Improve allocation fallback handling

Before this change in case of any allocation error ZFS always fallen
back to normal class.  But with more of different classes available
we migth want more sophisticated logic.  For example, it makes sense
to fall back from dedup first to special class (if it is allowed to
put DDT there) and only then to normal, since in a pool with dedup
and special classes populated normal class likely has performance
characteristics unsuitable for dedup.

This change implements general mechanism where fallback order is
controlled by the same spa_preferred_class() as the initial class
selection.  And as first application it implements the mentioned
dedup->special->normal fallbacks.  I have more plans for it later.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes openzfs#17391

Author: amotin

include/sys/spa.h

@@ -1116,7 +1116,9 @@ extern boolean_t spa_has_spare(spa_t *, uint64_t guid);
 extern uint64_t dva_get_dsize_sync(spa_t *spa, const dva_t *dva);
 extern uint64_t bp_get_dsize_sync(spa_t *spa, const blkptr_t *bp);
 extern uint64_t bp_get_dsize(spa_t *spa, const blkptr_t *bp);
+extern boolean_t spa_has_dedup(spa_t *spa);
 extern boolean_t spa_has_slogs(spa_t *spa);
+extern boolean_t spa_has_special(spa_t *spa);
 extern boolean_t spa_is_root(spa_t *spa);
 extern boolean_t spa_writeable(spa_t *spa);
 extern boolean_t spa_has_pending_synctask(spa_t *spa);

module/zfs/ddt.c

@@ -1037,29 +1037,18 @@ ddt_remove(ddt_t *ddt, ddt_entry_t *dde)
 	ddt_free(ddt, dde);
 }
 
+/*
+ * We're considered over quota when we hit 85% full, or for larger drives,
+ * when there is less than 8GB free.
+ */
 static boolean_t
-ddt_special_over_quota(spa_t *spa, metaslab_class_t *mc)
-{
-	if (mc != NULL && metaslab_class_get_space(mc) > 0) {
-		/* Over quota if allocating outside of this special class */
-		if (spa_syncing_txg(spa) <= spa->spa_dedup_class_full_txg +
-		    dedup_class_wait_txgs) {
-			/* Waiting for some deferred frees to be processed */
-			return (B_TRUE);
-		}
-
-		/*
-		 * We're considered over quota when we hit 85% full, or for
-		 * larger drives, when there is less than 8GB free.
-		 */
-		uint64_t allocated = metaslab_class_get_alloc(mc);
-		uint64_t capacity = metaslab_class_get_space(mc);
-		uint64_t limit = MAX(capacity * 85 / 100,
-		    (capacity > (1LL<<33)) ? capacity - (1LL<<33) : 0);
-
-		return (allocated >= limit);
-	}
-	return (B_FALSE);
+ddt_special_over_quota(metaslab_class_t *mc)
+{
+	uint64_t allocated = metaslab_class_get_alloc(mc);
+	uint64_t capacity = metaslab_class_get_space(mc);
+	uint64_t limit = MAX(capacity * 85 / 100,
+	    (capacity > (1LL<<33)) ? capacity - (1LL<<33) : 0);
+	return (allocated >= limit);
 }
 
 /*
@@ -1082,13 +1071,21 @@ ddt_over_quota(spa_t *spa)
 		return (ddt_get_ddt_dsize(spa) > spa->spa_dedup_table_quota);
 
 	/*
-	 * For automatic quota, table size is limited by dedup or special class
+	 * Over quota if have to allocate outside of the dedup/special class.
 	 */
-	if (ddt_special_over_quota(spa, spa_dedup_class(spa)))
-		return (B_TRUE);
-	else if (spa_special_has_ddt(spa) &&
-	    ddt_special_over_quota(spa, spa_special_class(spa)))
+	if (spa_syncing_txg(spa) <= spa->spa_dedup_class_full_txg +
+	    dedup_class_wait_txgs) {
+		/* Waiting for some deferred frees to be processed */
 		return (B_TRUE);
+	}
+
+	/*
+	 * For automatic quota, table size is limited by dedup or special class
+	 */
+	if (spa_has_dedup(spa))
+		return (ddt_special_over_quota(spa_dedup_class(spa)));
+	else if (spa_special_has_ddt(spa))
+		return (ddt_special_over_quota(spa_special_class(spa)));
 
 	return (B_FALSE);
 }

module/zfs/spa_misc.c

@@ -2008,8 +2008,7 @@ spa_dedup_class(spa_t *spa)
 boolean_t
 spa_special_has_ddt(spa_t *spa)
 {
-	return (zfs_ddt_data_is_special &&
-	    spa->spa_special_class->mc_groups != 0);
+	return (zfs_ddt_data_is_special && spa_has_special(spa));
 }
 
