts/common/fs/zfs/metaslab.c
+++ b/usr/src/uts/common/fs/zfs/metaslab.c
@@ -1129,85 +1129,6 @@ metaslab_rangesize_compare(const void *x1, const void *x2)
 }
 
 /*
- * Create any block allocator specific components. The current allocators
- * rely on using both a size-ordered range_tree_t and an array of uint64_t's.
- */
-static void
-metaslab_rt_create(range_tree_t *rt, void *arg)
-{
-	metaslab_t *msp = arg;
-
-	ASSERT3P(rt->rt_arg, ==, msp);
-	ASSERT(msp->ms_allocatable == NULL);
-
-	avl_create(&msp->ms_allocatable_by_size, metaslab_rangesize_compare,
-	    sizeof (range_seg_t), offsetof(range_seg_t, rs_pp_node));
-}
-
-/*
- * Destroy the block allocator specific components.
- */
-static void
-metaslab_rt_destroy(range_tree_t *rt, void *arg)
-{
-	metaslab_t *msp = arg;
-
-	ASSERT3P(rt->rt_arg, ==, msp);
-	ASSERT3P(msp->ms_allocatable, ==, rt);
-	ASSERT0(avl_numnodes(&msp->ms_allocatable_by_size));
-
-	avl_destroy(&msp->ms_allocatable_by_size);
-}
-
-static void
-metaslab_rt_add(range_tree_t *rt, range_seg_t *rs, void *arg)
-{
-	metaslab_t *msp = arg;
-
-	ASSERT3P(rt->rt_arg, ==, msp);
-	ASSERT3P(msp->ms_allocatable, ==, rt);
-	VERIFY(!msp->ms_condensing);
-	avl_add(&msp->ms_allocatable_by_size, rs);
-}
-
-static void
-metaslab_rt_remove(range_tree_t *rt, range_seg_t *rs, void *arg)
-{
-	metaslab_t *msp = arg;
-
-	ASSERT3P(rt->rt_arg, ==, msp);
-	ASSERT3P(msp->ms_allocatable, ==, rt);
-	VERIFY(!msp->ms_condensing);
-	avl_remove(&msp->ms_allocatable_by_size, rs);
-}
-
-static void
-metaslab_rt_vacate(range_tree_t *rt, void *arg)
-{
-	metaslab_t *msp = arg;
-
-	ASSERT3P(rt->rt_arg, ==, msp);
-	ASSERT3P(msp->ms_allocatable, ==, rt);
-
-	/*
-	 * Normally one would walk the tree freeing nodes along the way.
-	 * Since the nodes are shared with the range trees we can avoid
-	 * walking all nodes and just reinitialize the avl tree. The nodes
-	 * will be freed by the range tree, so we don't want to free them here.
-	 */
-	avl_create(&msp->ms_allocatable_by_size, metaslab_rangesize_compare,
-	    sizeof (range_seg_t), offsetof(range_seg_t, rs_pp_node));
-}
-
-static range_tree_ops_t metaslab_rt_ops = {
-	metaslab_rt_create,
-	metaslab_rt_destroy,
-	metaslab_rt_add,
-	metaslab_rt_remove,
-	metaslab_rt_vacate
-};
-
-/*
  * ==========================================================================
  * Common allocator routines
  * ==========================================================================
@@ -1891,7 +1812,8 @@ metaslab_init(metaslab_group_t *mg, uint64_t id, uint64_t object, uint64_t txg,
 	 * we'd data fault on any attempt to use this metaslab before
 	 * it's ready.
 	 */
-	ms->ms_allocatable = range_tree_create(&metaslab_rt_ops, ms);
+	ms->ms_allocatable = range_tree_create_impl(&rt_avl_ops,
+	    &ms->ms_allocatable_by_size, metaslab_rangesize_compare, 0);
 	metaslab_group_add(mg, ms);
 
 	metaslab_set_fragmentation(ms);
diff --git a/usr/src/uts/common/fs/zfs/range_tree.c b/usr/src/uts/common/fs/zfs/range_tree.c
index 0a852a9c8d..fc705e3796 100644
--- a/usr/src/uts/common/fs/zfs/range_tree.c
+++ b/usr/src/uts/common/fs/zfs/range_tree.c
@@ -33,8 +33,58 @@
 #include <sys/zio.h>
 #include <sys/range_tree.h>
 
+/*
+ * Range trees are tree-based data structures that can be used to
+ * track free space or generally any space allocation information.
+ * A range tree keeps track of individual segments and automatically
+ * provides facilities such as adjacent extent merging and extent
+ * splitting in response to range add/remove requests.
+ *
+ * A range tree starts out completely empty, with no segments in it.
+ * Adding an allocation via range_tree_add to the range tree can either:
+ * 1) create a new extent
+ * 2) extend an adjacent extent
+ * 3) merge two adjacent extents
+ * Conversely, removing an allocation via range_tree_remove can:
+ * 1) completely remove an extent
+ * 2) shorten an extent (if the allocation was near one of its ends)
+ * 3) split an extent into two extents, in effect punching a hole
+ *
+ * A range tree is also capable of 'bridging' gaps when adding
+ * allocations. This is useful for cases when close proximity of
+ * allocations is an important detail that needs to be represented
+ * in the range tree. See range_tree_set_gap(). The default behavior
+ * is not to bridge gaps (i.e. the maximum allowed gap size is 0).
+ *
+ * In order to traverse a range tree, use either the range_tree_walk()
+ * or range_tree_vacate() functions.
+ *
+ * To obtain more accurate information on individual segment
+ * operations that the range tree performs "under the hood", you can
+ * specify a set of callbacks by passing a range_tree_ops_t structure
+ * to the range_tree_create function. Any callbacks that are non-NULL
+ * are then called at the appropriate times.
+ *
+ * The range tree code also supports a special variant of range trees
+ * that can bridge small gaps between segments. This kind of tree is used
+ * by the dsl scanning code to group I/Os into mostly sequential chunks to
+ * optimize disk performance. The code here attempts to do this with as
+ * little memory and computational overhead as possible. One limitation of
+ * this implementation is that segments of range trees with gaps can only
+ * support removing complete segments.
+ */
+
 kmem_cache_t *range_seg_cache;
 
+/* Generic ops for managing an AVL tree alongside a range tree */
+struct range_tree_ops rt_avl_ops = {
+	.rtop_create = rt_avl_create,
+	.rtop_destroy = rt_avl_destroy,
+	.rtop_add = rt_avl_add,
+	.rtop_remove = rt_avl_remove,
+	.rtop_vacate = rt_avl_vacate,
+};
+
 void
 range_tree_init(void)
 {
@@ -119,30 +169,37 @@ range_tree_seg_compare(const void *x1, const void *x2)
 }
 
 range_tree_t *
-range_tree_create(range_tree_ops_t *ops, void *arg)
+range_tree_create_impl(range_tree_ops_t *ops, void *arg,
+    int (*avl_compare) (const void *, const void *), uint64_t gap)
 {
-	range_tree_t *rt;
-
-	rt = kmem_zalloc(sizeof (range_tree_t), KM_SLEEP);
+	range_tree_t *rt = kmem_zalloc(sizeof (range_tree_t), KM_SLEEP);
 
 	avl_create(&rt->rt_root, range_tree_seg_compare,
 	    sizeof (range_seg_t), offsetof(range_seg_t, rs_node));
 
 	rt->rt_ops = ops;
 	rt->rt_arg = arg;
+	rt->rt_gap = gap;
+	rt->rt_avl_compare = avl_compare;
 
-	if (rt->rt_ops != NULL)
+	if (rt->rt_ops != NULL && rt->rt_ops->rtop_create != NULL)
 		rt->rt_ops->rtop_create(rt, rt->rt_arg);
 
 	return (rt);
 }
 
+range_tree_t *
+range_tree_create(range_tree_ops_t *ops, void *arg)
+{
+	return (range_tree_create_impl(ops, arg, NULL, 0));
+}
+
 void
 range_tree_destroy(range_tree_t *rt)
 {
 	VERIFY0(rt->rt_space);
 
-	if (rt->rt_ops != NULL)
+	if (rt->rt_ops != NULL && rt->rt_ops->rtop_destroy != NULL)
 		rt->rt_ops->rtop_destroy(rt, rt->rt_arg);
 
 	avl_destroy(&rt->rt_root);
@@ -150,39 +207,99 @@ range_tree_destroy(range_tree_t *rt)
 }
 
 void
-range_tree_add(void *arg, uint64_t start, uint64_t size)
+range_tree_adjust_fill(range_tree_t *rt, range_seg_t *rs, int64_t delta)
+{
+	ASSERT3U(rs->rs_fill + delta, !=, 0);
+	ASSERT3U(rs->rs_fill + delta, <=, rs->rs_end - rs->rs_start);
+
+	if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL)
+		rt->rt_ops->rtop_remove(rt, rs, rt->rt_arg);
+	rs->rs_fill += delta;
+	if (rt->rt_ops != NULL && rt->rt_ops->rtop_add != NULL)
+		rt->rt_ops->rtop_add(rt, rs, rt->rt_arg);
+}
+
+static void
+range_tree_add_impl(void *arg, uint64_t start, uint64_t size, uint64_t fill)
 {
 	range_tree_t *rt = arg;
 	avl_index_t where;
 	range_seg_t rsearch, *rs_before, *rs_after, *rs;
-	uint64_t end = start + size;
+	uint64_t end = start + size, gap = rt->rt_gap;
+	uint64_t bridge_size = 0;
 	boolean_t merge_before, merge_after;
 
-	VERIFY(size != 0);
+	ASSERT3U(size, !=, 0);
+	ASSERT3U(fill, <=, size);
 
 	rsearch.rs_start = start;
 	rsearch.rs_end = end;
 	rs = avl_find(&rt->rt_root, &rsearch, &where);
 
-	if (rs != NULL && rs->rs_start <= start && rs->rs_end >= end) {
+	if (gap == 0 && rs != NULL &&
+	    rs->rs_start <= start && rs->rs_end >= end) {
 		zfs_panic_recover("zfs: allocating allocated segment"
-		    "(offset=%llu size=%llu)\n",
-		    (longlong_t)start, (longlong_t)size);
+		    "(offset=%llu size=%llu) of (offset=%llu size=%llu)\n",
+		    (longlong_t)start, (longlong_t)size,
+		    (longlong_t)rs->rs_start,
+		    (longlong_t)rs->rs_end - rs->rs_start);
 		return;
 	}
 
