Set aside a metaslab for ZIL blocks

Mixing ZIL and normal allocations has several problems:

1. The ZIL allocations are allocated, written to disk, and then a few
seconds later freed.  This leaves behind holes (free segments) where the
ZIL blocks used to be, which increases fragmentation, which negatively
impacts performance.

2. When under moderate load, ZIL allocations are of 128KB.  If the pool
is fairly fragmented, there may not be many free chunks of that size.
This causes ZFS to load more metaslabs to locate free segments of 128KB
or more.  The loading happens synchronously (from zil_commit()), and can
take around a second even if the metaslab's spacemap is cached in the
ARC.  All concurrent synchronous operations on this filesystem must wait
while the metaslab is loading.  This can cause a significant performance
impact.

3. If the pool is very fragmented, there may be zero free chunks of
128KB or more.  In this case, the ZIL falls back to txg_wait_synced(),
which has an enormous performance impact.

These problems can be eliminated by using a dedicated log device
("slog"), even one with the same performance characteristics as the
normal devices.

This change sets aside one metaslab from each top-level vdev that is
preferentially used for ZIL allocations (vdev_log_mg,
spa_embedded_log_class).  From an allocation perspective, this is
similar to having a dedicated log device, and it eliminates the
above-mentioned performance problems.

Log (ZIL) blocks can be allocated from the following locations.  Each one is
tried in order until the allocation succeeds:
1. dedicated log vdevs, aka "slog" (spa_log_class)
2. embedded slog metaslabs (spa_embedded_log_class)
3. other metaslabs in normal vdevs (spa_normal_class)

The space required for the embedded slog metaslabs is usually between 0.5% and
1.0% of the pool, and comes out of the existing 3.2% of "slop" space that is
not available for user data.

On an all-ssd system with 4TB storage, 87% fragmentation, 60% capacity, and
recordsize=8k, testing shows a ~50% performance increase on random 8k sync
writes.  On even more fragmented systems (which hit problem #3 above and call
txg_wait_synced()), the performance improvement can be arbitrarily large
(>100x).

Signed-off-by: Matthew Ahrens <mahrens@delphix.com>

cmd/zdb/zdb.c

@@ -5844,6 +5844,7 @@ zdb_leak_init_prepare_indirect_vdevs(spa_t *spa, zdb_cb_t *zcb)
 		 * metaslabs.  We want to set them up for
 		 * zio_claim().
 		 */
+		vdev_metaslab_group_create(vd);
 		VERIFY0(vdev_metaslab_init(vd, 0));
 
 		vdev_indirect_mapping_t *vim = vd->vdev_indirect_mapping;
@@ -5882,6 +5883,7 @@ zdb_leak_init(spa_t *spa, zdb_cb_t *zcb)
 	 */
 	spa->spa_normal_class->mc_ops = &zdb_metaslab_ops;
 	spa->spa_log_class->mc_ops = &zdb_metaslab_ops;
+	spa->spa_embedded_log_class->mc_ops = &zdb_metaslab_ops;
 
 	zcb->zcb_vd_obsolete_counts =
 	    umem_zalloc(rvd->vdev_children * sizeof (uint32_t *),
@@ -6015,15 +6017,16 @@ zdb_leak_fini(spa_t *spa, zdb_cb_t *zcb)
 	vdev_t *rvd = spa->spa_root_vdev;
 	for (unsigned c = 0; c < rvd->vdev_children; c++) {
 		vdev_t *vd = rvd->vdev_child[c];
-		metaslab_group_t *mg __maybe_unused = vd->vdev_mg;
 
 		if (zcb->zcb_vd_obsolete_counts[c] != NULL) {
 			leaks |= zdb_check_for_obsolete_leaks(vd, zcb);
 		}
 
 		for (uint64_t m = 0; m < vd->vdev_ms_count; m++) {
 			metaslab_t *msp = vd->vdev_ms[m];
-			ASSERT3P(mg, ==, msp->ms_group);
+			ASSERT3P(msp->ms_group, ==, (msp->ms_group->mg_class ==
+			    spa_embedded_log_class(spa)) ?
+			    vd->vdev_log_mg : vd->vdev_mg);
 