 /*
@@ -2018,6 +2017,9 @@ spa_special_has_ddt(spa_t *spa)
 metaslab_class_t *
 spa_preferred_class(spa_t *spa, const zio_t *zio)
 {
+	metaslab_class_t *mc = zio->io_metaslab_class;
+	boolean_t tried_dedup = (mc == spa_dedup_class(spa));
+	boolean_t tried_special = (mc == spa_special_class(spa));
 	const zio_prop_t *zp = &zio->io_prop;
 
 	/*
@@ -2035,12 +2037,10 @@ spa_preferred_class(spa_t *spa, const zio_t *zio)
 	 */
 	ASSERT(objtype != DMU_OT_INTENT_LOG);
 
-	boolean_t has_special_class = spa->spa_special_class->mc_groups != 0;
-
 	if (DMU_OT_IS_DDT(objtype)) {
-		if (spa->spa_dedup_class->mc_groups != 0)
+		if (spa_has_dedup(spa) && !tried_dedup && !tried_special)
 			return (spa_dedup_class(spa));
-		else if (has_special_class && zfs_ddt_data_is_special)
+		else if (spa_special_has_ddt(spa) && !tried_special)
 			return (spa_special_class(spa));
 		else
 			return (spa_normal_class(spa));
@@ -2049,14 +2049,15 @@ spa_preferred_class(spa_t *spa, const zio_t *zio)
 	/* Indirect blocks for user data can land in special if allowed */
 	if (zp->zp_level > 0 &&
 	    (DMU_OT_IS_FILE(objtype) || objtype == DMU_OT_ZVOL)) {
-		if (has_special_class && zfs_user_indirect_is_special)
+		if (zfs_user_indirect_is_special && spa_has_special(spa) &&
+		    !tried_special)
 			return (spa_special_class(spa));
 		else
 			return (spa_normal_class(spa));
 	}
 
 	if (DMU_OT_IS_METADATA(objtype) || zp->zp_level > 0) {
-		if (has_special_class)
+		if (spa_has_special(spa) && !tried_special)
 			return (spa_special_class(spa));
 		else
 			return (spa_normal_class(spa));
@@ -2068,7 +2069,8 @@ spa_preferred_class(spa_t *spa, const zio_t *zio)
 	 * zfs_special_class_metadata_reserve_pct exclusively for metadata.
 	 */
 	if ((DMU_OT_IS_FILE(objtype) || objtype == DMU_OT_ZVOL) &&
-	    has_special_class && zio->io_size <= zp->zp_zpl_smallblk) {
+	    spa_has_special(spa) && !tried_special &&
+	    zio->io_size <= zp->zp_zpl_smallblk) {
 		metaslab_class_t *special = spa_special_class(spa);
 		uint64_t alloc = metaslab_class_get_alloc(special);
 		uint64_t space = metaslab_class_get_space(special);
@@ -2639,6 +2641,12 @@ spa_fini(void)
 	mutex_destroy(&spa_l2cache_lock);
 }
 
+boolean_t
+spa_has_dedup(spa_t *spa)
+{
+	return (spa->spa_dedup_class->mc_groups != 0);
+}
+
 /*
  * Return whether this pool has a dedicated slog device. No locking needed.
  * It's not a problem if the wrong answer is returned as it's only for
@@ -2650,6 +2658,12 @@ spa_has_slogs(spa_t *spa)
 	return (spa->spa_log_class->mc_groups != 0);
 }
 
