1 files changed, 202 insertions, 474 deletions

diff --git a/usr/src/uts/common/fs/zfs/vdev_queue.c b/usr/src/uts/common/fs/zfs/vdev_queue.c
index ebf87cdb7f..8de4b324a2 100644
--- a/usr/src/uts/common/fs/zfs/vdev_queue.c
+++ b/usr/src/uts/common/fs/zfs/vdev_queue.c
@@ -25,7 +25,7 @@
  */
 
 /*
- * Copyright (c) 2013 by Delphix. All rights reserved.
+ * Copyright (c) 2012 by Delphix. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
@@ -33,130 +33,29 @@
 #include <sys/spa_impl.h>
 #include <sys/zio.h>
 #include <sys/avl.h>
-#include <sys/dsl_pool.h>
 #include <sys/zfs_zone.h>
 
 /*
- * ZFS I/O Scheduler
- * ---------------
- *
- * ZFS issues I/O operations to leaf vdevs to satisfy and complete zios.  The
- * I/O scheduler determines when and in what order those operations are
- * issued.  The I/O scheduler divides operations into five I/O classes
- * prioritized in the following order: sync read, sync write, async read,
- * async write, and scrub/resilver.  Each queue defines the minimum and
- * maximum number of concurrent operations that may be issued to the device.
- * In addition, the device has an aggregate maximum. Note that the sum of the
- * per-queue minimums must not exceed the aggregate maximum, and if the
- * aggregate maximum is equal to or greater than the sum of the per-queue
- * maximums, the per-queue minimum has no effect.
- *
- * For many physical devices, throughput increases with the number of
- * concurrent operations, but latency typically suffers. Further, physical
- * devices typically have a limit at which more concurrent operations have no
- * effect on throughput or can actually cause it to decrease.
- *
- * The scheduler selects the next operation to issue by first looking for an
- * I/O class whose minimum has not been satisfied. Once all are satisfied and
- * the aggregate maximum has not been hit, the scheduler looks for classes
- * whose maximum has not been satisfied. Iteration through the I/O classes is
- * done in the order specified above. No further operations are issued if the
- * aggregate maximum number of concurrent operations has been hit or if there
- * are no operations queued for an I/O class that has not hit its maximum.
- * Every time an i/o is queued or an operation completes, the I/O scheduler
- * looks for new operations to issue.
- *
- * All I/O classes have a fixed maximum number of outstanding operations
- * except for the async write class. Asynchronous writes represent the data
- * that is committed to stable storage during the syncing stage for
- * transaction groups (see txg.c). Transaction groups enter the syncing state
- * periodically so the number of queued async writes will quickly burst up and
- * then bleed down to zero. Rather than servicing them as quickly as possible,
- * the I/O scheduler changes the maximum number of active async write i/os
- * according to the amount of dirty data in the pool (see dsl_pool.c). Since
- * both throughput and latency typically increase with the number of
- * concurrent operations issued to physical devices, reducing the burstiness
- * in the number of concurrent operations also stabilizes the response time of
- * operations from other -- and in particular synchronous -- queues. In broad
- * strokes, the I/O scheduler will issue more concurrent operations from the
- * async write queue as there's more dirty data in the pool.
- *
- * Async Writes
- *
- * The number of concurrent operations issued for the async write I/O class
- * follows a piece-wise linear function defined by a few adjustable points.
- *
- *        |                   o---------| <-- zfs_vdev_async_write_max_active
- *   ^    |                  /^         |
- *   |    |                 / |         |
- * active |                /  |         |
- *  I/O   |               /   |         |
- * count  |              /    |         |
- *        |             /     |         |
- *        |------------o      |         | <-- zfs_vdev_async_write_min_active
- *       0|____________^______|_________|
- *        0%           |      |       100% of zfs_dirty_data_max
- *                     |      |
- *                     |      `-- zfs_vdev_async_write_active_max_dirty_percent
- *                     `--------- zfs_vdev_async_write_active_min_dirty_percent
- *
- * Until the amount of dirty data exceeds a minimum percentage of the dirty
- * data allowed in the pool, the I/O scheduler will limit the number of
- * concurrent operations to the minimum. As that threshold is crossed, the
- * number of concurrent operations issued increases linearly to the maximum at
- * the specified maximum percentage of the dirty data allowed in the pool.
- *
- * Ideally, the amount of dirty data on a busy pool will stay in the sloped
- * part of the function between zfs_vdev_async_write_active_min_dirty_percent
- * and zfs_vdev_async_write_active_max_dirty_percent. If it exceeds the
- * maximum percentage, this indicates that the rate of incoming data is
- * greater than the rate that the backend storage can handle. In this case, we
- * must further throttle incoming writes (see dmu_tx_delay() for details).
+ * These tunables are for performance analysis.
  */
 