-	/* Make sure we don't overlap with either of our neighbors */
-	VERIFY3P(rs, ==, NULL);
+	/*
+	 * If this is a gap-supporting range tree, it is possible that we
+	 * are inserting into an existing segment. In this case simply
+	 * bump the fill count and call the remove / add callbacks. If the
+	 * new range will extend an existing segment, we remove the
+	 * existing one, apply the new extent to it and re-insert it using
+	 * the normal code paths.
+	 */
+	if (rs != NULL) {
+		ASSERT3U(gap, !=, 0);
+		if (rs->rs_start <= start && rs->rs_end >= end) {
+			range_tree_adjust_fill(rt, rs, fill);
+			return;
+		}
+
+		avl_remove(&rt->rt_root, rs);
+		if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL)
+			rt->rt_ops->rtop_remove(rt, rs, rt->rt_arg);
+
+		range_tree_stat_decr(rt, rs);
+		rt->rt_space -= rs->rs_end - rs->rs_start;
+
+		fill += rs->rs_fill;
+		start = MIN(start, rs->rs_start);
+		end = MAX(end, rs->rs_end);
+		size = end - start;
+
+		range_tree_add_impl(rt, start, size, fill);
+
+		kmem_cache_free(range_seg_cache, rs);
+		return;
+	}
 
+	ASSERT3P(rs, ==, NULL);
+
+	/*
+	 * Determine whether or not we will have to merge with our neighbors.
+	 * If gap != 0, we might need to merge with our neighbors even if we
+	 * aren't directly touching.
+	 */
 	rs_before = avl_nearest(&rt->rt_root, where, AVL_BEFORE);
 	rs_after = avl_nearest(&rt->rt_root, where, AVL_AFTER);
 
-	merge_before = (rs_before != NULL && rs_before->rs_end == start);
-	merge_after = (rs_after != NULL && rs_after->rs_start == end);
+	merge_before = (rs_before != NULL && rs_before->rs_end >= start - gap);
+	merge_after = (rs_after != NULL && rs_after->rs_start <= end + gap);
+
+	if (merge_before && gap != 0)
+		bridge_size += start - rs_before->rs_end;
+	if (merge_after && gap != 0)
+		bridge_size += rs_after->rs_start - end;
 
 	if (merge_before && merge_after) {
 		avl_remove(&rt->rt_root, rs_before);
-		if (rt->rt_ops != NULL) {
+		if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL) {
 			rt->rt_ops->rtop_remove(rt, rs_before, rt->rt_arg);
 			rt->rt_ops->rtop_remove(rt, rs_after, rt->rt_arg);
 		}
@@ -190,43 +307,59 @@ range_tree_add(void *arg, uint64_t start, uint64_t size)
 		range_tree_stat_decr(rt, rs_before);
 		range_tree_stat_decr(rt, rs_after);
 
+		rs_after->rs_fill += rs_before->rs_fill + fill;
 		rs_after->rs_start = rs_before->rs_start;
 		kmem_cache_free(range_seg_cache, rs_before);
 		rs = rs_after;
 	} else if (merge_before) {
-		if (rt->rt_ops != NULL)
+		if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL)
 			rt->rt_ops->rtop_remove(rt, rs_before, rt->rt_arg);
 
 		range_tree_stat_decr(rt, rs_before);
 
+		rs_before->rs_fill += fill;
 		rs_before->rs_end = end;
 		rs = rs_before;
 	} else if (merge_after) {
-		if (rt->rt_ops != NULL)
+		if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL)
 			rt->rt_ops->rtop_remove(rt, rs_after, rt->rt_arg);
 
 		range_tree_stat_decr(rt, rs_after);
 
+		rs_after->rs_fill += fill;
 		rs_after->rs_start = start;
 		rs = rs_after;
 	} else {
 		rs = kmem_cache_alloc(range_seg_cache, KM_SLEEP);
+
+		rs->rs_fill = fill;
 		rs->rs_start = start;
 		rs->rs_end = end;
 		avl_insert(&rt->rt_root, rs, where);
 	}
 
-	if (rt->rt_ops != NULL)
+	if (gap != 0)
+		ASSERT3U(rs->rs_fill, <=, rs->rs_end - rs->rs_start);
+	else
+		ASSERT3U(rs->rs_fill, ==, rs->rs_end - rs->rs_start);
+
+	if (rt->rt_ops != NULL && rt->rt_ops->rtop_add != NULL)
 		rt->rt_ops->rtop_add(rt, rs, rt->rt_arg);
 
 	range_tree_stat_incr(rt, rs);
-	rt->rt_space += size;
+	rt->rt_space += size + bridge_size;
 }
 
 void
-range_tree_remove(void *arg, uint64_t start, uint64_t size)
+range_tree_add(void *arg, uint64_t start, uint64_t size)
+{
+	range_tree_add_impl(arg, start, size, size);
+}
+
+static void
+range_tree_remove_impl(range_tree_t *rt, uint64_t start, uint64_t size,
+    boolean_t do_fill)
 {
-	range_tree_t *rt = arg;
 	avl_index_t where;
 	range_seg_t rsearch, *rs, *newseg;
 	uint64_t end = start + size;
@@ -246,6 +379,34 @@ range_tree_remove(void *arg, uint64_t start, uint64_t size)
 		    (longlong_t)start, (longlong_t)size);
 		return;
 	}
+
+	/*
+	 * Range trees with gap support must only remove complete segments
+	 * from the tree. This allows us to maintain accurate fill accounting
+	 * and to ensure that bridged sections are not leaked. If we need to
+	 * remove less than the full segment, we can only adjust the fill count.
+	 */
+	if (rt->rt_gap != 0) {
+		if (do_fill) {
+			if (rs->rs_fill == size) {
+				start = rs->rs_start;
+				end = rs->rs_end;
+				size = end - start;
+			} else {
+				range_tree_adjust_fill(rt, rs, -size);
+				return;
+			}
+		} else if (rs->rs_start != start || rs->rs_end != end) {
+			zfs_panic_recover("zfs: freeing partial segment of "
+			    "gap tree (offset=%llu size=%llu) of "
+			    "(offset=%llu size=%llu)",
+			    (longlong_t)start, (longlong_t)size,
+			    (longlong_t)rs->rs_start,
+			    (longlong_t)rs->rs_end - rs->rs_start);
+			return;
+		}
+	}
+
 	VERIFY3U(rs->rs_start, <=, start);
 	VERIFY3U(rs->rs_end, >=, end);
 
@@ -254,19 +415,20 @@ range_tree_remove(void *arg, uint64_t start, uint64_t size)
 
 	range_tree_stat_decr(rt, rs);
 
-	if (rt->rt_ops != NULL)
+	if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL)
 		rt->rt_ops->rtop_remove(rt, rs, rt->rt_arg);
 
 	if (left_over && right_over) {
 		newseg = kmem_cache_alloc(range_seg_cache, KM_SLEEP);
 		newseg->rs_start = end;
 		newseg->rs_end = rs->rs_end;
+		newseg->rs_fill = newseg->rs_end - newseg->rs_start;
 		range_tree_stat_incr(rt, newseg);
 
 		rs->rs_end = start;
 
 		avl_insert_here(&rt->rt_root, newseg, rs, AVL_AFTER);
-		if (rt->rt_ops != NULL)
+		if (rt->rt_ops != NULL && rt->rt_ops->rtop_add != NULL)
 			rt->rt_ops->rtop_add(rt, newseg, rt->rt_arg);
 	} else if (left_over) {
 		rs->rs_end = start;
@@ -279,15 +441,53 @@ range_tree_remove(void *arg, uint64_t start, uint64_t size)
 	}
 
 	if (rs != NULL) {
+		/*
+		 * The fill of the leftover segment will always be equal to
+		 * the size, since we do not support removing partial segments
+		 * of range trees with gaps.
+		 */
+		rs->rs_fill = rs->rs_end - rs->rs_start;
 		range_tree_stat_incr(rt, rs);
 
-		if (rt->rt_ops != NULL)
+		if (rt->rt_ops != NULL && rt->rt_ops->rtop_add != NULL)
 			rt->rt_ops->rtop_add(rt, rs, rt->rt_arg);
 	}
 
 	rt->rt_space -= size;
 }
 
+void
+range_tree_remove(void *arg, uint64_t start, uint64_t size)
+{
+	range_tree_remove_impl(arg, start, size, B_FALSE);
+}
+
+void
+range_tree_remove_fill(range_tree_t *rt, uint64_t start, uint64_t size)
+{
+	range_tree_remove_impl(rt, start, size, B_TRUE);
+}
+
+void
+range_tree_resize_segment(range_tree_t *rt, range_seg_t *rs,
+    uint64_t newstart, uint64_t newsize)
+{
+	int64_t delta = newsize - (rs->rs_end - rs->rs_start);
+
+	range_tree_stat_decr(rt, rs);
+	if (rt->rt_ops != NULL && rt->rt_ops->rtop_remove != NULL)
+		rt->rt_ops->rtop_remove(rt, rs, rt->rt_arg);
+
+	rs->rs_start = newstart;
+	rs->rs_end = newstart + newsize;
+
+	range_tree_stat_incr(rt, rs);
+	if (rt->rt_ops != NULL && rt->rt_ops->rtop_add != NULL)
+		rt->rt_ops->rtop_add(rt, rs, rt->rt_arg);
+
+	rt->rt_space += delta;
+}
+
 static range_seg_t *
 range_tree_find_impl(range_tree_t *rt, uint64_t start, uint64_t size)
 {
@@ -301,7 +501,7 @@ range_tree_find_impl(range_tree_t *rt, uint64_t start, uint64_t size)
 	return (avl_find(&rt->rt_root, &rsearch, NULL));
 }
 
-static range_seg_t *
+range_seg_t *
 range_tree_find(range_tree_t *rt, uint64_t start, uint64_t size)
 {
 	range_seg_t *rs = range_tree_find_impl(rt, start, size);
@@ -363,7 +563,7 @@ range_tree_vacate(range_tree_t *rt, range_tree_func_t *func, void *arg)
 	void *cookie = NULL;
 
 
-	if (rt->rt_ops != NULL)
+	if (rt->rt_ops != NULL && rt->rt_ops->rtop_vacate != NULL)
 		rt->rt_ops->rtop_vacate(rt, rt->rt_arg);
 
 	while ((rs = avl_destroy_nodes(&rt->rt_root, &cookie)) != NULL) {
@@ -385,12 +585,63 @@ range_tree_walk(range_tree_t *rt, range_tree_func_t *func, void *arg)
 		func(arg, rs->rs_start, rs->rs_end - rs->rs_start);
 }
 
+range_seg_t *
+range_tree_first(range_tree_t *rt)
+{
+	return (avl_first(&rt->rt_root));
+}
+
 uint64_t
 range_tree_space(range_tree_t *rt)
 {
 	return (rt->rt_space);
 }
 
