1 files changed, 76 insertions, 0 deletions
diff --git a/usr/src/uts/common/fs/zfs/zap_micro.c b/usr/src/uts/common/fs/zfs/zap_micro.c
index 06d8128c7c..fbc93b423d 100644
--- a/usr/src/uts/common/fs/zfs/zap_micro.c
+++ b/usr/src/uts/common/fs/zfs/zap_micro.c
@@ -1065,3 +1065,79 @@ zap_get_stats(objset_t *os, uint64_t zapobj, zap_stats_t *zs)
 	zap_unlockdir(zap);
 	return (0);
 }
+
+int
+zap_count_write(objset_t *os, uint64_t zapobj, const char *name, int add,
+    uint64_t *towrite, uint64_t *tooverwrite, uint64_t dn_datablkshift)
+{
+	zap_t *zap;
+	int err = 0;
+
+
+	/*
+	 * Since, we don't have a name, we cannot figure out which blocks will
+	 * be affected in this operation. So, account for the worst case :
+	 * - 3 blocks overwritten: target leaf, ptrtbl block, header block
+	 * - 4 new blocks written if adding:
+	 * 	- 2 blocks for possibly split leaves,
+	 * 	- 2 grown ptrtbl blocks
+	 *
+	 * This also accomodates the case where an add operation to a fairly
+	 * large microzap results in a promotion to fatzap.
+	 */
+	if (name == NULL) {
+		*towrite += (3 + (add ? 4 : 0)) * SPA_MAXBLOCKSIZE;
+		return (err);
+	}
+
+	/*
+	 * We lock the zap with adding ==  FALSE. Because, if we pass
+	 * the actual value of add, it could trigger a mzap_upgrade().
+	 * At present we are just evaluating the possibility of this operation
+	 * and hence we donot want to trigger an upgrade.
+	 */
+	err = zap_lockdir(os, zapobj, NULL, RW_READER, TRUE, FALSE, &zap);
+	if (err)
+		return (err);
+
+	if (!zap->zap_ismicro) {
+		zap_name_t *zn = zap_name_alloc(zap, name, MT_EXACT);
+		if (zn) {
+			err = fzap_count_write(zn, add, towrite,
+			    tooverwrite);
+			zap_name_free(zn);
+		} else {
+			/*
+			 * We treat this case as similar to (name == NULL)
+			 */
+			*towrite += (3 + (add ? 4 : 0)) * SPA_MAXBLOCKSIZE;
+		}
+	} else {
+		if (!add) {
+			if (dmu_buf_freeable(zap->zap_dbuf))
+				*tooverwrite += SPA_MAXBLOCKSIZE;
+			else
+				*towrite += SPA_MAXBLOCKSIZE;
+		} else {
+			/*
+			 * We are here if we are adding and (name != NULL).
+			 * It is hard to find out if this add will promote this
+			 * microzap to fatzap. Hence, we assume the worst case
+			 * and account for the blocks assuming this microzap
+			 * would be promoted to a fatzap.
+			 *
+			 * 1 block overwritten  : header block
+			 * 4 new blocks written : 2 new split leaf, 2 grown
+			 *			ptrtbl blocks
+			 */
+			if (dmu_buf_freeable(zap->zap_dbuf))
+				*tooverwrite += 1 << dn_datablkshift;
+			else
+				*towrite += 1 << dn_datablkshift;
+			*towrite += 4 << dn_datablkshift;
+		}
+	}
+
+	zap_unlockdir(zap);
+	return (err);
+}