 			/*
 			 * ms_allocatable has been overloaded
@@ -6230,6 +6233,8 @@ dump_block_stats(spa_t *spa)
 	zcb.zcb_totalasize = metaslab_class_get_alloc(spa_normal_class(spa));
 	zcb.zcb_totalasize += metaslab_class_get_alloc(spa_special_class(spa));
 	zcb.zcb_totalasize += metaslab_class_get_alloc(spa_dedup_class(spa));
+	zcb.zcb_totalasize +=
+	    metaslab_class_get_alloc(spa_embedded_log_class(spa));
 	zcb.zcb_start = zcb.zcb_lastprint = gethrtime();
 	err = traverse_pool(spa, 0, flags, zdb_blkptr_cb, &zcb);
 
@@ -6277,6 +6282,7 @@ dump_block_stats(spa_t *spa)
 
 	total_alloc = norm_alloc +
 	    metaslab_class_get_alloc(spa_log_class(spa)) +
+	    metaslab_class_get_alloc(spa_embedded_log_class(spa)) +
 	    metaslab_class_get_alloc(spa_special_class(spa)) +
 	    metaslab_class_get_alloc(spa_dedup_class(spa)) +
 	    get_unflushed_alloc_space(spa);
@@ -6344,6 +6350,17 @@ dump_block_stats(spa_t *spa)
 		    100.0 * alloc / space);
 	}
 
+	if (spa_embedded_log_class(spa)->mc_allocator[0].mca_rotor != NULL) {
+		uint64_t alloc = metaslab_class_get_alloc(
+		    spa_embedded_log_class(spa));
+		uint64_t space = metaslab_class_get_space(
+		    spa_embedded_log_class(spa));
+
+		(void) printf("\t%-16s %14llu     used: %5.2f%%\n",
+		    "Embedded log class", (u_longlong_t)alloc,
+		    100.0 * alloc / space);
+	}
+
 	for (i = 0; i < NUM_BP_EMBEDDED_TYPES; i++) {
 		if (zcb.zcb_embedded_blocks[i] == 0)
 			continue;

include/sys/dmu.h

@@ -142,9 +142,6 @@ typedef enum dmu_object_byteswap {
 #define	DMU_OT_IS_DDT(ot) \
 	((ot) == DMU_OT_DDT_ZAP)
 
-#define	DMU_OT_IS_ZIL(ot) \
-	((ot) == DMU_OT_INTENT_LOG)
-
 /* Note: ztest uses DMU_OT_UINT64_OTHER as a proxy for file blocks */
 #define	DMU_OT_IS_FILE(ot) \
 	((ot) == DMU_OT_PLAIN_FILE_CONTENTS || (ot) == DMU_OT_UINT64_OTHER)

include/sys/spa.h

@@ -1047,6 +1047,7 @@ extern uint64_t spa_version(spa_t *spa);
 extern boolean_t spa_deflate(spa_t *spa);
 extern metaslab_class_t *spa_normal_class(spa_t *spa);
 extern metaslab_class_t *spa_log_class(spa_t *spa);
+extern metaslab_class_t *spa_embedded_log_class(spa_t *spa);
 extern metaslab_class_t *spa_special_class(spa_t *spa);
 extern metaslab_class_t *spa_dedup_class(spa_t *spa);
 extern metaslab_class_t *spa_preferred_class(spa_t *spa, uint64_t size,

include/sys/spa_impl.h

@@ -226,6 +226,7 @@ struct spa {
 	boolean_t	spa_is_exporting;	/* true while exporting pool */
 	metaslab_class_t *spa_normal_class;	/* normal data class */
 	metaslab_class_t *spa_log_class;	/* intent log data class */
+	metaslab_class_t *spa_embedded_log_class; /* log on normal vdevs */
 	metaslab_class_t *spa_special_class;	/* special allocation class */
 	metaslab_class_t *spa_dedup_class;	/* dedup allocation class */
 	uint64_t	spa_first_txg;		/* first txg after spa_open() */

include/sys/vdev.h

@@ -33,6 +33,7 @@
 #include <sys/zio.h>
 #include <sys/dmu.h>
 #include <sys/space_map.h>
+#include <sys/metaslab.h>
 #include <sys/fs/zfs.h>
 