+/* Generic range tree functions for maintaining segments in an AVL tree. */
+void
+rt_avl_create(range_tree_t *rt, void *arg)
+{
+	avl_tree_t *tree = arg;
+
+	avl_create(tree, rt->rt_avl_compare, sizeof (range_seg_t),
+	    offsetof(range_seg_t, rs_pp_node));
+}
+
+void
+rt_avl_destroy(range_tree_t *rt, void *arg)
+{
+	avl_tree_t *tree = arg;
+
+	ASSERT0(avl_numnodes(tree));
+	avl_destroy(tree);
+}
+
+void
+rt_avl_add(range_tree_t *rt, range_seg_t *rs, void *arg)
+{
+	avl_tree_t *tree = arg;
+	avl_add(tree, rs);
+}
+
+void
+rt_avl_remove(range_tree_t *rt, range_seg_t *rs, void *arg)
+{
+	avl_tree_t *tree = arg;
+	avl_remove(tree, rs);
+}
+
+void
+rt_avl_vacate(range_tree_t *rt, void *arg)
+{
+	/*
+	 * Normally one would walk the tree freeing nodes along the way.
+	 * Since the nodes are shared with the range trees we can avoid
+	 * walking all nodes and just reinitialize the avl tree. The nodes
+	 * will be freed by the range tree, so we don't want to free them here.
+	 */
+	rt_avl_create(rt, arg);
+}
+
 boolean_t
 range_tree_is_empty(range_tree_t *rt)
 {
diff --git a/usr/src/uts/common/fs/zfs/spa.c b/usr/src/uts/common/fs/zfs/spa.c
index 403ace2d9d..68abdf114c 100644
--- a/usr/src/uts/common/fs/zfs/spa.c
+++ b/usr/src/uts/common/fs/zfs/spa.c
@@ -1999,7 +1999,7 @@ spa_load_verify_done(zio_t *zio)
 	}
 
 	mutex_enter(&spa->spa_scrub_lock);
-	spa->spa_scrub_inflight--;
+	spa->spa_load_verify_ios--;
 	cv_broadcast(&spa->spa_scrub_io_cv);
 	mutex_exit(&spa->spa_scrub_lock);
 }
@@ -2033,9 +2033,9 @@ spa_load_verify_cb(spa_t *spa, zilog_t *zilog, const blkptr_t *bp,
 	size_t size = BP_GET_PSIZE(bp);
 
 	mutex_enter(&spa->spa_scrub_lock);
-	while (spa->spa_scrub_inflight >= spa_load_verify_maxinflight)
+	while (spa->spa_load_verify_ios >= spa_load_verify_maxinflight)
 		cv_wait(&spa->spa_scrub_io_cv, &spa->spa_scrub_lock);
-	spa->spa_scrub_inflight++;
+	spa->spa_load_verify_ios++;
 	mutex_exit(&spa->spa_scrub_lock);
 
 	zio_nowait(zio_read(rio, spa, bp, abd_alloc_for_io(size, B_FALSE), size,
diff --git a/usr/src/uts/common/fs/zfs/spa_misc.c b/usr/src/uts/common/fs/zfs/spa_misc.c
index 9a80f89a8a..7a44ac86b0 100644
--- a/usr/src/uts/common/fs/zfs/spa_misc.c
+++ b/usr/src/uts/common/fs/zfs/spa_misc.c
@@ -2041,6 +2041,7 @@ spa_init(int mode)
 	zpool_feature_init();
 	spa_config_load();
 	l2arc_start();
+	scan_init();
 }
 
 void
@@ -2058,6 +2059,7 @@ spa_fini(void)
 	range_tree_fini();
 	unique_fini();
 	zfs_refcount_fini();
+	scan_fini();
 
 	avl_destroy(&spa_namespace_avl);
 	avl_destroy(&spa_spare_avl);
@@ -2159,6 +2161,7 @@ spa_scan_stat_init(spa_t *spa)
 		spa->spa_scan_pass_scrub_pause = 0;
 	spa->spa_scan_pass_scrub_spent_paused = 0;
 	spa->spa_scan_pass_exam = 0;
+	spa->spa_scan_pass_issued = 0;
 	vdev_scan_stat_init(spa->spa_root_vdev);
 }
 
@@ -2176,18 +2179,22 @@ spa_scan_get_stats(spa_t *spa, pool_scan_stat_t *ps)
 
 	/* data stored on disk */
 	ps->pss_func = scn->scn_phys.scn_func;
+	ps->pss_state = scn->scn_phys.scn_state;
 	ps->pss_start_time = scn->scn_phys.scn_start_time;
 	ps->pss_end_time = scn->scn_phys.scn_end_time;
 	ps->pss_to_examine = scn->scn_phys.scn_to_examine;
-	ps->pss_examined = scn->scn_phys.scn_examined;
 	ps->pss_to_process = scn->scn_phys.scn_to_process;
 	ps->pss_processed = scn->scn_phys.scn_processed;
 	ps->pss_errors = scn->scn_phys.scn_errors;
+	ps->pss_examined = scn->scn_phys.scn_examined;
+	ps->pss_issued =
+	    scn->scn_issued_before_pass + spa->spa_scan_pass_issued;
 	ps->pss_state = scn->scn_phys.scn_state;
 
 	/* data not stored on disk */
 	ps->pss_pass_start = spa->spa_scan_pass_start;
 	ps->pss_pass_exam = spa->spa_scan_pass_exam;
+	ps->pss_pass_issued = spa->spa_scan_pass_issued;
 	ps->pss_pass_scrub_pause = spa->spa_scan_pass_scrub_pause;
 	ps->pss_pass_scrub_spent_paused = spa->spa_scan_pass_scrub_spent_paused;
 
diff --git a/usr/src/uts/common/fs/zfs/sys/arc.h b/usr/src/uts/common/fs/zfs/sys/arc.h
index 87d2d1d329..1ce4740fcf 100644
--- a/usr/src/uts/common/fs/zfs/sys/arc.h
+++ b/usr/src/uts/common/fs/zfs/sys/arc.h
@@ -58,11 +58,13 @@ _NOTE(CONSTCOND) } while (0)
 
 typedef struct arc_buf_hdr arc_buf_hdr_t;
 typedef struct arc_buf arc_buf_t;
-typedef void arc_done_func_t(zio_t *zio, arc_buf_t *buf, void *private);
+typedef void arc_read_done_func_t(zio_t *zio, const zbookmark_phys_t *zb,
+    const blkptr_t *bp, arc_buf_t *buf, void *private);
+typedef void arc_write_done_func_t(zio_t *zio, arc_buf_t *buf, void *priv);
 
 /* generic arc_done_func_t's which you can use */
-arc_done_func_t arc_bcopy_func;
-arc_done_func_t arc_getbuf_func;
+arc_read_done_func_t arc_bcopy_func;
+arc_read_done_func_t arc_getbuf_func;
 
 typedef enum arc_flags
 {
@@ -75,35 +77,36 @@ typedef enum arc_flags
 	ARC_FLAG_CACHED			= 1 << 3,	/* I/O was in cache */
 	ARC_FLAG_L2CACHE		= 1 << 4,	/* cache in L2ARC */
 	ARC_FLAG_PREDICTIVE_PREFETCH	= 1 << 5,	/* I/O from zfetch */
+	ARC_FLAG_PRESCIENT_PREFETCH	= 1 << 6,	/* long min lifespan */
 
 	/*
 	 * Private ARC flags.  These flags are private ARC only flags that
 	 * will show up in b_flags in the arc_hdr_buf_t. These flags should
 	 * only be set by ARC code.
 	 */
-	ARC_FLAG_IN_HASH_TABLE		= 1 << 6,	/* buffer is hashed */
-	ARC_FLAG_IO_IN_PROGRESS		= 1 << 7,	/* I/O in progress */
-	ARC_FLAG_IO_ERROR		= 1 << 8,	/* I/O failed for buf */
-	ARC_FLAG_INDIRECT		= 1 << 9,	/* indirect block */
+	ARC_FLAG_IN_HASH_TABLE		= 1 << 7,	/* buffer is hashed */
+	ARC_FLAG_IO_IN_PROGRESS		= 1 << 8,	/* I/O in progress */
+	ARC_FLAG_IO_ERROR		= 1 << 9,	/* I/O failed for buf */
+	ARC_FLAG_INDIRECT		= 1 << 10,	/* indirect block */
 	/* Indicates that block was read with ASYNC priority. */
-	ARC_FLAG_PRIO_ASYNC_READ	= 1 << 10,
-	ARC_FLAG_L2_WRITING		= 1 << 11,	/* write in progress */
-	ARC_FLAG_L2_EVICTED		= 1 << 12,	/* evicted during I/O */
-	ARC_FLAG_L2_WRITE_HEAD		= 1 << 13,	/* head of write list */
+	ARC_FLAG_PRIO_ASYNC_READ	= 1 << 11,
+	ARC_FLAG_L2_WRITING		= 1 << 12,	/* write in progress */
+	ARC_FLAG_L2_EVICTED		= 1 << 13,	/* evicted during I/O */
+	ARC_FLAG_L2_WRITE_HEAD		= 1 << 14,	/* head of write list */
 	/* indicates that the buffer contains metadata (otherwise, data) */
-	ARC_FLAG_BUFC_METADATA		= 1 << 14,
+	ARC_FLAG_BUFC_METADATA		= 1 << 15,
 
 	/* Flags specifying whether optional hdr struct fields are defined */
-	ARC_FLAG_HAS_L1HDR		= 1 << 15,
-	ARC_FLAG_HAS_L2HDR		= 1 << 16,
+	ARC_FLAG_HAS_L1HDR		= 1 << 16,
+	ARC_FLAG_HAS_L2HDR		= 1 << 17,
 
 	/*
 	 * Indicates the arc_buf_hdr_t's b_pdata matches the on-disk data.
 	 * This allows the l2arc to use the blkptr's checksum to verify
 	 * the data without having to store the checksum in the hdr.
 	 */
-	ARC_FLAG_COMPRESSED_ARC		= 1 << 17,
-	ARC_FLAG_SHARED_DATA		= 1 << 18,
+	ARC_FLAG_COMPRESSED_ARC		= 1 << 18,
+	ARC_FLAG_SHARED_DATA		= 1 << 19,
 