-/*
- * The maximum number of i/os active to each device.  Ideally, this will be >=
- * the sum of each queue's max_active.  It must be at least the sum of each
- * queue's min_active.
- */
-uint32_t zfs_vdev_max_active = 1000;
+/* The maximum number of I/Os concurrently pending to each device. */
+int zfs_vdev_max_pending = 10;
 
 /*
- * Per-queue limits on the number of i/os active to each device.  If the
- * sum of the queue's max_active is < zfs_vdev_max_active, then the
- * min_active comes into play.  We will send min_active from each queue,
- * and then select from queues in the order defined by zio_priority_t.
- *
- * In general, smaller max_active's will lead to lower latency of synchronous
- * operations.  Larger max_active's may lead to higher overall throughput,
- * depending on underlying storage.
- *
- * The ratio of the queues' max_actives determines the balance of performance
- * between reads, writes, and scrubs.  E.g., increasing
- * zfs_vdev_scrub_max_active will cause the scrub or resilver to complete
- * more quickly, but reads and writes to have higher latency and lower
- * throughput.
+ * The initial number of I/Os pending to each device, before it starts ramping
+ * up to zfs_vdev_max_pending.
  */
-uint32_t zfs_vdev_sync_read_min_active = 10;
-uint32_t zfs_vdev_sync_read_max_active = 10;
-uint32_t zfs_vdev_sync_write_min_active = 10;
-uint32_t zfs_vdev_sync_write_max_active = 10;
-uint32_t zfs_vdev_async_read_min_active = 1;
-uint32_t zfs_vdev_async_read_max_active = 3;
-uint32_t zfs_vdev_async_write_min_active = 1;
-uint32_t zfs_vdev_async_write_max_active = 10;
-uint32_t zfs_vdev_scrub_min_active = 1;
-uint32_t zfs_vdev_scrub_max_active = 2;
+int zfs_vdev_min_pending = 4;
 
 /*
- * When the pool has less than zfs_vdev_async_write_active_min_dirty_percent
- * dirty data, use zfs_vdev_async_write_min_active.  When it has more than
- * zfs_vdev_async_write_active_max_dirty_percent, use
- * zfs_vdev_async_write_max_active. The value is linearly interpolated
- * between min and max.
+ * The deadlines are grouped into buckets based on zfs_vdev_time_shift:
+ * deadline = pri + gethrtime() >> time_shift)
  */
-int zfs_vdev_async_write_active_min_dirty_percent = 30;
-int zfs_vdev_async_write_active_max_dirty_percent = 60;
+int zfs_vdev_time_shift = 29; /* each bucket is 0.537 seconds */
+
+/* exponential I/O issue ramp-up rate */
+int zfs_vdev_ramp_rate = 2;
 
 /*
  * To reduce IOPs, we aggregate small adjacent I/Os into one large I/O.
@@ -168,12 +67,20 @@ int zfs_vdev_aggregation_limit = SPA_MAXBLOCKSIZE;
 int zfs_vdev_read_gap_limit = 32 << 10;
 int zfs_vdev_write_gap_limit = 4 << 10;
 
+/*
+ * Virtual device vector for disk I/O scheduling.
+ */
 int
-vdev_queue_offset_compare(const void *x1, const void *x2)
+vdev_queue_deadline_compare(const void *x1, const void *x2)
 {
 	const zio_t *z1 = x1;
 	const zio_t *z2 = x2;
 
+	if (z1->io_deadline < z2->io_deadline)
+		return (-1);
+	if (z1->io_deadline > z2->io_deadline)
+		return (1);
+
 	if (z1->io_offset < z2->io_offset)
 		return (-1);
 	if (z1->io_offset > z2->io_offset)
@@ -188,14 +95,14 @@ vdev_queue_offset_compare(const void *x1, const void *x2)
 }
 
 int
-vdev_queue_timestamp_compare(const void *x1, const void *x2)
+vdev_queue_offset_compare(const void *x1, const void *x2)
 {
 	const zio_t *z1 = x1;
 	const zio_t *z2 = x2;
 
-	if (z1->io_timestamp < z2->io_timestamp)
+	if (z1->io_offset < z2->io_offset)
 		return (-1);
-	if (z1->io_timestamp > z2->io_timestamp)
+	if (z1->io_offset > z2->io_offset)
 		return (1);
 
 	if (z1 < z2)
@@ -212,10 +119,12 @@ vdev_queue_init(vdev_t *vd)
 	vdev_queue_t *vq = &vd->vdev_queue;
 
 	mutex_init(&vq->vq_lock, NULL, MUTEX_DEFAULT, NULL);
-	vq->vq_vdev = vd;
 