+boolean_t
+spa_has_special(spa_t *spa)
+{
+	return (spa->spa_special_class->mc_groups != 0);
+}
+
 spa_log_state_t
 spa_get_log_state(spa_t *spa)
 {

module/zfs/zio.c

@@ -4150,7 +4150,7 @@ static zio_t *
 zio_dva_allocate(zio_t *zio)
 {
 	spa_t *spa = zio->io_spa;
-	metaslab_class_t *mc;
+	metaslab_class_t *mc, *newmc;
 	blkptr_t *bp = zio->io_bp;
 	int error;
 	int flags = 0;
@@ -4193,7 +4193,7 @@ zio_dva_allocate(zio_t *zio)
 again:
 	/*
 	 * Try allocating the block in the usual metaslab class.
-	 * If that's full, allocate it in the normal class.
+	 * If that's full, allocate it in some other class(es).
 	 * If that's full, allocate as a gang block,
 	 * and if all are full, the allocation fails (which shouldn't happen).
 	 *
@@ -4208,29 +4208,29 @@ zio_dva_allocate(zio_t *zio)
 	    &zio->io_alloc_list, zio->io_allocator, zio);
 
 	/*
-	 * Fallback to normal class when an alloc class is full
+	 * When the dedup or special class is spilling into the normal class,
+	 * there can still be significant space available due to deferred
+	 * frees that are in-flight.  We track the txg when this occurred and
+	 * back off adding new DDT entries for a few txgs to allow the free
+	 * blocks to be processed.
 	 */
-	if (error == ENOSPC && mc != spa_normal_class(spa)) {
-		/*
-		 * When the dedup or special class is spilling into the  normal
-		 * class, there can still be significant space available due
-		 * to deferred frees that are in-flight.  We track the txg when
-		 * this occurred and back off adding new DDT entries for a few
-		 * txgs to allow the free blocks to be processed.
-		 */
-		if ((mc == spa_dedup_class(spa) || (spa_special_has_ddt(spa) &&
-		    mc == spa_special_class(spa))) &&
-		    spa->spa_dedup_class_full_txg != zio->io_txg) {
-			spa->spa_dedup_class_full_txg = zio->io_txg;
-			zfs_dbgmsg("%s[%d]: %s class spilling, req size %d, "
-			    "%llu allocated of %llu",
-			    spa_name(spa), (int)zio->io_txg,
-			    mc == spa_dedup_class(spa) ? "dedup" : "special",
-			    (int)zio->io_size,
-			    (u_longlong_t)metaslab_class_get_alloc(mc),
-			    (u_longlong_t)metaslab_class_get_space(mc));
-		}
+	if (error == ENOSPC && spa->spa_dedup_class_full_txg != zio->io_txg &&
+	    (mc == spa_dedup_class(spa) || (mc == spa_special_class(spa) &&
+	    !spa_has_dedup(spa) && spa_special_has_ddt(spa)))) {
+		spa->spa_dedup_class_full_txg = zio->io_txg;
+		zfs_dbgmsg("%s[%llu]: %s class spilling, req size %llu, "
+		    "%llu allocated of %llu",
+		    spa_name(spa), (u_longlong_t)zio->io_txg,
+		    mc == spa_dedup_class(spa) ? "dedup" : "special",
+		    (u_longlong_t)zio->io_size,
+		    (u_longlong_t)metaslab_class_get_alloc(mc),
+		    (u_longlong_t)metaslab_class_get_space(mc));
+	}
 
+	/*
+	 * Fall back to some other class when this one is full.
+	 */
+	if (error == ENOSPC && (newmc = spa_preferred_class(spa, zio)) != mc) {
 		/*
 		 * If we are holding old class reservation, drop it.
 		 * Dispatch the next ZIO(s) there if some are waiting.
@@ -4246,15 +4246,15 @@ zio_dva_allocate(zio_t *zio)
 
 		if (zfs_flags & ZFS_DEBUG_METASLAB_ALLOC) {
 			zfs_dbgmsg("%s: metaslab allocation failure, "
-			    "trying normal class: zio %px, size %llu, error %d",
+			    "trying fallback: zio %px, size %llu, error %d",
 			    spa_name(spa), zio, (u_longlong_t)zio->io_size,
 			    error);
 		}
-		zio->io_metaslab_class = mc = spa_normal_class(spa);
+		zio->io_metaslab_class = mc = newmc;
 		ZIOSTAT_BUMP(ziostat_alloc_class_fallbacks);
 
 		/*
-		 * If normal class uses throttling, return to that pipeline
+		 * If the new class uses throttling, return to that pipeline
 		 * stage.  Otherwise just do another allocation attempt.
 		 */
 		if (zio->io_priority != ZIO_PRIORITY_SYNC_WRITE &&