 	/*
 	 * The arc buffer's compression mode is stored in the top 7 bits of the
@@ -180,12 +183,12 @@ int arc_referenced(arc_buf_t *buf);
 #endif
 
 int arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
-    arc_done_func_t *done, void *private, zio_priority_t priority, int flags,
-    arc_flags_t *arc_flags, const zbookmark_phys_t *zb);
+    arc_read_done_func_t *done, void *private, zio_priority_t priority,
+    int flags, arc_flags_t *arc_flags, const zbookmark_phys_t *zb);
 zio_t *arc_write(zio_t *pio, spa_t *spa, uint64_t txg,
     blkptr_t *bp, arc_buf_t *buf, boolean_t l2arc, const zio_prop_t *zp,
-    arc_done_func_t *ready, arc_done_func_t *child_ready,
-    arc_done_func_t *physdone, arc_done_func_t *done,
+    arc_write_done_func_t *ready, arc_write_done_func_t *child_ready,
+    arc_write_done_func_t *physdone, arc_write_done_func_t *done,
     void *private, zio_priority_t priority, int zio_flags,
     const zbookmark_phys_t *zb);
 void arc_freed(spa_t *spa, const blkptr_t *bp);
diff --git a/usr/src/uts/common/fs/zfs/sys/dsl_pool.h b/usr/src/uts/common/fs/zfs/sys/dsl_pool.h
index b23d19eef5..66098900db 100644
--- a/usr/src/uts/common/fs/zfs/sys/dsl_pool.h
+++ b/usr/src/uts/common/fs/zfs/sys/dsl_pool.h
@@ -77,6 +77,7 @@ typedef struct zfs_blkstat {
 
 typedef struct zfs_all_blkstats {
 	zfs_blkstat_t	zab_type[DN_MAX_LEVELS + 1][DMU_OT_TOTAL + 1];
+	kmutex_t	zab_lock;
 } zfs_all_blkstats_t;
 
 
diff --git a/usr/src/uts/common/fs/zfs/sys/dsl_scan.h b/usr/src/uts/common/fs/zfs/sys/dsl_scan.h
index a738ebe3ee..1b600405ae 100644
--- a/usr/src/uts/common/fs/zfs/sys/dsl_scan.h
+++ b/usr/src/uts/common/fs/zfs/sys/dsl_scan.h
@@ -74,6 +74,8 @@ typedef enum dsl_scan_flags {
 	DSF_SCRUB_PAUSED = 1<<1,
 } dsl_scan_flags_t;
 
+#define	DSL_SCAN_FLAGS_MASK (DSF_VISIT_DS_AGAIN)
+
 /*
  * Every pool will have one dsl_scan_t and this structure will contain
  * in-memory information about the scan and a pointer to the on-disk
@@ -106,12 +108,10 @@ typedef enum dsl_scan_flags {
  */
 typedef struct dsl_scan {
 	struct dsl_pool *scn_dp;
-
-	boolean_t scn_suspending;
 	uint64_t scn_restart_txg;
 	uint64_t scn_done_txg;
 	uint64_t scn_sync_start_time;
-	zio_t *scn_zio_root;
+	uint64_t scn_issued_before_pass;
 
 	/* for freeing blocks */
 	boolean_t scn_is_bptree;
@@ -119,12 +119,46 @@ typedef struct dsl_scan {
 	boolean_t scn_async_stalled;
 	uint64_t  scn_async_block_min_time_ms;
 
-	/* for debugging / information */
-	uint64_t scn_visited_this_txg;
+	/* flags and stats for controlling scan state */
+	boolean_t scn_is_sorted;	/* doing sequential scan */
+	boolean_t scn_clearing;		/* scan is issuing sequential extents */
+	boolean_t scn_checkpointing;	/* scan is issuing all queued extents */
+	boolean_t scn_suspending;	/* scan is suspending until next txg */
+	uint64_t scn_last_checkpoint;	/* time of last checkpoint */
+
+	/* members for thread synchronization */
+	zio_t *scn_zio_root;		/* root zio for waiting on IO */
+	taskq_t *scn_taskq;		/* task queue for issuing extents */
 
-	dsl_scan_phys_t scn_phys;
+	/* for controlling scan prefetch, protected by spa_scrub_lock */
+	boolean_t scn_prefetch_stop;	/* prefetch should stop */
+	zbookmark_phys_t scn_prefetch_bookmark;	/* prefetch start bookmark */
+	avl_tree_t scn_prefetch_queue;	/* priority queue of prefetch IOs */
+	uint64_t scn_maxinflight_bytes;	/* max bytes in flight for pool */
+
+	/* per txg statistics */
+	uint64_t scn_visited_this_txg;	/* total bps visited this txg */
+	uint64_t scn_holes_this_txg;
+	uint64_t scn_lt_min_this_txg;
+	uint64_t scn_gt_max_this_txg;
+	uint64_t scn_ddt_contained_this_txg;
+	uint64_t scn_objsets_visited_this_txg;
+	uint64_t scn_avg_seg_size_this_txg;
+	uint64_t scn_segs_this_txg;
+	uint64_t scn_avg_zio_size_this_txg;
+	uint64_t scn_zios_this_txg;
+
+	/* members needed for syncing scan status to disk */
+	dsl_scan_phys_t scn_phys;	/* on disk representation of scan */
+	dsl_scan_phys_t scn_phys_cached;
+	avl_tree_t scn_queue;		/* queue of datasets to scan */
+	uint64_t scn_bytes_pending;	/* outstanding data to issue */
 } dsl_scan_t;
 
+typedef struct dsl_scan_io_queue dsl_scan_io_queue_t;
+
+void scan_init(void);
+void scan_fini(void);
 int dsl_scan_init(struct dsl_pool *dp, uint64_t txg);
 void dsl_scan_fini(struct dsl_pool *dp);
 void dsl_scan_sync(struct dsl_pool *, dmu_tx_t *);
@@ -143,6 +177,9 @@ void dsl_scan_ds_clone_swapped(struct dsl_dataset *ds1, struct dsl_dataset *ds2,
     struct dmu_tx *tx);
 boolean_t dsl_scan_active(dsl_scan_t *scn);
 boolean_t dsl_scan_is_paused_scrub(const dsl_scan_t *scn);
+void dsl_scan_freed(spa_t *spa, const blkptr_t *bp);
+void dsl_scan_io_queue_destroy(dsl_scan_io_queue_t *queue);
+void dsl_scan_io_queue_vdev_xfer(vdev_t *svd, vdev_t *tvd);
 
 #ifdef	__cplusplus
 }
diff --git a/usr/src/uts/common/fs/zfs/sys/range_tree.h b/usr/src/uts/common/fs/zfs/sys/range_tree.h
index 3816dabf7c..588f41fcb7 100644
--- a/usr/src/uts/common/fs/zfs/sys/range_tree.h
+++ b/usr/src/uts/common/fs/zfs/sys/range_tree.h
@@ -48,9 +48,13 @@ typedef struct range_tree_ops range_tree_ops_t;
 typedef struct range_tree {
 	avl_tree_t	rt_root;	/* offset-ordered segment AVL tree */
 	uint64_t	rt_space;	/* sum of all segments in the map */
+	uint64_t	rt_gap;		/* allowable inter-segment gap */
 	range_tree_ops_t *rt_ops;
 	void		*rt_arg;
 
+	/* rt_avl_compare should only be set it rt_arg is an AVL tree */
+	int (*rt_avl_compare)(const void *, const void *);
+
 	/*
 	 * The rt_histogram maintains a histogram of ranges. Each bucket,
 	 * rt_histogram[i], contains the number of ranges whose size is:
@@ -64,6 +68,7 @@ typedef struct range_seg {
 	avl_node_t	rs_pp_node;	/* AVL picker-private node */
 	uint64_t	rs_start;	/* starting offset of this segment */
 	uint64_t	rs_end;		/* ending offset (non-inclusive) */
+	uint64_t	rs_fill;	/* actual fill if gap mode is on */
 } range_seg_t;
 
 struct range_tree_ops {
@@ -78,11 +83,16 @@ typedef void range_tree_func_t(void *arg, uint64_t start, uint64_t size);
 
 void range_tree_init(void);
 void range_tree_fini(void);
+range_tree_t *range_tree_create_impl(range_tree_ops_t *ops, void *arg,
+    int (*avl_compare)(const void*, const void*), uint64_t gap);
 range_tree_t *range_tree_create(range_tree_ops_t *ops, void *arg);
 void range_tree_destroy(range_tree_t *rt);
 boolean_t range_tree_contains(range_tree_t *rt, uint64_t start, uint64_t size);
 void range_tree_verify_not_present(range_tree_t *rt,
     uint64_t start, uint64_t size);
+range_seg_t *range_tree_find(range_tree_t *rt, uint64_t start, uint64_t size);
+void range_tree_resize_segment(range_tree_t *rt, range_seg_t *rs,
+    uint64_t newstart, uint64_t newsize);
 uint64_t range_tree_space(range_tree_t *rt);
 boolean_t range_tree_is_empty(range_tree_t *rt);
 void range_tree_swap(range_tree_t **rtsrc, range_tree_t **rtdst);
@@ -93,10 +103,27 @@ uint64_t range_tree_span(range_tree_t *rt);
 
 void range_tree_add(void *arg, uint64_t start, uint64_t size);
 void range_tree_remove(void *arg, uint64_t start, uint64_t size);
+void range_tree_remove_fill(range_tree_t *rt, uint64_t start, uint64_t size);
+void range_tree_adjust_fill(range_tree_t *rt, range_seg_t *rs, int64_t delta);
 void range_tree_clear(range_tree_t *rt, uint64_t start, uint64_t size);
 
 void range_tree_vacate(range_tree_t *rt, range_tree_func_t *func, void *arg);
 void range_tree_walk(range_tree_t *rt, range_tree_func_t *func, void *arg);
+range_seg_t *range_tree_first(range_tree_t *rt);
+
+void rt_avl_create(range_tree_t *rt, void *arg);
+void rt_avl_destroy(range_tree_t *rt, void *arg);
+void rt_avl_add(range_tree_t *rt, range_seg_t *rs, void *arg);
+void rt_avl_remove(range_tree_t *rt, range_seg_t *rs, void *arg);
+void rt_avl_vacate(range_tree_t *rt, void *arg);
+extern struct range_tree_ops rt_avl_ops;
+
+void rt_avl_create(range_tree_t *rt, void *arg);
+void rt_avl_destroy(range_tree_t *rt, void *arg);
+void rt_avl_add(range_tree_t *rt, range_seg_t *rs, void *arg);
+void rt_avl_remove(range_tree_t *rt, range_seg_t *rs, void *arg);
+void rt_avl_vacate(range_tree_t *rt, void *arg);
+extern struct range_tree_ops rt_avl_ops;
 