-	avl_create(&vq->vq_active_tree, vdev_queue_offset_compare,
-	    sizeof (zio_t), offsetof(struct zio, io_queue_node));
+	avl_create(&vq->vq_deadline_tree, vdev_queue_deadline_compare,
+	    sizeof (zio_t), offsetof(struct zio, io_deadline_node));
+
+	avl_create(&vq->vq_read_tree, vdev_queue_offset_compare,
+	    sizeof (zio_t), offsetof(struct zio, io_offset_node));
 
 	avl_create(&vq->vq_write_tree, vdev_queue_offset_compare,
 	    sizeof (zio_t), offsetof(struct zio, io_offset_node));
@@ -224,21 +133,6 @@ vdev_queue_init(vdev_t *vd)
 	    sizeof (zio_t), offsetof(struct zio, io_offset_node));
 
 	vq->vq_last_zone_id = 0;
-
-	for (zio_priority_t p = 0; p < ZIO_PRIORITY_NUM_QUEUEABLE; p++) {
-		/*
-		 * The synchronous i/o queues are FIFO rather than LBA ordered.
-		 * This provides more consistent latency for these i/os, and
-		 * they tend to not be tightly clustered anyway so there is
-		 * little to no throughput loss.
-		 */
-		boolean_t fifo = (p == ZIO_PRIORITY_SYNC_READ ||
-		    p == ZIO_PRIORITY_SYNC_WRITE);
-		avl_create(&vq->vq_class[p].vqc_queued_tree,
-		    fifo ? vdev_queue_timestamp_compare :
-		    vdev_queue_offset_compare,
-		    sizeof (zio_t), offsetof(struct zio, io_queue_node));
-	}
 }
 
 void
@@ -246,9 +140,10 @@ vdev_queue_fini(vdev_t *vd)
 {
 	vdev_queue_t *vq = &vd->vdev_queue;
 
-	for (zio_priority_t p = 0; p < ZIO_PRIORITY_NUM_QUEUEABLE; p++)
-		avl_destroy(&vq->vq_class[p].vqc_queued_tree);
-	avl_destroy(&vq->vq_active_tree);
+	avl_destroy(&vq->vq_deadline_tree);
+	avl_destroy(&vq->vq_read_tree);
+	avl_destroy(&vq->vq_write_tree);
+	avl_destroy(&vq->vq_pending_tree);
 
 	mutex_destroy(&vq->vq_lock);
 }
@@ -261,14 +156,11 @@ vdev_queue_io_add(vdev_queue_t *vq, zio_t *zio)
 	zfs_zone_zio_enqueue(zio);
 	avl_add(zio->io_vdev_tree, zio);
 
-	ASSERT3U(zio->io_priority, <, ZIO_PRIORITY_NUM_QUEUEABLE);
-	avl_add(&vq->vq_class[zio->io_priority].vqc_queued_tree, zio);
-
-	mutex_enter(&spa->spa_iokstat_lock);
-	spa->spa_queue_stats[zio->io_priority].spa_queued++;
-	if (spa->spa_iokstat != NULL)
+	if (spa->spa_iokstat != NULL) {
+		mutex_enter(&spa->spa_iokstat_lock);
 		kstat_waitq_enter(spa->spa_iokstat->ks_data);
-	mutex_exit(&spa->spa_iokstat_lock);
+		mutex_exit(&spa->spa_iokstat_lock);
+	}
 }
 
 static void
@@ -279,48 +171,34 @@ vdev_queue_io_remove(vdev_queue_t *vq, zio_t *zio)
 	zfs_zone_zio_dequeue(zio);
 	avl_remove(zio->io_vdev_tree, zio);
 
-	ASSERT3U(zio->io_priority, <, ZIO_PRIORITY_NUM_QUEUEABLE);
-	avl_remove(&vq->vq_class[zio->io_priority].vqc_queued_tree, zio);
-
-	mutex_enter(&spa->spa_iokstat_lock);
-	ASSERT3U(spa->spa_queue_stats[zio->io_priority].spa_queued, >, 0);
-	spa->spa_queue_stats[zio->io_priority].spa_queued--;
-	if (spa->spa_iokstat != NULL)
+	if (spa->spa_iokstat != NULL) {
+		mutex_enter(&spa->spa_iokstat_lock);
 		kstat_waitq_exit(spa->spa_iokstat->ks_data);
-	mutex_exit(&spa->spa_iokstat_lock);
+		mutex_exit(&spa->spa_iokstat_lock);
+	}
 }
 
 static void
 vdev_queue_pending_add(vdev_queue_t *vq, zio_t *zio)
 {
 	spa_t *spa = zio->io_spa;
-	ASSERT(MUTEX_HELD(&vq->vq_lock));
-	ASSERT3U(zio->io_priority, <, ZIO_PRIORITY_NUM_QUEUEABLE);
-	vq->vq_class[zio->io_priority].vqc_active++;
-	avl_add(&vq->vq_active_tree, zio);
-
-	mutex_enter(&spa->spa_iokstat_lock);
-	spa->spa_queue_stats[zio->io_priority].spa_active++;
-	if (spa->spa_iokstat != NULL)
+	avl_add(&vq->vq_pending_tree, zio);
+	if (spa->spa_iokstat != NULL) {
+		mutex_enter(&spa->spa_iokstat_lock);
 		kstat_runq_enter(spa->spa_iokstat->ks_data);
-	mutex_exit(&spa->spa_iokstat_lock);
+		mutex_exit(&spa->spa_iokstat_lock);
+	}
 }
 
