diff --git a/include/sys/metaslab_impl.h b/include/sys/metaslab_impl.h
index 658359478..386f6240c 100644
--- a/include/sys/metaslab_impl.h
+++ b/include/sys/metaslab_impl.h
@@ -21,7 +21,10 @@
 /*
  * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
  * Use is subject to license terms.
- * Copyright (c) 2011 by Delphix. All rights reserved.
+ */
+
+/*
+ * Copyright (c) 2012 by Delphix. All rights reserved.
  */
 
 #ifndef _SYS_METASLAB_IMPL_H
diff --git a/include/sys/spa.h b/include/sys/spa.h
index ca9fb2438..8f2af8af2 100644
--- a/include/sys/spa.h
+++ b/include/sys/spa.h
@@ -486,13 +486,7 @@ extern int spa_scan_stop(spa_t *spa);
 extern void spa_sync(spa_t *spa, uint64_t txg); /* only for DMU use */
 extern void spa_sync_allpools(void);
 
-/*
- * DEFERRED_FREE must be large enough that regular blocks are not
- * deferred.  XXX so can't we change it back to 1?
- */
-#define	SYNC_PASS_DEFERRED_FREE	2	/* defer frees after this pass */
-#define	SYNC_PASS_DONT_COMPRESS	4	/* don't compress after this pass */
-#define	SYNC_PASS_REWRITE	1	/* rewrite new bps after this pass */
+extern int zfs_sync_pass_deferred_free;
 
 /* spa namespace global mutex */
 extern kmutex_t spa_namespace_lock;
diff --git a/include/sys/space_map.h b/include/sys/space_map.h
index 332299724..cbe75afd1 100644
--- a/include/sys/space_map.h
+++ b/include/sys/space_map.h
@@ -23,6 +23,10 @@
  * Use is subject to license terms.
  */
 
+/*
+ * Copyright (c) 2012 by Delphix. All rights reserved.
+ */
+
 #ifndef _SYS_SPACE_MAP_H
 #define	_SYS_SPACE_MAP_H
 
@@ -136,6 +140,8 @@ struct space_map_ops {
 
 typedef void space_map_func_t(space_map_t *sm, uint64_t start, uint64_t size);
 
+extern void space_map_init(void);
+extern void space_map_fini(void);
 extern void space_map_create(space_map_t *sm, uint64_t start, uint64_t size,
     uint8_t shift, kmutex_t *lp);
 extern void space_map_destroy(space_map_t *sm);
diff --git a/include/sys/zio_impl.h b/include/sys/zio_impl.h
index d90bd8bd5..2d062d091 100644
--- a/include/sys/zio_impl.h
+++ b/include/sys/zio_impl.h
@@ -23,6 +23,10 @@
  * Use is subject to license terms.
  */
 
+/*
+ * Copyright (c) 2012 by Delphix. All rights reserved.
+ */
+
 #ifndef _ZIO_IMPL_H
 #define	_ZIO_IMPL_H
 
@@ -143,6 +147,7 @@ enum zio_stage {
 #define	ZIO_FREE_PIPELINE			\
 	(ZIO_INTERLOCK_STAGES |			\
 	ZIO_STAGE_FREE_BP_INIT |		\
+	ZIO_STAGE_ISSUE_ASYNC |			\
 	ZIO_STAGE_DVA_FREE)
 
 #define	ZIO_DDT_FREE_PIPELINE			\
diff --git a/module/zfs/metaslab.c b/module/zfs/metaslab.c
index 03d5a1d9c..76dc4f637 100644
--- a/module/zfs/metaslab.c
+++ b/module/zfs/metaslab.c
@@ -899,8 +899,9 @@ metaslab_activate(metaslab_t *msp, uint64_t activation_weight)
 	if ((msp->ms_weight & METASLAB_ACTIVE_MASK) == 0) {
 		space_map_load_wait(sm);
 		if (!sm->sm_loaded) {
-			int error = space_map_load(sm, sm_ops, SM_FREE,
-			    &msp->ms_smo,
+			space_map_obj_t *smo = &msp->ms_smo;
+
+			int error = space_map_load(sm, sm_ops, SM_FREE, smo,
 			    spa_meta_objset(msp->ms_group->mg_vd->vdev_spa));
 			if (error)  {
 				metaslab_group_sort(msp->ms_group, msp, 0);
diff --git a/module/zfs/spa.c b/module/zfs/spa.c
index 12c24879b..22fa7878c 100644
--- a/module/zfs/spa.c
+++ b/module/zfs/spa.c
@@ -6078,7 +6078,7 @@ spa_sync(spa_t *spa, uint64_t txg)
 		spa_errlog_sync(spa, txg);
 		dsl_pool_sync(dp, txg);
 