 #ifdef	__cplusplus
 }
diff --git a/usr/src/uts/common/fs/zfs/sys/spa_impl.h b/usr/src/uts/common/fs/zfs/sys/spa_impl.h
index dcb6cc9f19..b3840a98a3 100644
--- a/usr/src/uts/common/fs/zfs/sys/spa_impl.h
+++ b/usr/src/uts/common/fs/zfs/sys/spa_impl.h
@@ -265,9 +265,9 @@ struct spa {
 	uberblock_t	spa_ubsync;		/* last synced uberblock */
 	uberblock_t	spa_uberblock;		/* current uberblock */
 	boolean_t	spa_extreme_rewind;	/* rewind past deferred frees */
-	uint64_t	spa_last_io;		/* lbolt of last non-scan I/O */
 	kmutex_t	spa_scrub_lock;		/* resilver/scrub lock */
-	uint64_t	spa_scrub_inflight;	/* in-flight scrub I/Os */
+	uint64_t	spa_scrub_inflight;	/* in-flight scrub bytes */
+	uint64_t	spa_load_verify_ios;	/* in-flight verification IOs */
 	kcondvar_t	spa_scrub_io_cv;	/* scrub I/O completion */
 	uint8_t		spa_scrub_active;	/* active or suspended? */
 	uint8_t		spa_scrub_type;		/* type of scrub we're doing */
@@ -278,6 +278,7 @@ struct spa {
 	uint64_t	spa_scan_pass_scrub_pause; /* scrub pause time */
 	uint64_t	spa_scan_pass_scrub_spent_paused; /* total paused */
 	uint64_t	spa_scan_pass_exam;	/* examined bytes per pass */
+	uint64_t	spa_scan_pass_issued;	/* issued bytes per pass */
 	kmutex_t	spa_async_lock;		/* protect async state */
 	kthread_t	*spa_async_thread;	/* thread doing async task */
 	int		spa_async_suspended;	/* async tasks suspended */
diff --git a/usr/src/uts/common/fs/zfs/sys/vdev.h b/usr/src/uts/common/fs/zfs/sys/vdev.h
index ef3bc5dd09..0c0bc874c1 100644
--- a/usr/src/uts/common/fs/zfs/sys/vdev.h
+++ b/usr/src/uts/common/fs/zfs/sys/vdev.h
@@ -71,6 +71,7 @@ extern void vdev_dtl_dirty(vdev_t *vd, vdev_dtl_type_t d,
 extern boolean_t vdev_dtl_contains(vdev_t *vd, vdev_dtl_type_t d,
     uint64_t txg, uint64_t size);
 extern boolean_t vdev_dtl_empty(vdev_t *vd, vdev_dtl_type_t d);
+extern boolean_t vdev_dtl_need_resilver(vdev_t *vd, uint64_t off, size_t size);
 extern void vdev_dtl_reassess(vdev_t *vd, uint64_t txg, uint64_t scrub_txg,
     int scrub_done);
 extern boolean_t vdev_dtl_required(vdev_t *vd);
@@ -136,6 +137,7 @@ extern void vdev_queue_init(vdev_t *vd);
 extern void vdev_queue_fini(vdev_t *vd);
 extern zio_t *vdev_queue_io(zio_t *zio);
 extern void vdev_queue_io_done(zio_t *zio);
+extern void vdev_queue_change_io_priority(zio_t *zio, zio_priority_t priority);
 
 extern void vdev_config_dirty(vdev_t *vd);
 extern void vdev_config_clean(vdev_t *vd);
diff --git a/usr/src/uts/common/fs/zfs/sys/vdev_impl.h b/usr/src/uts/common/fs/zfs/sys/vdev_impl.h
index 6ddbe55a0c..4e1b09c27d 100644
--- a/usr/src/uts/common/fs/zfs/sys/vdev_impl.h
+++ b/usr/src/uts/common/fs/zfs/sys/vdev_impl.h
@@ -73,6 +73,7 @@ typedef uint64_t vdev_asize_func_t(vdev_t *vd, uint64_t psize);
 typedef void	vdev_io_start_func_t(zio_t *zio);
 typedef void	vdev_io_done_func_t(zio_t *zio);
 typedef void	vdev_state_change_func_t(vdev_t *vd, int, int);
+typedef boolean_t vdev_need_resilver_func_t(vdev_t *vd, uint64_t, size_t);
 typedef void	vdev_hold_func_t(vdev_t *vd);
 typedef void	vdev_rele_func_t(vdev_t *vd);
 
@@ -94,6 +95,7 @@ typedef struct vdev_ops {
 	vdev_io_start_func_t		*vdev_op_io_start;
 	vdev_io_done_func_t		*vdev_op_io_done;
 	vdev_state_change_func_t	*vdev_op_state_change;
+	vdev_need_resilver_func_t	*vdev_op_need_resilver;
 	vdev_hold_func_t		*vdev_op_hold;
 	vdev_rele_func_t		*vdev_op_rele;
 	vdev_remap_func_t		*vdev_op_remap;
@@ -305,6 +307,13 @@ struct vdev {
 	space_map_t	*vdev_obsolete_sm;
 
 	/*
+	 * Protects the vdev_scan_io_queue field itself as well as the
+	 * structure's contents (when present).
+	 */
+	kmutex_t	vdev_scan_io_queue_lock;
+	struct dsl_scan_io_queue	*vdev_scan_io_queue;
+
+	/*
 	 * Leaf vdev state.
 	 */
 	range_tree_t	*vdev_dtl[DTL_TYPES]; /* dirty time logs	*/
diff --git a/usr/src/uts/common/fs/zfs/sys/zio.h b/usr/src/uts/common/fs/zfs/sys/zio.h
index 517764f1ce..00d2ebbebb 100644
--- a/usr/src/uts/common/fs/zfs/sys/zio.h
+++ b/usr/src/uts/common/fs/zfs/sys/zio.h
@@ -565,6 +565,8 @@ extern void zio_vdev_io_bypass(zio_t *zio);
 extern void zio_vdev_io_reissue(zio_t *zio);
 extern void zio_vdev_io_redone(zio_t *zio);
 
+extern void zio_change_priority(zio_t *pio, zio_priority_t priority);
+
 extern void zio_checksum_verified(zio_t *zio);
 extern int zio_worst_error(int e1, int e2);
 
diff --git a/usr/src/uts/common/fs/zfs/vdev.c b/usr/src/uts/common/fs/zfs/vdev.c
index b8a385d391..c7dca83777 100644
--- a/usr/src/uts/common/fs/zfs/vdev.c
+++ b/usr/src/uts/common/fs/zfs/vdev.c
@@ -501,6 +501,7 @@ vdev_alloc_common(spa_t *spa, uint_t id, uint64_t guid, vdev_ops_t *ops)
 	mutex_init(&vd->vdev_dtl_lock, NULL, MUTEX_DEFAULT, NULL);
 	mutex_init(&vd->vdev_stat_lock, NULL, MUTEX_DEFAULT, NULL);
 	mutex_init(&vd->vdev_probe_lock, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&vd->vdev_scan_io_queue_lock, NULL, MUTEX_DEFAULT, NULL);
 	mutex_init(&vd->vdev_initialize_lock, NULL, MUTEX_DEFAULT, NULL);
 	mutex_init(&vd->vdev_initialize_io_lock, NULL, MUTEX_DEFAULT, NULL);
 	cv_init(&vd->vdev_initialize_cv, NULL, CV_DEFAULT, NULL);
@@ -802,6 +803,18 @@ vdev_free(vdev_t *vd)
 	ASSERT3P(vd->vdev_initialize_thread, ==, NULL);
 
 	/*
+	 * Scan queues are normally destroyed at the end of a scan. If the
+	 * queue exists here, that implies the vdev is being removed while
+	 * the scan is still running.
+	 */
+	if (vd->vdev_scan_io_queue != NULL) {
+		mutex_enter(&vd->vdev_scan_io_queue_lock);
+		dsl_scan_io_queue_destroy(vd->vdev_scan_io_queue);
+		vd->vdev_scan_io_queue = NULL;
+		mutex_exit(&vd->vdev_scan_io_queue_lock);
+	}
+
+	/*
 	 * vdev_free() implies closing the vdev first.  This is simpler than
 	 * trying to ensure complicated semantics for all callers.
 	 */
@@ -891,6 +904,7 @@ vdev_free(vdev_t *vd)
 	mutex_destroy(&vd->vdev_dtl_lock);
 	mutex_destroy(&vd->vdev_stat_lock);
 	mutex_destroy(&vd->vdev_probe_lock);
+	mutex_destroy(&vd->vdev_scan_io_queue_lock);
 	mutex_destroy(&vd->vdev_initialize_lock);
 	mutex_destroy(&vd->vdev_initialize_io_lock);
 	cv_destroy(&vd->vdev_initialize_io_cv);
@@ -1000,6 +1014,8 @@ vdev_top_transfer(vdev_t *svd, vdev_t *tvd)
 
 	tvd->vdev_islog = svd->vdev_islog;
 	svd->vdev_islog = 0;
+
+	dsl_scan_io_queue_vdev_xfer(svd, tvd);
 }
 
 static void
@@ -2161,6 +2177,7 @@ vdev_metaslab_set_size(vdev_t *vd)
 	uint64_t ms_count = asize >> zfs_vdev_default_ms_shift;
 	uint64_t ms_shift;
 
+	/* BEGIN CSTYLED */
 	/*
 	 * There are two dimensions to the metaslab sizing calculation:
 	 * the size of the metaslab and the count of metaslabs per vdev.
@@ -2204,6 +2221,7 @@ vdev_metaslab_set_size(vdev_t *vd)
 	 *  in additional metaslabs being allocated making it possible
 	 *  to exceed the zfs_vdev_ms_count_limit.
 	 */
+	/* END CSTYLED */
 
 	if (ms_count < zfs_vdev_min_ms_count)
 		ms_shift = highbit64(asize / zfs_vdev_min_ms_count);
@@ -2348,6 +2366,21 @@ vdev_dtl_empty(vdev_t *vd, vdev_dtl_type_t t)
 }
 
 /*
+ * Returns B_TRUE if vdev determines offset needs to be resilvered.
+ */
+boolean_t
+vdev_dtl_need_resilver(vdev_t *vd, uint64_t offset, size_t psize)
+{
+	ASSERT(vd != vd->vdev_spa->spa_root_vdev);
+
+	if (vd->vdev_ops->vdev_op_need_resilver == NULL ||
+	    vd->vdev_ops->vdev_op_leaf)
+		return (B_TRUE);
+
+	return (vd->vdev_ops->vdev_op_need_resilver(vd, offset, psize));
+}
+
+/*
  * Returns the lowest txg in the DTL range.
  */
 static uint64_t
diff --git a/usr/src/uts/common/fs/zfs/vdev_disk.c b/usr/src/uts/common/fs/zfs/vdev_disk.c
index 2d431373ce..93462ee2ba 100644
--- a/usr/src/uts/common/fs/zfs/vdev_disk.c
+++ b/usr/src/uts/common/fs/zfs/vdev_disk.c
@@ -884,18 +884,19 @@ vdev_disk_io_done(zio_t *zio)
 }
 
 vdev_ops_t vdev_disk_ops = {
-	vdev_disk_open,
-	vdev_disk_close,
-	vdev_default_asize,
-	vdev_disk_io_start,
-	vdev_disk_io_done,
-	NULL,
-	vdev_disk_hold,
-	vdev_disk_rele,
-	NULL,
-	vdev_default_xlate,
-	VDEV_TYPE_DISK,		/* name of this vdev type */
-	B_TRUE			/* leaf vdev */
+	.vdev_op_open = vdev_disk_open,
+	.vdev_op_close = vdev_disk_close,
+	.vdev_op_asize = vdev_default_asize,
+	.vdev_op_io_start = vdev_disk_io_start,
+	.vdev_op_io_done = vdev_disk_io_done,
+	.vdev_op_state_change = NULL,
+	.vdev_op_need_resilver = NULL,
+	.vdev_op_hold = vdev_disk_hold,
+	.vdev_op_rele = vdev_disk_rele,
+	.vdev_op_remap = NULL,
+	.vdev_op_xlate = vdev_default_xlate,
+	.vdev_op_type = VDEV_TYPE_DISK,		/* name of this vdev type */
+	.vdev_op_leaf = B_TRUE			/* leaf vdev */
 };
 