 static void
 vdev_queue_pending_remove(vdev_queue_t *vq, zio_t *zio)
 {
 	spa_t *spa = zio->io_spa;
-	ASSERT(MUTEX_HELD(&vq->vq_lock));
-	ASSERT3U(zio->io_priority, <, ZIO_PRIORITY_NUM_QUEUEABLE);
-	vq->vq_class[zio->io_priority].vqc_active--;
-	avl_remove(&vq->vq_active_tree, zio);
-
-	mutex_enter(&spa->spa_iokstat_lock);
-	ASSERT3U(spa->spa_queue_stats[zio->io_priority].spa_active, >, 0);
-	spa->spa_queue_stats[zio->io_priority].spa_active--;
+	avl_remove(&vq->vq_pending_tree, zio);
 	if (spa->spa_iokstat != NULL) {
 		kstat_io_t *ksio = spa->spa_iokstat->ks_data;
 
+		mutex_enter(&spa->spa_iokstat_lock);
 		kstat_runq_exit(spa->spa_iokstat->ks_data);
 		if (zio->io_type == ZIO_TYPE_READ) {
 			ksio->reads++;
@@ -329,131 +207,23 @@ vdev_queue_pending_remove(vdev_queue_t *vq, zio_t *zio)
 			ksio->writes++;
 			ksio->nwritten += zio->io_size;
 		}
+		mutex_exit(&spa->spa_iokstat_lock);
 	}
-	mutex_exit(&spa->spa_iokstat_lock);
 }
 
 static void
 vdev_queue_agg_io_done(zio_t *aio)
 {
-	if (aio->io_type == ZIO_TYPE_READ) {
-		zio_t *pio;
-		while ((pio = zio_walk_parents(aio)) != NULL) {
+	zio_t *pio;
+
+	while ((pio = zio_walk_parents(aio)) != NULL)
+		if (aio->io_type == ZIO_TYPE_READ)
 			bcopy((char *)aio->io_data + (pio->io_offset -
 			    aio->io_offset), pio->io_data, pio->io_size);
-		}
-	}
 
 	zio_buf_free(aio->io_data, aio->io_size);
 }
 
-static int
-vdev_queue_class_min_active(zio_priority_t p)
-{
-	switch (p) {
-	case ZIO_PRIORITY_SYNC_READ:
-		return (zfs_vdev_sync_read_min_active);
-	case ZIO_PRIORITY_SYNC_WRITE:
-		return (zfs_vdev_sync_write_min_active);
-	case ZIO_PRIORITY_ASYNC_READ:
-		return (zfs_vdev_async_read_min_active);
-	case ZIO_PRIORITY_ASYNC_WRITE:
-		return (zfs_vdev_async_write_min_active);
-	case ZIO_PRIORITY_SCRUB:
-		return (zfs_vdev_scrub_min_active);
-	default:
-		panic("invalid priority %u", p);
-		return (0);
-	}
-}
-
-static int
-vdev_queue_max_async_writes(uint64_t dirty)
-{
-	int writes;
-	uint64_t min_bytes = zfs_dirty_data_max *
-	    zfs_vdev_async_write_active_min_dirty_percent / 100;
-	uint64_t max_bytes = zfs_dirty_data_max *
-	    zfs_vdev_async_write_active_max_dirty_percent / 100;
-
-	if (dirty < min_bytes)
-		return (zfs_vdev_async_write_min_active);
-	if (dirty > max_bytes)
-		return (zfs_vdev_async_write_max_active);
-
-	/*
-	 * linear interpolation:
-	 * slope = (max_writes - min_writes) / (max_bytes - min_bytes)
-	 * move right by min_bytes
-	 * move up by min_writes
-	 */
-	writes = (dirty - min_bytes) *
-	    (zfs_vdev_async_write_max_active -
-	    zfs_vdev_async_write_min_active) /
-	    (max_bytes - min_bytes) +
-	    zfs_vdev_async_write_min_active;
-	ASSERT3U(writes, >=, zfs_vdev_async_write_min_active);
-	ASSERT3U(writes, <=, zfs_vdev_async_write_max_active);
-	return (writes);
-}
-
-static int
-vdev_queue_class_max_active(spa_t *spa, zio_priority_t p)
-{
-	switch (p) {
-	case ZIO_PRIORITY_SYNC_READ:
-		return (zfs_vdev_sync_read_max_active);
-	case ZIO_PRIORITY_SYNC_WRITE:
-		return (zfs_vdev_sync_write_max_active);
-	case ZIO_PRIORITY_ASYNC_READ:
-		return (zfs_vdev_async_read_max_active);
-	case ZIO_PRIORITY_ASYNC_WRITE:
-		return (vdev_queue_max_async_writes(
-		    spa->spa_dsl_pool->dp_dirty_total));
-	case ZIO_PRIORITY_SCRUB:
-		return (zfs_vdev_scrub_max_active);
-	default:
-		panic("invalid priority %u", p);
-		return (0);
-	}
-}
-
-/*
- * Return the i/o class to issue from, or ZIO_PRIORITY_MAX_QUEUEABLE if
- * there is no eligible class.
- */
-static zio_priority_t
-vdev_queue_class_to_issue(vdev_queue_t *vq)
-{
-	spa_t *spa = vq->vq_vdev->vdev_spa;
-	zio_priority_t p;
-
-	if (avl_numnodes(&vq->vq_active_tree) >= zfs_vdev_max_active)
-		return (ZIO_PRIORITY_NUM_QUEUEABLE);
-
-	/* find a queue that has not reached its minimum # outstanding i/os */
-	for (p = 0; p < ZIO_PRIORITY_NUM_QUEUEABLE; p++) {
-		if (avl_numnodes(&vq->vq_class[p].vqc_queued_tree) > 0 &&
-		    vq->vq_class[p].vqc_active <
-		    vdev_queue_class_min_active(p))
-			return (p);
-	}
-
-	/*
-	 * If we haven't found a queue, look for one that hasn't reached its
-	 * maximum # outstanding i/os.
-	 */
-	for (p = 0; p < ZIO_PRIORITY_NUM_QUEUEABLE; p++) {
-		if (avl_numnodes(&vq->vq_class[p].vqc_queued_tree) > 0 &&
-		    vq->vq_class[p].vqc_active <
-		    vdev_queue_class_max_active(spa, p))
-			return (p);
-	}
-
-	/* No eligible queued i/os */
-	return (ZIO_PRIORITY_NUM_QUEUEABLE);
-}
-
 /*
  * Compute the range spanned by two i/os, which is the endpoint of the last
  * (lio->io_offset + lio->io_size) minus start of the first (fio->io_offset).
@@ -464,26 +234,20 @@ vdev_queue_class_to_issue(vdev_queue_t *vq)
 #define	IO_GAP(fio, lio) (-IO_SPAN(lio, fio))
 