 #ifdef	__cplusplus
@@ -113,6 +114,9 @@ extern void vdev_xlate_walk(vdev_t *vd, const range_seg64_t *logical_rs,
     vdev_xlate_func_t *func, void *arg);
 
 extern void vdev_get_stats_ex(vdev_t *vd, vdev_stat_t *vs, vdev_stat_ex_t *vsx);
+
+extern metaslab_group_t *vdev_get_mg(vdev_t *vd, metaslab_class_t *mc);
+
 extern void vdev_get_stats(vdev_t *vd, vdev_stat_t *vs);
 extern void vdev_clear_stats(vdev_t *vd);
 extern void vdev_stat_update(zio_t *zio, uint64_t psize);

include/sys/vdev_impl.h

@@ -280,6 +280,7 @@ struct vdev {
 	uint64_t	vdev_ms_shift;	/* metaslab size shift		*/
 	uint64_t	vdev_ms_count;	/* number of metaslabs		*/
 	metaslab_group_t *vdev_mg;	/* metaslab group		*/
+	metaslab_group_t *vdev_log_mg;	/* embedded slog metaslab group	*/
 	metaslab_t	**vdev_ms;	/* metaslab array		*/
 	uint64_t	vdev_pending_fastwrite; /* allocated fastwrites */
 	txg_list_t	vdev_ms_list;	/* per-txg dirty metaslab lists	*/
@@ -636,6 +637,7 @@ extern int vdev_obsolete_counts_are_precise(vdev_t *vd, boolean_t *are_precise);
  * Other miscellaneous functions
  */
 int vdev_checkpoint_sm_object(vdev_t *vd, uint64_t *sm_obj);
+void vdev_metaslab_group_create(vdev_t *vd);
 
 /*
  * Vdev ashift optimization tunables

include/sys/zfs_debug.h

@@ -56,6 +56,7 @@ extern int zfs_dbgmsg_enable;
 #define	ZFS_DEBUG_INDIRECT_REMAP	(1 << 10)
 #define	ZFS_DEBUG_TRIM			(1 << 11)
 #define	ZFS_DEBUG_LOG_SPACEMAP		(1 << 12)
+#define	ZFS_DEBUG_METASLAB_ALLOC	(1 << 13)
 
 extern void __set_error(const char *file, const char *func, int line, int err);
 extern void __zfs_dbgmsg(char *buf);

man/man5/zfs-module-parameters.5

@@ -3936,6 +3936,22 @@ to limit potential SLOG device abuse by single active ZIL writer.
 Default value: \fB786,432\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_embedded_slog_min_ms\fR (int)
+.ad
+.RS 12n
+Usually, one metaslab from each (normal-class) vdev is dedicated for use by
+the ZIL (to log synchronous writes).
+However, if there are fewer than zfs_embedded_slog_min_ms metaslabs in the
+vdev, this functionality is disabled.
+This ensures that we don't set aside an unreasonable amount of space for the
+ZIL.
+.sp
+Default value: \fB64\fR.
+.RE
+
 .sp
 .ne 2
 .na

module/zfs/metaslab.c

@@ -524,7 +524,7 @@ metaslab_class_histogram_verify(metaslab_class_t *mc)
 
 	for (int c = 0; c < rvd->vdev_children; c++) {
 		vdev_t *tvd = rvd->vdev_child[c];
-		metaslab_group_t *mg = tvd->vdev_mg;
+		metaslab_group_t *mg = vdev_get_mg(tvd, mc);
 
 		/*
 		 * Skip any holes, uninitialized top-levels, or
@@ -535,12 +535,16 @@ metaslab_class_histogram_verify(metaslab_class_t *mc)
 			continue;
 		}
 
