diff --git a/include/sys/uberblock.h b/include/sys/uberblock.h
index b5bb91573..21e7ae0de 100644
--- a/include/sys/uberblock.h
+++ b/include/sys/uberblock.h
@@ -22,6 +22,9 @@
  * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
  * Use is subject to license terms.
  */
+/*
+ * Copyright (c) 2014 by Delphix. All rights reserved.
+ */
 
 #ifndef _SYS_UBERBLOCK_H
 #define	_SYS_UBERBLOCK_H
@@ -36,8 +39,8 @@ extern "C" {
 
 typedef struct uberblock uberblock_t;
 
-extern int uberblock_verify(uberblock_t *ub);
-extern int uberblock_update(uberblock_t *ub, vdev_t *rvd, uint64_t txg);
+extern int uberblock_verify(uberblock_t *);
+extern boolean_t uberblock_update(uberblock_t *, vdev_t *, uint64_t);
 
 #ifdef	__cplusplus
 }
diff --git a/module/zfs/dsl_scan.c b/module/zfs/dsl_scan.c
index 048935971..b989e7633 100644
--- a/module/zfs/dsl_scan.c
+++ b/module/zfs/dsl_scan.c
@@ -1528,11 +1528,15 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
 			    dp->dp_bptree_obj, tx));
 			dp->dp_bptree_obj = 0;
 			scn->scn_async_destroying = B_FALSE;
+			scn->scn_async_stalled = B_FALSE;
 		} else {
 			/*
-			 * If we didn't make progress, mark the async destroy as
-			 * stalled, so that we will not initiate a spa_sync() on
-			 * its behalf.
+			 * If we didn't make progress, mark the async
+			 * destroy as stalled, so that we will not initiate
+			 * a spa_sync() on its behalf.  Note that we only
+			 * check this if we are not finished, because if the
+			 * bptree had no blocks for us to visit, we can
+			 * finish without "making progress".
 			 */
 			scn->scn_async_stalled =
 			    (scn->scn_visited_this_txg == 0);
diff --git a/module/zfs/spa.c b/module/zfs/spa.c
index b5e024c3f..d8eaf9979 100644
--- a/module/zfs/spa.c
+++ b/module/zfs/spa.c
@@ -6393,21 +6393,6 @@ spa_sync(spa_t *spa, uint64_t txg)
 		}
 	}
 
-	/*
-	 * If anything has changed in this txg, or if someone is waiting
-	 * for this txg to sync (eg, spa_vdev_remove()), push the
-	 * deferred frees from the previous txg.  If not, leave them
-	 * alone so that we don't generate work on an otherwise idle
-	 * system.
-	 */
-	if (!txg_list_empty(&dp->dp_dirty_datasets, txg) ||
-	    !txg_list_empty(&dp->dp_dirty_dirs, txg) ||
-	    !txg_list_empty(&dp->dp_sync_tasks, txg) ||
-	    ((dsl_scan_active(dp->dp_scan) ||
-	    txg_sync_waiting(dp)) && !spa_shutting_down(spa))) {
-		spa_sync_deferred_frees(spa, tx);
-	}
-
 	/*
 	 * Iterate to convergence.
 	 */
@@ -6425,6 +6410,11 @@ spa_sync(spa_t *spa, uint64_t txg)
 		if (pass < zfs_sync_pass_deferred_free) {
 			spa_sync_frees(spa, free_bpl, tx);
 		} else {
+			/*
+			 * We can not defer frees in pass 1, because
+			 * we sync the deferred frees later in pass 1.
+			 */
+			ASSERT3U(pass, >, 1);
 			bplist_iterate(free_bpl, bpobj_enqueue_cb,
 			    &spa->spa_deferred_bpobj, tx);
 		}
@@ -6435,8 +6425,37 @@ spa_sync(spa_t *spa, uint64_t txg)
 		while ((vd = txg_list_remove(&spa->spa_vdev_txg_list, txg)))
 			vdev_sync(vd, txg);
 
-		if (pass == 1)
+		if (pass == 1) {
 			spa_sync_upgrades(spa, tx);
+			ASSERT3U(txg, >=,
+			    spa->spa_uberblock.ub_rootbp.blk_birth);
+			/*
+			 * Note: We need to check if the MOS is dirty
+			 * because we could have marked the MOS dirty
+			 * without updating the uberblock (e.g. if we
+			 * have sync tasks but no dirty user data).  We
+			 * need to check the uberblock's rootbp because
+			 * it is updated if we have synced out dirty
+			 * data (though in this case the MOS will most
+			 * likely also be dirty due to second order
+			 * effects, we don't want to rely on that here).
+			 */
+			if (spa->spa_uberblock.ub_rootbp.blk_birth < txg &&
+			    !dmu_objset_is_dirty(mos, txg)) {
+				/*
+				 * Nothing changed on the first pass,
+				 * therefore this TXG is a no-op.  Avoid
+				 * syncing deferred frees, so that we
+				 * can keep this TXG as a no-op.
+				 */
+				ASSERT(txg_list_empty(&dp->dp_dirty_datasets,
+				    txg));
+				ASSERT(txg_list_empty(&dp->dp_dirty_dirs, txg));
+				ASSERT(txg_list_empty(&dp->dp_sync_tasks, txg));
+				break;
+			}
+			spa_sync_deferred_frees(spa, tx);
+		}
 
 	} while (dmu_objset_is_dirty(mos, txg));
 
diff --git a/module/zfs/uberblock.c b/module/zfs/uberblock.c
index a07dc00ae..f8bdecdf5 100644
--- a/module/zfs/uberblock.c
+++ b/module/zfs/uberblock.c
@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2013 by Delphix. All rights reserved.
+ * Copyright (c) 2013, 2014 by Delphix. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
@@ -40,10 +40,10 @@ uberblock_verify(uberblock_t *ub)
 }
 
 /*
- * Update the uberblock and return a boolean value indicating whether
- * anything changed in this transaction group.
+ * Update the uberblock and return TRUE if anything changed in this
+ * transaction group.
  */
-int
+boolean_t
 uberblock_update(uberblock_t *ub, vdev_t *rvd, uint64_t txg)
 {
 	ASSERT(ub->ub_txg < txg);