 static zio_t *
-vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
+vdev_queue_io_to_issue(vdev_queue_t *vq, uint64_t pending_limit)
 {
-	zio_t *first, *last, *aio, *dio, *mandatory, *nio;
-	uint64_t maxgap = 0;
-	uint64_t size;
-	boolean_t stretch = B_FALSE;
-	vdev_queue_class_t *vqc = &vq->vq_class[zio->io_priority];
-	avl_tree_t *t = &vqc->vqc_queued_tree;
-	enum zio_flag flags = zio->io_flags & ZIO_FLAG_AGG_INHERIT;
-
-	if (zio->io_flags & ZIO_FLAG_DONT_AGGREGATE)
-		return (NULL);
+	zio_t *fio, *lio, *aio, *dio, *nio, *mio;
+	avl_tree_t *t;
+	int flags;
+	uint64_t maxspan = zfs_vdev_aggregation_limit;
+	uint64_t maxgap;
+	int stretch;
 
-	/*
-	 * The synchronous i/o queues are not sorted by LBA, so we can't
-	 * find adjacent i/os.  These i/os tend to not be tightly clustered,
-	 * or too large to aggregate, so this has little impact on performance.
-	 */
-	if (zio->io_priority == ZIO_PRIORITY_SYNC_READ ||
-	    zio->io_priority == ZIO_PRIORITY_SYNC_WRITE)
+again:
+	ASSERT(MUTEX_HELD(&vq->vq_lock));
+
+	if (avl_numnodes(&vq->vq_pending_tree) >= pending_limit ||
+	    avl_numnodes(&vq->vq_deadline_tree) == 0)
 		return (NULL);
 
 #ifdef _KERNEL
@@ -492,170 +256,136 @@ vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
 	fio = lio = avl_first(&vq->vq_deadline_tree);
 #endif
 
-	first = last = zio;
-
-	if (zio->io_type == ZIO_TYPE_READ)
-		maxgap = zfs_vdev_read_gap_limit;
+	t = fio->io_vdev_tree;
+	flags = fio->io_flags & ZIO_FLAG_AGG_INHERIT;
+	maxgap = (t == &vq->vq_read_tree) ? zfs_vdev_read_gap_limit : 0;
 
-	/*
-	 * We can aggregate I/Os that are sufficiently adjacent and of
-	 * the same flavor, as expressed by the AGG_INHERIT flags.
-	 * The latter requirement is necessary so that certain
-	 * attributes of the I/O, such as whether it's a normal I/O
-	 * or a scrub/resilver, can be preserved in the aggregate.
-	 * We can include optional I/Os, but don't allow them
-	 * to begin a range as they add no benefit in that situation.
-	 */
+	if (!(flags & ZIO_FLAG_DONT_AGGREGATE)) {
+		/*
+		 * We can aggregate I/Os that are sufficiently adjacent and of
+		 * the same flavor, as expressed by the AGG_INHERIT flags.
+		 * The latter requirement is necessary so that certain
+		 * attributes of the I/O, such as whether it's a normal I/O
+		 * or a scrub/resilver, can be preserved in the aggregate.
+		 * We can include optional I/Os, but don't allow them
+		 * to begin a range as they add no benefit in that situation.
+		 */
 