+		IMPLY(mg == mg->mg_vd->vdev_log_mg,
+		    mc == spa_embedded_log_class(mg->mg_vd->vdev_spa));
+
 		for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i++)
 			mc_hist[i] += mg->mg_histogram[i];
 	}
 
-	for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i++)
+	for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i++) {
 		VERIFY3U(mc_hist[i], ==, mc->mc_histogram[i]);
+	}
 
 	kmem_free(mc_hist, sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE);
 }
@@ -1004,16 +1008,23 @@ metaslab_group_initialized(metaslab_group_t *mg)
 uint64_t
 metaslab_group_get_space(metaslab_group_t *mg)
 {
-	return ((1ULL << mg->mg_vd->vdev_ms_shift) * mg->mg_vd->vdev_ms_count);
+	/*
+	 * Note that the number of nodes in mg_metaslab_tree may be one less
+	 * than vdev_ms_count, due to the embedded log metaslab.
+	 */
+	mutex_enter(&mg->mg_lock);
+	uint64_t space = (1ULL << mg->mg_vd->vdev_ms_shift) *
+	    avl_numnodes(&mg->mg_metaslab_tree);
+	mutex_exit(&mg->mg_lock);
+	return (space);
 }
 
 void
 metaslab_group_histogram_verify(metaslab_group_t *mg)
 {
 	uint64_t *mg_hist;
-	vdev_t *vd = mg->mg_vd;
-	uint64_t ashift = vd->vdev_ashift;
-	int i;
+	avl_tree_t *t = &mg->mg_metaslab_tree;
+	uint64_t ashift = mg->mg_vd->vdev_ashift;
 
 	if ((zfs_flags & ZFS_DEBUG_HISTOGRAM_VERIFY) == 0)
 		return;
@@ -1024,21 +1035,25 @@ metaslab_group_histogram_verify(metaslab_group_t *mg)
 	ASSERT3U(RANGE_TREE_HISTOGRAM_SIZE, >=,
 	    SPACE_MAP_HISTOGRAM_SIZE + ashift);
 
-	for (int m = 0; m < vd->vdev_ms_count; m++) {
-		metaslab_t *msp = vd->vdev_ms[m];
-
-		/* skip if not active or not a member */
-		if (msp->ms_sm == NULL || msp->ms_group != mg)
+	mutex_enter(&mg->mg_lock);
+	for (metaslab_t *msp = avl_first(t);
+	    msp != NULL; msp = AVL_NEXT(t, msp)) {
+		VERIFY3P(msp->ms_group, ==, mg);
+		/* skip if not active */
+		if (msp->ms_sm == NULL)
 			continue;
 
-		for (i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++)
+		for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
 			mg_hist[i + ashift] +=
 			    msp->ms_sm->sm_phys->smp_histogram[i];
+		}
 	}
 
-	for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i ++)
+	for (int i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i ++)
 		VERIFY3U(mg_hist[i], ==, mg->mg_histogram[i]);
 
+	mutex_exit(&mg->mg_lock);
+
 	kmem_free(mg_hist, sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE);
 }
 
@@ -1054,6 +1069,8 @@ metaslab_group_histogram_add(metaslab_group_t *mg, metaslab_t *msp)
 
 	mutex_enter(&mg->mg_lock);
 	for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
+		IMPLY(mg == mg->mg_vd->vdev_log_mg,
+		    mc == spa_embedded_log_class(mg->mg_vd->vdev_spa));
 		mg->mg_histogram[i + ashift] +=
 		    msp->ms_sm->sm_phys->smp_histogram[i];
 		mc->mc_histogram[i + ashift] +=
@@ -1078,6 +1095,8 @@ metaslab_group_histogram_remove(metaslab_group_t *mg, metaslab_t *msp)
 		    msp->ms_sm->sm_phys->smp_histogram[i]);
 		ASSERT3U(mc->mc_histogram[i + ashift], >=,
 		    msp->ms_sm->sm_phys->smp_histogram[i]);
+		IMPLY(mg == mg->mg_vd->vdev_log_mg,
+		    mc == spa_embedded_log_class(mg->mg_vd->vdev_spa));
 