-		if (pass <= SYNC_PASS_DEFERRED_FREE) {
+		if (pass < zfs_sync_pass_deferred_free) {
 			zio_t *zio = zio_root(spa, NULL, NULL, 0);
 			bplist_iterate(free_bpl, spa_free_sync_cb,
 			    zio, tx);
diff --git a/module/zfs/spa_misc.c b/module/zfs/spa_misc.c
index 476f9c02c..0ca9f3a7a 100644
--- a/module/zfs/spa_misc.c
+++ b/module/zfs/spa_misc.c
@@ -1633,6 +1633,7 @@ spa_init(int mode)
 	fm_init();
 	refcount_init();
 	unique_init();
+	space_map_init();
 	zio_init();
 	dmu_init();
 	zil_init();
@@ -1655,6 +1656,7 @@ spa_fini(void)
 	zil_fini();
 	dmu_fini();
 	zio_fini();
+	space_map_fini();
 	unique_fini();
 	refcount_fini();
 	fm_fini();
diff --git a/module/zfs/space_map.c b/module/zfs/space_map.c
index 9c0cdb6be..d99c7c0b3 100644
--- a/module/zfs/space_map.c
+++ b/module/zfs/space_map.c
@@ -29,6 +29,23 @@
 #include <sys/zio.h>
 #include <sys/space_map.h>
 
+static kmem_cache_t *space_seg_cache;
+
+void
+space_map_init(void)
+{
+	ASSERT(space_seg_cache == NULL);
+	space_seg_cache = kmem_cache_create("space_seg_cache",
+	    sizeof (space_seg_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
+}
+
+void
+space_map_fini(void)
+{
+	kmem_cache_destroy(space_seg_cache);
+	space_seg_cache = NULL;
+}
+
 /*
  * Space map routines.
  * NOTE: caller is responsible for all locking.
@@ -121,7 +138,7 @@ space_map_add(space_map_t *sm, uint64_t start, uint64_t size)
 			avl_remove(sm->sm_pp_root, ss_after);
 		}
 		ss_after->ss_start = ss_before->ss_start;
-		kmem_free(ss_before, sizeof (*ss_before));
+		kmem_cache_free(space_seg_cache, ss_before);
 		ss = ss_after;
 	} else if (merge_before) {
 		ss_before->ss_end = end;
@@ -134,7 +151,7 @@ space_map_add(space_map_t *sm, uint64_t start, uint64_t size)
 			avl_remove(sm->sm_pp_root, ss_after);
 		ss = ss_after;
 	} else {
-		ss = kmem_alloc(sizeof (*ss), KM_PUSHPAGE);
+		ss = kmem_cache_alloc(space_seg_cache, KM_PUSHPAGE);
 		ss->ss_start = start;
 		ss->ss_end = end;
 		avl_insert(&sm->sm_root, ss, where);
@@ -181,7 +198,7 @@ space_map_remove(space_map_t *sm, uint64_t start, uint64_t size)
 		avl_remove(sm->sm_pp_root, ss);
 
 	if (left_over && right_over) {
-		newseg = kmem_alloc(sizeof (*newseg), KM_PUSHPAGE);
+		newseg = kmem_cache_alloc(space_seg_cache, KM_PUSHPAGE);
 		newseg->ss_start = end;
 		newseg->ss_end = ss->ss_end;
 		ss->ss_end = start;
@@ -194,7 +211,7 @@ space_map_remove(space_map_t *sm, uint64_t start, uint64_t size)
 		ss->ss_start = end;
 	} else {
 		avl_remove(&sm->sm_root, ss);
-		kmem_free(ss, sizeof (*ss));
+		kmem_cache_free(space_seg_cache, ss);
 		ss = NULL;
 	}
 
@@ -234,7 +251,7 @@ space_map_vacate(space_map_t *sm, space_map_func_t *func, space_map_t *mdest)
 	while ((ss = avl_destroy_nodes(&sm->sm_root, &cookie)) != NULL) {
 		if (func != NULL)
 			func(mdest, ss->ss_start, ss->ss_end - ss->ss_start);
-		kmem_free(ss, sizeof (*ss));
+		kmem_cache_free(space_seg_cache, ss);
 	}
 	sm->sm_space = 0;
 }
@@ -406,7 +423,7 @@ space_map_sync(space_map_t *sm, uint8_t maptype,
 	spa_t *spa = dmu_objset_spa(os);
 	void *cookie = NULL;
 	space_seg_t *ss;
-	uint64_t bufsize, start, size, run_len;
+	uint64_t bufsize, start, size, run_len, delta, sm_space;
 	uint64_t *entry, *entry_map, *entry_map_end;
 