-	/*
-	 * We keep track of the last non-optional I/O.
-	 */
-	mandatory = (first->io_flags & ZIO_FLAG_OPTIONAL) ? NULL : first;
+		/*
+		 * We keep track of the last non-optional I/O.
+		 */
+		mio = (fio->io_flags & ZIO_FLAG_OPTIONAL) ? NULL : fio;
 
-	/*
-	 * Walk backwards through sufficiently contiguous I/Os
-	 * recording the last non-option I/O.
-	 */
-	while ((dio = AVL_PREV(t, first)) != NULL &&
-	    (dio->io_flags & ZIO_FLAG_AGG_INHERIT) == flags &&
-	    IO_SPAN(dio, last) <= zfs_vdev_aggregation_limit &&
-	    IO_GAP(dio, first) <= maxgap) {
-		first = dio;
-		if (mandatory == NULL && !(first->io_flags & ZIO_FLAG_OPTIONAL))
-			mandatory = first;
-	}
+		/*
+		 * Walk backwards through sufficiently contiguous I/Os
+		 * recording the last non-option I/O.
+		 */
+		while ((dio = AVL_PREV(t, fio)) != NULL &&
+		    (dio->io_flags & ZIO_FLAG_AGG_INHERIT) == flags &&
+		    IO_SPAN(dio, lio) <= maxspan &&
+		    IO_GAP(dio, fio) <= maxgap) {
+			fio = dio;
+			if (mio == NULL && !(fio->io_flags & ZIO_FLAG_OPTIONAL))
+				mio = fio;
+		}
 
-	/*
-	 * Skip any initial optional I/Os.
-	 */
-	while ((first->io_flags & ZIO_FLAG_OPTIONAL) && first != last) {
-		first = AVL_NEXT(t, first);
-		ASSERT(first != NULL);
-	}
+		/*
+		 * Skip any initial optional I/Os.
+		 */
+		while ((fio->io_flags & ZIO_FLAG_OPTIONAL) && fio != lio) {
+			fio = AVL_NEXT(t, fio);
+			ASSERT(fio != NULL);
+		}
 
-	/*
-	 * Walk forward through sufficiently contiguous I/Os.
-	 */
-	while ((dio = AVL_NEXT(t, last)) != NULL &&
-	    (dio->io_flags & ZIO_FLAG_AGG_INHERIT) == flags &&
-	    IO_SPAN(first, dio) <= zfs_vdev_aggregation_limit &&
-	    IO_GAP(last, dio) <= maxgap) {
-		last = dio;
-		if (!(last->io_flags & ZIO_FLAG_OPTIONAL))
-			mandatory = last;
-	}
+		/*
+		 * Walk forward through sufficiently contiguous I/Os.
+		 */
+		while ((dio = AVL_NEXT(t, lio)) != NULL &&
+		    (dio->io_flags & ZIO_FLAG_AGG_INHERIT) == flags &&
+		    IO_SPAN(fio, dio) <= maxspan &&
+		    IO_GAP(lio, dio) <= maxgap) {
+			lio = dio;
+			if (!(lio->io_flags & ZIO_FLAG_OPTIONAL))
+				mio = lio;
+		}
 
-	/*
-	 * Now that we've established the range of the I/O aggregation
-	 * we must decide what to do with trailing optional I/Os.
-	 * For reads, there's nothing to do. While we are unable to
-	 * aggregate further, it's possible that a trailing optional
-	 * I/O would allow the underlying device to aggregate with
-	 * subsequent I/Os. We must therefore determine if the next
-	 * non-optional I/O is close enough to make aggregation
-	 * worthwhile.
-	 */
-	if (zio->io_type == ZIO_TYPE_WRITE && mandatory != NULL) {
-		zio_t *nio = last;
-		while ((dio = AVL_NEXT(t, nio)) != NULL &&
-		    IO_GAP(nio, dio) == 0 &&
-		    IO_GAP(mandatory, dio) <= zfs_vdev_write_gap_limit) {
-			nio = dio;
-			if (!(nio->io_flags & ZIO_FLAG_OPTIONAL)) {
-				stretch = B_TRUE;
-				break;
+		/*
+		 * Now that we've established the range of the I/O aggregation
+		 * we must decide what to do with trailing optional I/Os.
+		 * For reads, there's nothing to do. While we are unable to
+		 * aggregate further, it's possible that a trailing optional
+		 * I/O would allow the underlying device to aggregate with
+		 * subsequent I/Os. We must therefore determine if the next
+		 * non-optional I/O is close enough to make aggregation
+		 * worthwhile.
+		 */
+		stretch = B_FALSE;
+		if (t != &vq->vq_read_tree && mio != NULL) {
+			nio = lio;
+			while ((dio = AVL_NEXT(t, nio)) != NULL &&
+			    IO_GAP(nio, dio) == 0 &&
+			    IO_GAP(mio, dio) <= zfs_vdev_write_gap_limit) {
+				nio = dio;
+				if (!(nio->io_flags & ZIO_FLAG_OPTIONAL)) {
+					stretch = B_TRUE;
+					break;
+				}
 			}
 		}
-	}
 
