diff --git a/configure.ac b/configure.ac
index 46a27f7f1..6fcc89044 100644
--- a/configure.ac
+++ b/configure.ac
@@ -323,6 +323,7 @@ AC_CONFIG_FILES([
 	tests/zfs-tests/tests/functional/rename_dirs/Makefile
 	tests/zfs-tests/tests/functional/replacement/Makefile
 	tests/zfs-tests/tests/functional/reservation/Makefile
+	tests/zfs-tests/tests/functional/resilver/Makefile
 	tests/zfs-tests/tests/functional/rootpool/Makefile
 	tests/zfs-tests/tests/functional/rsend/Makefile
 	tests/zfs-tests/tests/functional/scrub_mirror/Makefile
diff --git a/include/sys/dsl_scan.h b/include/sys/dsl_scan.h
index 345d2754f..032f7f3e2 100644
--- a/include/sys/dsl_scan.h
+++ b/include/sys/dsl_scan.h
@@ -21,7 +21,7 @@
 /*
  * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2012, 2017 by Delphix. All rights reserved.
- * Copyright (c) 2017 Datto Inc.
+ * Copyright (c) 2017, 2019, Datto Inc. All rights reserved.
  */
 
 #ifndef	_SYS_DSL_SCAN_H
@@ -164,10 +164,12 @@ void dsl_scan_fini(struct dsl_pool *dp);
 void dsl_scan_sync(struct dsl_pool *, dmu_tx_t *);
 int dsl_scan_cancel(struct dsl_pool *);
 int dsl_scan(struct dsl_pool *, pool_scan_func_t);
+void dsl_scan_assess_vdev(struct dsl_pool *dp, vdev_t *vd);
 boolean_t dsl_scan_scrubbing(const struct dsl_pool *dp);
 int dsl_scrub_set_pause_resume(const struct dsl_pool *dp, pool_scrub_cmd_t cmd);
-void dsl_resilver_restart(struct dsl_pool *, uint64_t txg);
+void dsl_scan_restart_resilver(struct dsl_pool *, uint64_t txg);
 boolean_t dsl_scan_resilvering(struct dsl_pool *dp);
+boolean_t dsl_scan_resilver_scheduled(struct dsl_pool *dp);
 boolean_t dsl_dataset_unstable(struct dsl_dataset *ds);
 void dsl_scan_ddt_entry(dsl_scan_t *scn, enum zio_checksum checksum,
     ddt_entry_t *dde, dmu_tx_t *tx);
diff --git a/include/sys/spa.h b/include/sys/spa.h
index 42bf9dcc1..4c1dcdcc1 100644
--- a/include/sys/spa.h
+++ b/include/sys/spa.h
@@ -26,7 +26,7 @@
  * Copyright 2013 Saso Kiselkov. All rights reserved.
  * Copyright (c) 2014 Integros [integros.com]
  * Copyright 2017 Joyent, Inc.
- * Copyright (c) 2017 Datto Inc.
+ * Copyright (c) 2017, 2019, Datto Inc. All rights reserved.
  * Copyright (c) 2017, Intel Corporation.
  */
 
@@ -777,6 +777,7 @@ extern void spa_async_request(spa_t *spa, int flag);
 extern void spa_async_unrequest(spa_t *spa, int flag);
 extern void spa_async_suspend(spa_t *spa);
 extern void spa_async_resume(spa_t *spa);
+extern int spa_async_tasks(spa_t *spa);
 extern spa_t *spa_inject_addref(char *pool);
 extern void spa_inject_delref(spa_t *spa);
 extern void spa_scan_stat_init(spa_t *spa);
diff --git a/include/sys/vdev.h b/include/sys/vdev.h
index 67ca0d116..339a48861 100644
--- a/include/sys/vdev.h
+++ b/include/sys/vdev.h
@@ -23,6 +23,7 @@
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2011, 2017 by Delphix. All rights reserved.
  * Copyright (c) 2017, Intel Corporation.
+ * Copyright (c) 2019, Datto Inc. All rights reserved.
  */
 