 	ASSERT(MUTEX_HELD(sm->sm_lock));
@@ -435,11 +452,13 @@ space_map_sync(space_map_t *sm, uint8_t maptype,
 	    SM_DEBUG_SYNCPASS_ENCODE(spa_sync_pass(spa)) |
 	    SM_DEBUG_TXG_ENCODE(dmu_tx_get_txg(tx));
 
+	delta = 0;
+	sm_space = sm->sm_space;
 	while ((ss = avl_destroy_nodes(&sm->sm_root, &cookie)) != NULL) {
 		size = ss->ss_end - ss->ss_start;
 		start = (ss->ss_start - sm->sm_start) >> sm->sm_shift;
 
-		sm->sm_space -= size;
+		delta += size;
 		size >>= sm->sm_shift;
 
 		while (size) {
@@ -461,7 +480,7 @@ space_map_sync(space_map_t *sm, uint8_t maptype,
 			start += run_len;
 			size -= run_len;
 		}
-		kmem_free(ss, sizeof (*ss));
+		kmem_cache_free(space_seg_cache, ss);
 	}
 
 	if (entry != entry_map) {
@@ -473,8 +492,15 @@ space_map_sync(space_map_t *sm, uint8_t maptype,
 		smo->smo_objsize += size;
 	}
 
+	/*
+	 * Ensure that the space_map's accounting wasn't changed
+	 * while we were in the middle of writing it out.
+	 */
+	VERIFY3U(sm->sm_space, ==, sm_space);
+
 	zio_buf_free(entry_map, bufsize);
 
+	sm->sm_space -= delta;
 	VERIFY3U(sm->sm_space, ==, 0);
 }
 
diff --git a/module/zfs/zio.c b/module/zfs/zio.c
index a721903a3..fda76123e 100644
--- a/module/zfs/zio.c
+++ b/module/zfs/zio.c
@@ -84,6 +84,22 @@ extern vmem_t *zio_alloc_arena;
 #endif
 extern int zfs_mg_alloc_failures;
 
+/*
+ * The following actions directly effect the spa's sync-to-convergence logic.
+ * The values below define the sync pass when we start performing the action.
+ * Care should be taken when changing these values as they directly impact
+ * spa_sync() performance. Tuning these values may introduce subtle performance
+ * pathologies and should only be done in the context of performance analysis.
+ * These tunables will eventually be removed and replaced with #defines once
+ * enough analysis has been done to determine optimal values.
+ *
+ * The 'zfs_sync_pass_deferred_free' pass must be greater than 1 to ensure that
+ * regular blocks are not deferred.
+ */
+int zfs_sync_pass_deferred_free = 2; /* defer frees starting in this pass */
+int zfs_sync_pass_dont_compress = 5; /* don't compress starting in this pass */
+int zfs_sync_pass_rewrite = 2; /* rewrite new bps starting in this pass */
+
 /*
  * An allocating zio is one that either currently has the DVA allocate
  * stage set or will have it later in its lifetime.
@@ -771,7 +787,7 @@ zio_free_sync(zio_t *pio, spa_t *spa, uint64_t txg, const blkptr_t *bp,
 
 	ASSERT(!BP_IS_HOLE(bp));
 	ASSERT(spa_syncing_txg(spa) == txg);
-	ASSERT(spa_sync_pass(spa) <= SYNC_PASS_DEFERRED_FREE);
+	ASSERT(spa_sync_pass(spa) < zfs_sync_pass_deferred_free);
 
 	zio = zio_create(pio, spa, txg, bp, NULL, BP_GET_PSIZE(bp),
 	    NULL, NULL, ZIO_TYPE_FREE, ZIO_PRIORITY_FREE, flags,
@@ -1068,7 +1084,7 @@ zio_write_bp_init(zio_t *zio)
 		ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
 		ASSERT(!BP_GET_DEDUP(bp));
 
-		if (pass > SYNC_PASS_DONT_COMPRESS)
+		if (pass >= zfs_sync_pass_dont_compress)
 			compress = ZIO_COMPRESS_OFF;
 
 		/* Make sure someone doesn't change their mind on overwrites */
@@ -1097,7 +1113,7 @@ zio_write_bp_init(zio_t *zio)
 	 * There should only be a handful of blocks after pass 1 in any case.
 	 */
 	if (bp->blk_birth == zio->io_txg && BP_GET_PSIZE(bp) == psize &&
-	    pass > SYNC_PASS_REWRITE) {
+	    pass >= zfs_sync_pass_rewrite) {
 		enum zio_stage gang_stages = zio->io_pipeline & ZIO_GANG_STAGES;
 		ASSERT(psize != 0);
 		zio->io_pipeline = ZIO_REWRITE_PIPELINE | gang_stages;