-	if (stretch) {
-		/* This may be a no-op. */
-		dio = AVL_NEXT(t, last);
-		dio->io_flags &= ~ZIO_FLAG_OPTIONAL;
-	} else {
-		while (last != mandatory && last != first) {
-			ASSERT(last->io_flags & ZIO_FLAG_OPTIONAL);
-			last = AVL_PREV(t, last);
-			ASSERT(last != NULL);
+		if (stretch) {
+			/* This may be a no-op. */
+			VERIFY((dio = AVL_NEXT(t, lio)) != NULL);
+			dio->io_flags &= ~ZIO_FLAG_OPTIONAL;
+		} else {
+			while (lio != mio && lio != fio) {
+				ASSERT(lio->io_flags & ZIO_FLAG_OPTIONAL);
+				lio = AVL_PREV(t, lio);
+				ASSERT(lio != NULL);
+			}
 		}
 	}
 
-	if (first == last)
-		return (NULL);
-
-	size = IO_SPAN(first, last);
-	ASSERT3U(size, <=, zfs_vdev_aggregation_limit);
-
-	aio = zio_vdev_delegated_io(first->io_vd, first->io_offset,
-	    zio_buf_alloc(size), size, first->io_type, zio->io_priority,
-	    flags | ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_QUEUE,
-	    vdev_queue_agg_io_done, NULL);
-	aio->io_timestamp = first->io_timestamp;
-
-	nio = first;
-	do {
-		dio = nio;
-		nio = AVL_NEXT(t, dio);
-		ASSERT3U(dio->io_type, ==, aio->io_type);
-
-		if (dio->io_flags & ZIO_FLAG_NODATA) {
-			ASSERT3U(dio->io_type, ==, ZIO_TYPE_WRITE);
-			bzero((char *)aio->io_data + (dio->io_offset -
-			    aio->io_offset), dio->io_size);
-		} else if (dio->io_type == ZIO_TYPE_WRITE) {
-			bcopy(dio->io_data, (char *)aio->io_data +
-			    (dio->io_offset - aio->io_offset),
-			    dio->io_size);
-		}
-
-		zio_add_child(dio, aio);
-		vdev_queue_io_remove(vq, dio);
-		zio_vdev_io_bypass(dio);
-		zio_execute(dio);
-	} while (dio != last);
-
-	return (aio);
-}
-
-static zio_t *
-vdev_queue_io_to_issue(vdev_queue_t *vq)
-{
-	zio_t *zio, *aio;
-	zio_priority_t p;
-	avl_index_t idx;
-	vdev_queue_class_t *vqc;
-	zio_t search;
+	if (fio != lio) {
+		uint64_t size = IO_SPAN(fio, lio);
+		ASSERT(size <= zfs_vdev_aggregation_limit);
+
+		aio = zio_vdev_delegated_io(fio->io_vd, fio->io_offset,
+		    zio_buf_alloc(size), size, fio->io_type, ZIO_PRIORITY_AGG,
+		    flags | ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_QUEUE,
+		    vdev_queue_agg_io_done, NULL);
+		aio->io_timestamp = fio->io_timestamp;
+
+		nio = fio;
+		do {
+			dio = nio;
+			nio = AVL_NEXT(t, dio);
+			ASSERT(dio->io_type == aio->io_type);
+			ASSERT(dio->io_vdev_tree == t);
+
+			if (dio->io_flags & ZIO_FLAG_NODATA) {
+				ASSERT(dio->io_type == ZIO_TYPE_WRITE);
+				bzero((char *)aio->io_data + (dio->io_offset -
+				    aio->io_offset), dio->io_size);
+			} else if (dio->io_type == ZIO_TYPE_WRITE) {
+				bcopy(dio->io_data, (char *)aio->io_data +
+				    (dio->io_offset - aio->io_offset),
+				    dio->io_size);
+			}
 
-again:
-	ASSERT(MUTEX_HELD(&vq->vq_lock));
+			zio_add_child(dio, aio);
+			vdev_queue_io_remove(vq, dio);
+			zio_vdev_io_bypass(dio);
+			zio_execute(dio);
+		} while (dio != lio);
 
-	p = vdev_queue_class_to_issue(vq);
+		vdev_queue_pending_add(vq, aio);
 
-	if (p == ZIO_PRIORITY_NUM_QUEUEABLE) {
-		/* No eligible queued i/os */
-		return (NULL);
+		return (aio);
 	}
 