 		mg->mg_histogram[i + ashift] -=
 		    msp->ms_sm->sm_phys->smp_histogram[i];
@@ -2741,37 +2760,47 @@ metaslab_fini(metaslab_t *msp)
 
 	mutex_enter(&msp->ms_lock);
 	VERIFY(msp->ms_group == NULL);
-	metaslab_space_update(vd, mg->mg_class,
-	    -metaslab_allocated_space(msp), 0, -msp->ms_size);
+	/*
+	 * If the range trees haven't been allocated, this metaslab hasn't
+	 * been through metaslab_sync_done() for the first time yet, so its
+	 * space hasn't been accounted for in its vdev and doesn't need to be
+	 * subtracted.
+	 */
+	if (msp->ms_freed != NULL) {
+		metaslab_space_update(vd, mg->mg_class,
+		    -metaslab_allocated_space(msp), 0, -msp->ms_size);
 
+	}
 	space_map_close(msp->ms_sm);
 	msp->ms_sm = NULL;
 
 	metaslab_unload(msp);
+
 	range_tree_destroy(msp->ms_allocatable);
-	range_tree_destroy(msp->ms_freeing);
-	range_tree_destroy(msp->ms_freed);
 
-	ASSERT3U(spa->spa_unflushed_stats.sus_memused, >=,
-	    metaslab_unflushed_changes_memused(msp));
-	spa->spa_unflushed_stats.sus_memused -=
-	    metaslab_unflushed_changes_memused(msp);
-	range_tree_vacate(msp->ms_unflushed_allocs, NULL, NULL);
-	range_tree_destroy(msp->ms_unflushed_allocs);
-	range_tree_vacate(msp->ms_unflushed_frees, NULL, NULL);
-	range_tree_destroy(msp->ms_unflushed_frees);
+	if (msp->ms_freed != NULL) {
+		range_tree_destroy(msp->ms_freeing);
+		range_tree_destroy(msp->ms_freed);
 
-	for (int t = 0; t < TXG_SIZE; t++) {
-		range_tree_destroy(msp->ms_allocating[t]);
-	}
+		ASSERT3U(spa->spa_unflushed_stats.sus_memused, >=,
+		    metaslab_unflushed_changes_memused(msp));
+		spa->spa_unflushed_stats.sus_memused -=
+		    metaslab_unflushed_changes_memused(msp);
+		range_tree_vacate(msp->ms_unflushed_allocs, NULL, NULL);
+		range_tree_destroy(msp->ms_unflushed_allocs);
+		range_tree_destroy(msp->ms_checkpointing);
+		range_tree_vacate(msp->ms_unflushed_frees, NULL, NULL);
+		range_tree_destroy(msp->ms_unflushed_frees);
 
-	for (int t = 0; t < TXG_DEFER_SIZE; t++) {
-		range_tree_destroy(msp->ms_defer[t]);
+		for (int t = 0; t < TXG_SIZE; t++) {
+			range_tree_destroy(msp->ms_allocating[t]);
+		}
+		for (int t = 0; t < TXG_DEFER_SIZE; t++) {
+			range_tree_destroy(msp->ms_defer[t]);
+		}
 	}
 	ASSERT0(msp->ms_deferspace);
 
-	range_tree_destroy(msp->ms_checkpointing);
-
 	for (int t = 0; t < TXG_SIZE; t++)
 		ASSERT(!txg_list_member(&vd->vdev_ms_list, msp, t));
 
@@ -5113,7 +5142,7 @@ metaslab_alloc_dva(spa_t *spa, metaslab_class_t *mc, uint64_t psize,
 		 * all else fails.
 		 */
 		if (vd != NULL && vd->vdev_mg != NULL) {
-			mg = vd->vdev_mg;
+			mg = vdev_get_mg(vd, mc);
 
 			if (flags & METASLAB_HINTBP_AVOID &&
 			    mg->mg_next != NULL)