 #ifndef _SYS_VDEV_H
@@ -151,7 +152,8 @@ extern int vdev_config_sync(vdev_t **svd, int svdcount, uint64_t txg);
 extern void vdev_state_dirty(vdev_t *vd);
 extern void vdev_state_clean(vdev_t *vd);
 
-extern void vdev_set_deferred_resilver(spa_t *spa, vdev_t *vd);
+extern void vdev_defer_resilver(vdev_t *vd);
+extern boolean_t vdev_clear_resilver_deferred(vdev_t *vd, dmu_tx_t *tx);
 
 typedef enum vdev_config_flag {
 	VDEV_CONFIG_SPARE = 1 << 0,
diff --git a/module/zfs/dsl_scan.c b/module/zfs/dsl_scan.c
index f1d995b3d..d71113681 100644
--- a/module/zfs/dsl_scan.c
+++ b/module/zfs/dsl_scan.c
@@ -22,7 +22,7 @@
  * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2011, 2017 by Delphix. All rights reserved.
  * Copyright 2016 Gary Mills
- * Copyright (c) 2017 Datto Inc.
+ * Copyright (c) 2017, 2019, Datto Inc. All rights reserved.
  * Copyright 2019 Joyent, Inc.
  */
 
@@ -591,6 +591,13 @@ dsl_scan_restarting(dsl_scan_t *scn, dmu_tx_t *tx)
 	    scn->scn_restart_txg <= tx->tx_txg);
 }
 
+boolean_t
+dsl_scan_resilver_scheduled(dsl_pool_t *dp)
+{
+	return ((dp->dp_scan && dp->dp_scan->scn_restart_txg != 0) ||
+	    (spa_async_tasks(dp->dp_spa) & SPA_ASYNC_RESILVER));
+}
+
 boolean_t
 dsl_scan_scrubbing(const dsl_pool_t *dp)
 {
@@ -786,7 +793,7 @@ dsl_scan(dsl_pool_t *dp, pool_scan_func_t func)
 	(void) spa_vdev_state_exit(spa, NULL, 0);
 
 	if (func == POOL_SCAN_RESILVER) {
-		dsl_resilver_restart(spa->spa_dsl_pool, 0);
+		dsl_scan_restart_resilver(spa->spa_dsl_pool, 0);
 		return (0);
 	}
 
@@ -806,41 +813,6 @@ dsl_scan(dsl_pool_t *dp, pool_scan_func_t func)
 	    dsl_scan_setup_sync, &func, 0, ZFS_SPACE_CHECK_EXTRA_RESERVED));
 }
 
-/*
- * Sets the resilver defer flag to B_FALSE on all leaf devs under vd. Returns
- * B_TRUE if we have devices that need to be resilvered and are available to
- * accept resilver I/Os.
- */
-static boolean_t
-dsl_scan_clear_deferred(vdev_t *vd, dmu_tx_t *tx)
-{
-	boolean_t resilver_needed = B_FALSE;
-	spa_t *spa = vd->vdev_spa;
-
-	for (int c = 0; c < vd->vdev_children; c++) {
-		resilver_needed |=
-		    dsl_scan_clear_deferred(vd->vdev_child[c], tx);
-	}
-
-	if (vd == spa->spa_root_vdev &&
-	    spa_feature_is_active(spa, SPA_FEATURE_RESILVER_DEFER)) {
-		spa_feature_decr(spa, SPA_FEATURE_RESILVER_DEFER, tx);
-		vdev_config_dirty(vd);
-		spa->spa_resilver_deferred = B_FALSE;
-		return (resilver_needed);
-	}
-
-	if (!vdev_is_concrete(vd) || vd->vdev_aux ||
-	    !vd->vdev_ops->vdev_op_leaf)
-		return (resilver_needed);
-
-	if (vd->vdev_resilver_deferred)
-		vd->vdev_resilver_deferred = B_FALSE;
-
-	return (!vdev_is_dead(vd) && !vd->vdev_offline &&
-	    vdev_resilver_needed(vd, NULL, NULL));
-}
-
 /* ARGSUSED */
 static void
 dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
@@ -943,25 +915,21 @@ dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
 		spa_async_request(spa, SPA_ASYNC_RESILVER_DONE);
 