 /*
diff --git a/usr/src/uts/common/fs/zfs/vdev_file.c b/usr/src/uts/common/fs/zfs/vdev_file.c
index 4dbac6321b..3aaebe8505 100644
--- a/usr/src/uts/common/fs/zfs/vdev_file.c
+++ b/usr/src/uts/common/fs/zfs/vdev_file.c
@@ -255,18 +255,19 @@ vdev_file_io_done(zio_t *zio)
 }
 
 vdev_ops_t vdev_file_ops = {
-	vdev_file_open,
-	vdev_file_close,
-	vdev_default_asize,
-	vdev_file_io_start,
-	vdev_file_io_done,
-	NULL,
-	vdev_file_hold,
-	vdev_file_rele,
-	NULL,
-	vdev_default_xlate,
-	VDEV_TYPE_FILE,		/* name of this vdev type */
-	B_TRUE			/* leaf vdev */
+	.vdev_op_open = vdev_file_open,
+	.vdev_op_close = vdev_file_close,
+	.vdev_op_asize = vdev_default_asize,
+	.vdev_op_io_start = vdev_file_io_start,
+	.vdev_op_io_done = vdev_file_io_done,
+	.vdev_op_state_change = NULL,
+	.vdev_op_need_resilver = NULL,
+	.vdev_op_hold = vdev_file_hold,
+	.vdev_op_rele = vdev_file_rele,
+	.vdev_op_remap = NULL,
+	.vdev_op_xlate = vdev_default_xlate,
+	.vdev_op_type = VDEV_TYPE_FILE,		/* name of this vdev type */
+	.vdev_op_leaf = B_TRUE			/* leaf vdev */
 };
 
 /*
@@ -275,18 +276,19 @@ vdev_ops_t vdev_file_ops = {
 #ifndef _KERNEL
 
 vdev_ops_t vdev_disk_ops = {
-	vdev_file_open,
-	vdev_file_close,
-	vdev_default_asize,
-	vdev_file_io_start,
-	vdev_file_io_done,
-	NULL,
-	vdev_file_hold,
-	vdev_file_rele,
-	NULL,
-	vdev_default_xlate,
-	VDEV_TYPE_DISK,		/* name of this vdev type */
-	B_TRUE			/* leaf vdev */
+	.vdev_op_open = vdev_file_open,
+	.vdev_op_close = vdev_file_close,
+	.vdev_op_asize = vdev_default_asize,
+	.vdev_op_io_start = vdev_file_io_start,
+	.vdev_op_io_done = vdev_file_io_done,
+	.vdev_op_state_change = NULL,
+	.vdev_op_need_resilver = NULL,
+	.vdev_op_hold = vdev_file_hold,
+	.vdev_op_rele = vdev_file_rele,
+	.vdev_op_remap = NULL,
+	.vdev_op_xlate = vdev_default_xlate,
+	.vdev_op_type = VDEV_TYPE_DISK,		/* name of this vdev type */
+	.vdev_op_leaf = B_TRUE			/* leaf vdev */
 };
 
 #endif
diff --git a/usr/src/uts/common/fs/zfs/vdev_indirect.c b/usr/src/uts/common/fs/zfs/vdev_indirect.c
index 5b6415937f..958e8cd858 100644
--- a/usr/src/uts/common/fs/zfs/vdev_indirect.c
+++ b/usr/src/uts/common/fs/zfs/vdev_indirect.c
@@ -1812,16 +1812,17 @@ vdev_indirect_io_done(zio_t *zio)
 }
 
 vdev_ops_t vdev_indirect_ops = {
-	vdev_indirect_open,
-	vdev_indirect_close,
-	vdev_default_asize,
-	vdev_indirect_io_start,
-	vdev_indirect_io_done,
-	NULL,
-	NULL,
-	NULL,
-	vdev_indirect_remap,
-	NULL,
-	VDEV_TYPE_INDIRECT,	/* name of this vdev type */
-	B_FALSE			/* leaf vdev */
+	.vdev_op_open = vdev_indirect_open,
+	.vdev_op_close = vdev_indirect_close,
+	.vdev_op_asize = vdev_default_asize,
+	.vdev_op_io_start = vdev_indirect_io_start,
+	.vdev_op_io_done = vdev_indirect_io_done,
+	.vdev_op_state_change = NULL,
+	.vdev_op_need_resilver = NULL,
+	.vdev_op_hold = NULL,
+	.vdev_op_rele = NULL,
+	.vdev_op_remap = vdev_indirect_remap,
+	.vdev_op_xlate = NULL,
+	.vdev_op_type = VDEV_TYPE_INDIRECT,	/* name of this vdev type */
+	.vdev_op_leaf = B_FALSE			/* leaf vdev */
 };
diff --git a/usr/src/uts/common/fs/zfs/vdev_mirror.c b/usr/src/uts/common/fs/zfs/vdev_mirror.c
index 133558d3d3..f489bb1967 100644
--- a/usr/src/uts/common/fs/zfs/vdev_mirror.c
+++ b/usr/src/uts/common/fs/zfs/vdev_mirror.c
@@ -555,46 +555,49 @@ vdev_mirror_state_change(vdev_t *vd, int faulted, int degraded)
 }
 
 vdev_ops_t vdev_mirror_ops = {
-	vdev_mirror_open,
-	vdev_mirror_close,
-	vdev_default_asize,
-	vdev_mirror_io_start,
-	vdev_mirror_io_done,
-	vdev_mirror_state_change,
-	NULL,
-	NULL,
-	NULL,
-	vdev_default_xlate,
-	VDEV_TYPE_MIRROR,	/* name of this vdev type */
-	B_FALSE			/* not a leaf vdev */
+	.vdev_op_open = vdev_mirror_open,
+	.vdev_op_close = vdev_mirror_close,
+	.vdev_op_asize = vdev_default_asize,
+	.vdev_op_io_start = vdev_mirror_io_start,
+	.vdev_op_io_done = vdev_mirror_io_done,
+	.vdev_op_state_change = vdev_mirror_state_change,
+	.vdev_op_need_resilver = NULL,
+	.vdev_op_hold = NULL,
+	.vdev_op_rele = NULL,
+	.vdev_op_remap = NULL,
+	.vdev_op_xlate = vdev_default_xlate,
+	.vdev_op_type = VDEV_TYPE_MIRROR,	/* name of this vdev type */
+	.vdev_op_leaf = B_FALSE			/* not a leaf vdev */
 };
 
 vdev_ops_t vdev_replacing_ops = {
-	vdev_mirror_open,
-	vdev_mirror_close,
-	vdev_default_asize,
-	vdev_mirror_io_start,
-	vdev_mirror_io_done,
-	vdev_mirror_state_change,
-	NULL,
-	NULL,
-	NULL,
-	vdev_default_xlate,
-	VDEV_TYPE_REPLACING,	/* name of this vdev type */
-	B_FALSE			/* not a leaf vdev */
+	.vdev_op_open = vdev_mirror_open,
+	.vdev_op_close = vdev_mirror_close,
+	.vdev_op_asize = vdev_default_asize,
+	.vdev_op_io_start = vdev_mirror_io_start,
+	.vdev_op_io_done = vdev_mirror_io_done,
+	.vdev_op_state_change = vdev_mirror_state_change,
+	.vdev_op_need_resilver = NULL,
+	.vdev_op_hold = NULL,
+	.vdev_op_rele = NULL,
+	.vdev_op_remap = NULL,
+	.vdev_op_xlate = vdev_default_xlate,
+	.vdev_op_type = VDEV_TYPE_REPLACING,	/* name of this vdev type */
+	.vdev_op_leaf = B_FALSE			/* not a leaf vdev */
 };
 
 vdev_ops_t vdev_spare_ops = {
-	vdev_mirror_open,
-	vdev_mirror_close,
-	vdev_default_asize,
-	vdev_mirror_io_start,
-	vdev_mirror_io_done,
-	vdev_mirror_state_change,
-	NULL,
-	NULL,
-	NULL,
-	vdev_default_xlate,
-	VDEV_TYPE_SPARE,	/* name of this vdev type */
-	B_FALSE			/* not a leaf vdev */
+	.vdev_op_open = vdev_mirror_open,
+	.vdev_op_close = vdev_mirror_close,
+	.vdev_op_asize = vdev_default_asize,
+	.vdev_op_io_start = vdev_mirror_io_start,
+	.vdev_op_io_done = vdev_mirror_io_done,
+	.vdev_op_state_change = vdev_mirror_state_change,
+	.vdev_op_need_resilver = NULL,
+	.vdev_op_hold = NULL,
+	.vdev_op_rele = NULL,
+	.vdev_op_remap = NULL,
+	.vdev_op_xlate = vdev_default_xlate,
+	.vdev_op_type = VDEV_TYPE_SPARE,	/* name of this vdev type */
+	.vdev_op_leaf = B_FALSE			/* not a leaf vdev */
 };
diff --git a/usr/src/uts/common/fs/zfs/vdev_missing.c b/usr/src/uts/common/fs/zfs/vdev_missing.c
index c761de8a20..205b23eba7 100644
--- a/usr/src/uts/common/fs/zfs/vdev_missing.c
+++ b/usr/src/uts/common/fs/zfs/vdev_missing.c
@@ -80,31 +80,33 @@ vdev_missing_io_done(zio_t *zio)
 }
 