-	/*
-	 * For LBA-ordered queues (async / scrub), issue the i/o which follows
-	 * the most recently issued i/o in LBA (offset) order.
-	 *
-	 * For FIFO queues (sync), issue the i/o with the lowest timestamp.
-	 */
-	vqc = &vq->vq_class[p];
-	search.io_timestamp = 0;
-	search.io_offset = vq->vq_last_offset + 1;
-	VERIFY3P(avl_find(&vqc->vqc_queued_tree, &search, &idx), ==, NULL);
-	zio = avl_nearest(&vqc->vqc_queued_tree, idx, AVL_AFTER);
-	if (zio == NULL)
-		zio = avl_first(&vqc->vqc_queued_tree);
-	ASSERT3U(zio->io_priority, ==, p);
-
-	aio = vdev_queue_aggregate(vq, zio);
-	if (aio != NULL)
-		zio = aio;
-	else
-		vdev_queue_io_remove(vq, zio);
+	ASSERT(fio->io_vdev_tree == t);
+	vdev_queue_io_remove(vq, fio);
 
 	/*
 	 * If the I/O is or was optional and therefore has no data, we need to
@@ -663,18 +393,17 @@ again:
 	 * deadlock that we could encounter since this I/O will complete
 	 * immediately.
 	 */
-	if (zio->io_flags & ZIO_FLAG_NODATA) {
+	if (fio->io_flags & ZIO_FLAG_NODATA) {
 		mutex_exit(&vq->vq_lock);
-		zio_vdev_io_bypass(zio);
-		zio_execute(zio);
+		zio_vdev_io_bypass(fio);
+		zio_execute(fio);
 		mutex_enter(&vq->vq_lock);
 		goto again;
 	}
 
-	vdev_queue_pending_add(vq, zio);
-	vq->vq_last_offset = zio->io_offset;
+	vdev_queue_pending_add(vq, fio);
 
-	return (zio);
+	return (fio);
 }
 
 zio_t *
@@ -683,31 +412,28 @@ vdev_queue_io(zio_t *zio)
 	vdev_queue_t *vq = &zio->io_vd->vdev_queue;
 	zio_t *nio;
 
+	ASSERT(zio->io_type == ZIO_TYPE_READ || zio->io_type == ZIO_TYPE_WRITE);
+
 	if (zio->io_flags & ZIO_FLAG_DONT_QUEUE)
 		return (zio);
 
-	/*
-	 * Children i/os inherent their parent's priority, which might
-	 * not match the child's i/o type.  Fix it up here.
-	 */
-	if (zio->io_type == ZIO_TYPE_READ) {
-		if (zio->io_priority != ZIO_PRIORITY_SYNC_READ &&
-		    zio->io_priority != ZIO_PRIORITY_ASYNC_READ &&
-		    zio->io_priority != ZIO_PRIORITY_SCRUB)
-			zio->io_priority = ZIO_PRIORITY_ASYNC_READ;
-	} else {
-		ASSERT(zio->io_type == ZIO_TYPE_WRITE);
-		if (zio->io_priority != ZIO_PRIORITY_SYNC_WRITE &&
-		    zio->io_priority != ZIO_PRIORITY_ASYNC_WRITE)
-			zio->io_priority = ZIO_PRIORITY_ASYNC_WRITE;
-	}
-
 	zio->io_flags |= ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_QUEUE;
 
+	if (zio->io_type == ZIO_TYPE_READ)
+		zio->io_vdev_tree = &vq->vq_read_tree;
+	else
+		zio->io_vdev_tree = &vq->vq_write_tree;
+
 	mutex_enter(&vq->vq_lock);
+
 	zio->io_timestamp = gethrtime();
+	zio->io_deadline = (zio->io_timestamp >> zfs_vdev_time_shift) +
+	    zio->io_priority;
+
 	vdev_queue_io_add(vq, zio);
-	nio = vdev_queue_io_to_issue(vq);
+
+	nio = vdev_queue_io_to_issue(vq, zfs_vdev_min_pending);
+
 	mutex_exit(&vq->vq_lock);
 
 	if (nio == NULL)
@@ -725,7 +451,6 @@ void
 vdev_queue_io_done(zio_t *zio)
 {
 	vdev_queue_t *vq = &zio->io_vd->vdev_queue;
-	zio_t *nio;
 
 	if (zio_injection_enabled)
 		delay(SEC_TO_TICK(zio_handle_io_delay(zio)));
@@ -736,7 +461,10 @@ vdev_queue_io_done(zio_t *zio)
 
 	vq->vq_io_complete_ts = gethrtime();
 
-	while ((nio = vdev_queue_io_to_issue(vq)) != NULL) {
+	for (int i = 0; i < zfs_vdev_ramp_rate; i++) {
+		zio_t *nio = vdev_queue_io_to_issue(vq, zfs_vdev_max_pending);
+		if (nio == NULL)
+			break;
 		mutex_exit(&vq->vq_lock);
 		if (nio->io_done == vdev_queue_agg_io_done) {
 			zio_nowait(nio);