 vdev_ops_t vdev_missing_ops = {
-	vdev_missing_open,
-	vdev_missing_close,
-	vdev_default_asize,
-	vdev_missing_io_start,
-	vdev_missing_io_done,
-	NULL,
-	NULL,
-	NULL,
-	NULL,
-	NULL,
-	VDEV_TYPE_MISSING,	/* name of this vdev type */
-	B_TRUE			/* leaf vdev */
+	.vdev_op_open = vdev_missing_open,
+	.vdev_op_close = vdev_missing_close,
+	.vdev_op_asize = vdev_default_asize,
+	.vdev_op_io_start = vdev_missing_io_start,
+	.vdev_op_io_done = vdev_missing_io_done,
+	.vdev_op_state_change = NULL,
+	.vdev_op_need_resilver = NULL,
+	.vdev_op_hold = NULL,
+	.vdev_op_rele = NULL,
+	.vdev_op_remap = NULL,
+	.vdev_op_xlate = NULL,
+	.vdev_op_type = VDEV_TYPE_MISSING,	/* name of this vdev type */
+	.vdev_op_leaf = B_TRUE			/* leaf vdev */
 };
 
 vdev_ops_t vdev_hole_ops = {
-	vdev_missing_open,
-	vdev_missing_close,
-	vdev_default_asize,
-	vdev_missing_io_start,
-	vdev_missing_io_done,
-	NULL,
-	NULL,
-	NULL,
-	NULL,
-	NULL,
-	VDEV_TYPE_HOLE,		/* name of this vdev type */
-	B_TRUE			/* leaf vdev */
+	.vdev_op_open = vdev_missing_open,
+	.vdev_op_close = vdev_missing_close,
+	.vdev_op_asize = vdev_default_asize,
+	.vdev_op_io_start = vdev_missing_io_start,
+	.vdev_op_io_done = vdev_missing_io_done,
+	.vdev_op_state_change = NULL,
+	.vdev_op_need_resilver = NULL,
+	.vdev_op_hold = NULL,
+	.vdev_op_rele = NULL,
+	.vdev_op_remap = NULL,
+	.vdev_op_xlate = NULL,
+	.vdev_op_type = VDEV_TYPE_HOLE,		/* name of this vdev type */
+	.vdev_op_leaf = B_TRUE			/* leaf vdev */
 };
diff --git a/usr/src/uts/common/fs/zfs/vdev_queue.c b/usr/src/uts/common/fs/zfs/vdev_queue.c
index dff83e3108..0643c05f57 100644
--- a/usr/src/uts/common/fs/zfs/vdev_queue.c
+++ b/usr/src/uts/common/fs/zfs/vdev_queue.c
@@ -169,7 +169,7 @@ int zfs_vdev_async_write_active_max_dirty_percent = 60;
  * we include spans of optional I/Os to aid aggregation at the disk even when
  * they aren't able to help us aggregate at this level.
  */
-int zfs_vdev_aggregation_limit = SPA_OLD_MAXBLOCKSIZE;
+int zfs_vdev_aggregation_limit = 1 << 20;
 int zfs_vdev_read_gap_limit = 32 << 10;
 int zfs_vdev_write_gap_limit = 4 << 10;
 
@@ -519,6 +519,7 @@ static zio_t *
 vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
 {
 	zio_t *first, *last, *aio, *dio, *mandatory, *nio;
+	zio_link_t *zl = NULL;
 	uint64_t maxgap = 0;
 	uint64_t size;
 	boolean_t stretch = B_FALSE;
@@ -657,9 +658,18 @@ vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
 
 		zio_add_child(dio, aio);
 		vdev_queue_io_remove(vq, dio);
+	} while (dio != last);
+
+	/*
+	 * We need to drop the vdev queue's lock to avoid a deadlock that we
+	 * could encounter since this I/O will complete immediately.
+	 */
+	mutex_exit(&vq->vq_lock);
+	while ((dio = zio_walk_parents(aio, &zl)) != NULL) {
 		zio_vdev_io_bypass(dio);
 		zio_execute(dio);
-	} while (dio != last);
+	}
+	mutex_enter(&vq->vq_lock);
 
 	return (aio);
 }
@@ -797,3 +807,54 @@ vdev_queue_io_done(zio_t *zio)
 
 	mutex_exit(&vq->vq_lock);
 }
+
+void
+vdev_queue_change_io_priority(zio_t *zio, zio_priority_t priority)
+{
+	vdev_queue_t *vq = &zio->io_vd->vdev_queue;
+	avl_tree_t *tree;
+
+	/*
+	 * ZIO_PRIORITY_NOW is used by the vdev cache code and the aggregate zio
+	 * code to issue IOs without adding them to the vdev queue. In this
+	 * case, the zio is already going to be issued as quickly as possible
+	 * and so it doesn't need any reprioitization to help.
+	 */
+	if (zio->io_priority == ZIO_PRIORITY_NOW)
+		return;
+
+	ASSERT3U(zio->io_priority, <, ZIO_PRIORITY_NUM_QUEUEABLE);
+	ASSERT3U(priority, <, ZIO_PRIORITY_NUM_QUEUEABLE);
+
+	if (zio->io_type == ZIO_TYPE_READ) {
+		if (priority != ZIO_PRIORITY_SYNC_READ &&
+		    priority != ZIO_PRIORITY_ASYNC_READ &&
+		    priority != ZIO_PRIORITY_SCRUB)
+			priority = ZIO_PRIORITY_ASYNC_READ;
+	} else {
+		ASSERT(zio->io_type == ZIO_TYPE_WRITE);
+		if (priority != ZIO_PRIORITY_SYNC_WRITE &&
+		    priority != ZIO_PRIORITY_ASYNC_WRITE)
+			priority = ZIO_PRIORITY_ASYNC_WRITE;
+	}
+
+	mutex_enter(&vq->vq_lock);
+
+	/*
+	 * If the zio is in none of the queues we can simply change
+	 * the priority. If the zio is waiting to be submitted we must
+	 * remove it from the queue and re-insert it with the new priority.
+	 * Otherwise, the zio is currently active and we cannot change its
+	 * priority.
+	 */
+	tree = vdev_queue_class_tree(vq, zio->io_priority);
+	if (avl_find(tree, zio, NULL) == zio) {
+		avl_remove(vdev_queue_class_tree(vq, zio->io_priority), zio);
+		zio->io_priority = priority;
+		avl_add(vdev_queue_class_tree(vq, zio->io_priority), zio);
+	} else if (avl_find(&vq->vq_active_tree, zio, NULL) != zio) {
+		zio->io_priority = priority;
+	}
+
+	mutex_exit(&vq->vq_lock);
+}
diff --git a/usr/src/uts/common/fs/zfs/vdev_raidz.c b/usr/src/uts/common/fs/zfs/vdev_raidz.c
index 0e6dfcc2c0..6502353542 100644
--- a/usr/src/uts/common/fs/zfs/vdev_raidz.c
+++ b/usr/src/uts/common/fs/zfs/vdev_raidz.c
@@ -2598,6 +2598,44 @@ vdev_raidz_state_change(vdev_t *vd, int faulted, int degraded)
 		vdev_set_state(vd, B_FALSE, VDEV_STATE_HEALTHY, VDEV_AUX_NONE);
 }
 
+/*
+ * Determine if any portion of the provided block resides on a child vdev
+ * with a dirty DTL and therefore needs to be resilvered.  The function
+ * assumes that at least one DTL is dirty which imples that full stripe
+ * width blocks must be resilvered.
+ */
+static boolean_t
+vdev_raidz_need_resilver(vdev_t *vd, uint64_t offset, size_t psize)
+{
+	uint64_t dcols = vd->vdev_children;
+	uint64_t nparity = vd->vdev_nparity;
+	uint64_t ashift = vd->vdev_top->vdev_ashift;
+	/* The starting RAIDZ (parent) vdev sector of the block. */
+	uint64_t b = offset >> ashift;
+	/* The zio's size in units of the vdev's minimum sector size. */
+	uint64_t s = ((psize - 1) >> ashift) + 1;
+	/* The first column for this stripe. */
+	uint64_t f = b % dcols;
+
+	if (s + nparity >= dcols)
+		return (B_TRUE);
+
+	for (uint64_t c = 0; c < s + nparity; c++) {
+		uint64_t devidx = (f + c) % dcols;
+		vdev_t *cvd = vd->vdev_child[devidx];
+
+		/*
+		 * dsl_scan_need_resilver() already checked vd with
+		 * vdev_dtl_contains(). So here just check cvd with
+		 * vdev_dtl_empty(), cheaper and a good approximation.
+		 */
+		if (!vdev_dtl_empty(cvd, DTL_PARTIAL))
+			return (B_TRUE);
+	}
+
+	return (B_FALSE);
+}
+
 static void
 vdev_raidz_xlate(vdev_t *cvd, const range_seg_t *in, range_seg_t *res)
 {
@@ -2630,16 +2668,17 @@ vdev_raidz_xlate(vdev_t *cvd, const range_seg_t *in, range_seg_t *res)
 }
 
 vdev_ops_t vdev_raidz_ops = {
-	vdev_raidz_open,
-	vdev_raidz_close,
-	vdev_raidz_asize,
-	vdev_raidz_io_start,
-	vdev_raidz_io_done,
-	vdev_raidz_state_change,
-	NULL,
-	NULL,
-	NULL,
-	vdev_raidz_xlate,
-	VDEV_TYPE_RAIDZ,	/* name of this vdev type */
-	B_FALSE			/* not a leaf vdev */
+	.vdev_op_open = vdev_raidz_open,
+	.vdev_op_close = vdev_raidz_close,
+	.vdev_op_asize = vdev_raidz_asize,
+	.vdev_op_io_start = vdev_raidz_io_start,
+	.vdev_op_io_done = vdev_raidz_io_done,
+	.vdev_op_state_change = vdev_raidz_state_change,
+	.vdev_op_need_resilver = vdev_raidz_need_resilver,
+	.vdev_op_hold = NULL,
+	.vdev_op_rele = NULL,
+	.vdev_op_remap = NULL,
+	.vdev_op_xlate = vdev_raidz_xlate,
+	.vdev_op_type = VDEV_TYPE_RAIDZ,	/* name of this vdev type */
+	.vdev_op_leaf = B_FALSE			/* not a leaf vdev */
 };
diff --git a/usr/src/uts/common/fs/zfs/vdev_root.c b/usr/src/uts/common/fs/zfs/vdev_root.c
index edb52d6ca7..7170f70136 100644
--- a/usr/src/uts/common/fs/zfs/vdev_root.c
+++ b/usr/src/uts/common/fs/zfs/vdev_root.c
@@ -140,16 +140,17 @@ vdev_root_state_change(vdev_t *vd, int faulted, int degraded)
 }
 
 vdev_ops_t vdev_root_ops = {
-	vdev_root_open,
-	vdev_root_close,
-	vdev_default_asize,
-	NULL,			/* io_start - not applicable to the root */
-	NULL,			/* io_done - not applicable to the root */
-	vdev_root_state_change,
-	NULL,
-	NULL,
-	NULL,
-	NULL,
-	VDEV_TYPE_ROOT,		/* name of this vdev type */
-	B_FALSE			/* not a leaf vdev */
+	.vdev_op_open = vdev_root_open,
+	.vdev_op_close = vdev_root_close,
+	.vdev_op_asize = vdev_default_asize,
+	.vdev_op_io_start = NULL,	/* not applicable to the root */
+	.vdev_op_io_done = NULL,	/* not applicable to the root */
+	.vdev_op_state_change = vdev_root_state_change,
+	.vdev_op_need_resilver = NULL,
+	.vdev_op_hold = NULL,
+	.vdev_op_rele = NULL,
+	.vdev_op_remap = NULL,
+	.vdev_op_xlate = NULL,
+	.vdev_op_type = VDEV_TYPE_ROOT,	/* name of this vdev type */
+	.vdev_op_leaf = B_FALSE		/* not a leaf vdev */
 };
diff --git a/usr/src/uts/common/fs/zfs/zio.c b/usr/src/uts/common/fs/zfs/zio.c
index 619dad47f3..76312e6a74 100644
--- a/usr/src/uts/common/fs/zfs/zio.c
+++ b/usr/src/uts/common/fs/zfs/zio.c
@@ -41,6 +41,7 @@
 #include <sys/ddt.h>
 #include <sys/blkptr.h>
 #include <sys/zfeature.h>
+#include <sys/dsl_scan.h>
 #include <sys/metaslab_impl.h>
 #include <sys/abd.h>
 #include <sys/cityhash.h>
@@ -370,6 +371,8 @@ zio_walk_children(zio_t *pio, zio_link_t **zl)
 {
 	list_t *cl = &pio->io_child_list;
 
+	ASSERT(MUTEX_HELD(&pio->io_lock));
+
 	*zl = (*zl == NULL) ? list_head(cl) : list_next(cl, *zl);
 	if (*zl == NULL)
 		return (NULL);
@@ -404,8 +407,8 @@ zio_add_child(zio_t *pio, zio_t *cio)
 	zl->zl_parent = pio;
 	zl->zl_child = cio;
 
-	mutex_enter(&cio->io_lock);
 	mutex_enter(&pio->io_lock);
+	mutex_enter(&cio->io_lock);
 
 	ASSERT(pio->io_state[ZIO_WAIT_DONE] == 0);
 
@@ -418,8 +421,8 @@ zio_add_child(zio_t *pio, zio_t *cio)
 	pio->io_child_count++;
 	cio->io_parent_count++;
 
-	mutex_exit(&pio->io_lock);
 	mutex_exit(&cio->io_lock);
+	mutex_exit(&pio->io_lock);
 }
 
 static void
@@ -428,8 +431,8 @@ zio_remove_child(zio_t *pio, zio_t *cio, zio_link_t *zl)
 	ASSERT(zl->zl_parent == pio);
 	ASSERT(zl->zl_child == cio);
 
-	mutex_enter(&cio->io_lock);
 	mutex_enter(&pio->io_lock);
+	mutex_enter(&cio->io_lock);
 
 	list_remove(&pio->io_child_list, zl);
 	list_remove(&cio->io_parent_list, zl);
@@ -437,8 +440,8 @@ zio_remove_child(zio_t *pio, zio_t *cio, zio_link_t *zl)
 	pio->io_child_count--;
 	cio->io_parent_count--;
 
-	mutex_exit(&pio->io_lock);
 	mutex_exit(&cio->io_lock);
+	mutex_exit(&pio->io_lock);
 
 	kmem_cache_free(zio_link_cache, zl);
 }
@@ -921,6 +924,7 @@ zio_free_sync(zio_t *pio, spa_t *spa, uint64_t txg, const blkptr_t *bp,
 
 	metaslab_check_free(spa, bp);
 	arc_freed(spa, bp);
+	dsl_scan_freed(spa, bp);
 
 	/*
 	 * GANG and DEDUP blocks can induce a read (for the gang block header,
@@ -1760,14 +1764,16 @@ zio_reexecute(zio_t *pio)
 	 * cannot be affected by any side effects of reexecuting 'cio'.
 	 */
 	zio_link_t *zl = NULL;
+	mutex_enter(&pio->io_lock);
 	for (cio = zio_walk_children(pio, &zl); cio != NULL; cio = cio_next) {
 		cio_next = zio_walk_children(pio, &zl);
-		mutex_enter(&pio->io_lock);
 		for (int w = 0; w < ZIO_WAIT_TYPES; w++)
 			pio->io_children[cio->io_child_type][w]++;
 		mutex_exit(&pio->io_lock);
 		zio_reexecute(cio);
+		mutex_enter(&pio->io_lock);
 	}
+	mutex_exit(&pio->io_lock);
 
 	/*
 	 * Now that all children have been reexecuted, execute the parent.
@@ -3142,37 +3148,17 @@ zio_vdev_io_start(zio_t *zio)
 	if (zio->io_type == ZIO_TYPE_WRITE) {
 		ASSERT(spa->spa_trust_config);
 
+		/*
+		 * Note: the code can handle other kinds of writes,
+		 * but we don't expect them.
+		 */
 		if (zio->io_vd->vdev_removing) {
-			/*
-			 * Note: the code can handle other kinds of writes,
-			 * but we don't expect them.
-			 */
 			ASSERT(zio->io_flags &
 			    (ZIO_FLAG_PHYSICAL | ZIO_FLAG_SELF_HEAL |
 			    ZIO_FLAG_RESILVER | ZIO_FLAG_INDUCE_DAMAGE));
 		}
 	}
 
-	/*
-	 * We keep track of time-sensitive I/Os so that the scan thread
-	 * can quickly react to certain workloads.  In particular, we care
-	 * about non-scrubbing, top-level reads and writes with the following
-	 * characteristics:
-	 *	- synchronous writes of user data to non-slog devices
-	 *	- any reads of user data
-	 * When these conditions are met, adjust the timestamp of spa_last_io
-	 * which allows the scan thread to adjust its workload accordingly.
-	 */
-	if (!(zio->io_flags & ZIO_FLAG_SCAN_THREAD) && zio->io_bp != NULL &&
-	    vd == vd->vdev_top && !vd->vdev_islog &&
-	    zio->io_bookmark.zb_objset != DMU_META_OBJSET &&
-	    zio->io_txg != spa_syncing_txg(spa)) {
-		uint64_t old = spa->spa_last_io;
-		uint64_t new = ddi_get_lbolt64();
-		if (old != new)
-			(void) atomic_cas_64(&spa->spa_last_io, old, new);
-	}
-
 	align = 1ULL << vd->vdev_top->vdev_ashift;
 
 	if (!(zio->io_flags & ZIO_FLAG_PHYSICAL) &&
@@ -3312,6 +3298,35 @@ zio_vdev_io_done(zio_t *zio)
 }
 
 /*
+ * This function is used to change the priority of an existing zio that is
+ * currently in-flight. This is used by the arc to upgrade priority in the
+ * event that a demand read is made for a block that is currently queued
+ * as a scrub or async read IO. Otherwise, the high priority read request
+ * would end up having to wait for the lower priority IO.
+ */
+void
+zio_change_priority(zio_t *pio, zio_priority_t priority)
+{
+	zio_t *cio, *cio_next;
+	zio_link_t *zl = NULL;
+
+	ASSERT3U(priority, <, ZIO_PRIORITY_NUM_QUEUEABLE);
+
+	if (pio->io_vd != NULL && pio->io_vd->vdev_ops->vdev_op_leaf) {
+		vdev_queue_change_io_priority(pio, priority);
+	} else {
+		pio->io_priority = priority;
+	}
+
+	mutex_enter(&pio->io_lock);
+	for (cio = zio_walk_children(pio, &zl); cio != NULL; cio = cio_next) {
+		cio_next = zio_walk_children(pio, &zl);
+		zio_change_priority(cio, priority);
+	}
+	mutex_exit(&pio->io_lock);
+}
+
+/*
  * For non-raidz ZIOs, we can just copy aside the bad data read from the
  * disk, and use that to finish the checksum ereport later.
  */
diff --git a/usr/src/uts/common/os/taskq.c b/usr/src/uts/common/os/taskq.c
index d9fa450c99..fcdfe292dd 100644
--- a/usr/src/uts/common/os/taskq.c
+++ b/usr/src/uts/common/os/taskq.c
@@ -1366,6 +1366,12 @@ taskq_wait(taskq_t *tq)
 	}
 }
 
+void
+taskq_wait_id(taskq_t *tq, taskqid_t id __unused)
+{
+	taskq_wait(tq);
+}
+
 /*
  * Suspend execution of tasks.
  *
diff --git a/usr/src/uts/common/sys/fs/zfs.h b/usr/src/uts/common/sys/fs/zfs.h
index ed80492dd5..b54fb27d1e 100644
--- a/usr/src/uts/common/sys/fs/zfs.h
+++ b/usr/src/uts/common/sys/fs/zfs.h
@@ -806,17 +806,22 @@ typedef struct pool_scan_stat {
 	uint64_t	pss_start_time;	/* scan start time */
 	uint64_t	pss_end_time;	/* scan end time */
 	uint64_t	pss_to_examine;	/* total bytes to scan */
-	uint64_t	pss_examined;	/* total examined bytes	*/
+	uint64_t	pss_examined;	/* total bytes located by scanner */
 	uint64_t	pss_to_process; /* total bytes to process */
 	uint64_t	pss_processed;	/* total processed bytes */
 	uint64_t	pss_errors;	/* scan errors	*/
 
 	/* values not stored on disk */
 	uint64_t	pss_pass_exam;	/* examined bytes per scan pass */
+	uint64_t	pss_pass_issued; /* issued bytes per scan pass */
 	uint64_t	pss_pass_start;	/* start time of a scan pass */
 	uint64_t	pss_pass_scrub_pause; /* pause time of a scurb pass */
 	/* cumulative time scrub spent paused, needed for rate calculation */
 	uint64_t	pss_pass_scrub_spent_paused;
+
+	/* Sorted scrubbing new fields */
+	/* Stored on disk */
+	uint64_t	pss_issued;	/* total bytes checked by scanner */
 } pool_scan_stat_t;
 
 typedef struct pool_removal_stat {
diff --git a/usr/src/uts/common/sys/taskq.h b/usr/src/uts/common/sys/taskq.h
index 0fd72bd891..adc260bc70 100644
--- a/usr/src/uts/common/sys/taskq.h
+++ b/usr/src/uts/common/sys/taskq.h
@@ -83,6 +83,7 @@ extern taskqid_t taskq_dispatch(taskq_t *, task_func_t, void *, uint_t);
 extern void	nulltask(void *);
 extern void	taskq_destroy(taskq_t *);
 extern void	taskq_wait(taskq_t *);
+void	taskq_wait_id(taskq_t *, taskqid_t);
 extern boolean_t taskq_empty(taskq_t *);
 extern void	taskq_suspend(taskq_t *);
 extern int	taskq_suspended(taskq_t *);