 		/*
-		 * Clear any deferred_resilver flags in the config.
+		 * Clear any resilver_deferred flags in the config.
 		 * If there are drives that need resilvering, kick
 		 * off an asynchronous request to start resilver.
-		 * dsl_scan_clear_deferred() may update the config
+		 * vdev_clear_resilver_deferred() may update the config
 		 * before the resilver can restart. In the event of
 		 * a crash during this period, the spa loading code
 		 * will find the drives that need to be resilvered
-		 * when the machine reboots and start the resilver then.
+		 * and start the resilver then.
 		 */
-		if (spa_feature_is_enabled(spa, SPA_FEATURE_RESILVER_DEFER)) {
-			boolean_t resilver_needed =
-			    dsl_scan_clear_deferred(spa->spa_root_vdev, tx);
-			if (resilver_needed) {
-				spa_history_log_internal(spa,
-				    "starting deferred resilver", tx,
-				    "errors=%llu",
-				    (u_longlong_t)spa_get_errlog_size(spa));
-				spa_async_request(spa, SPA_ASYNC_RESILVER);
-			}
+		if (spa_feature_is_enabled(spa, SPA_FEATURE_RESILVER_DEFER) &&
+		    vdev_clear_resilver_deferred(spa->spa_root_vdev, tx)) {
+			spa_history_log_internal(spa,
+			    "starting deferred resilver", tx, "errors=%llu",
+			    (u_longlong_t)spa_get_errlog_size(spa));
+			spa_async_request(spa, SPA_ASYNC_RESILVER);
 		}
 	}
 
@@ -1071,7 +1039,7 @@ dsl_scrub_set_pause_resume(const dsl_pool_t *dp, pool_scrub_cmd_t cmd)
 
 /* start a new scan, or restart an existing one. */
 void
-dsl_resilver_restart(dsl_pool_t *dp, uint64_t txg)
+dsl_scan_restart_resilver(dsl_pool_t *dp, uint64_t txg)
 {
 	if (txg == 0) {
 		dmu_tx_t *tx;
@@ -4232,6 +4200,36 @@ dsl_scan_freed(spa_t *spa, const blkptr_t *bp)
 		dsl_scan_freed_dva(spa, bp, i);
 }
 
+/*
+ * Check if a vdev needs resilvering (non-empty DTL), if so, and resilver has
+ * not started, start it. Otherwise, only restart if max txg in DTL range is
+ * greater than the max txg in the current scan. If the DTL max is less than
+ * the scan max, then the vdev has not missed any new data since the resilver
+ * started, so a restart is not needed.
+ */
+void
+dsl_scan_assess_vdev(dsl_pool_t *dp, vdev_t *vd)
+{
+	uint64_t min, max;
+
+	if (!vdev_resilver_needed(vd, &min, &max))
+		return;
+
+	if (!dsl_scan_resilvering(dp)) {
+		spa_async_request(dp->dp_spa, SPA_ASYNC_RESILVER);
+		return;
+	}
+
+	if (max <= dp->dp_scan->scn_phys.scn_max_txg)
+		return;
+
+	/* restart is needed, check if it can be deferred */
+	if (spa_feature_is_enabled(dp->dp_spa, SPA_FEATURE_RESILVER_DEFER))
+		vdev_defer_resilver(vd);
+	else
+		spa_async_request(dp->dp_spa, SPA_ASYNC_RESILVER);
+}
+
 #if defined(_KERNEL)
 /* CSTYLED */
 module_param(zfs_scan_vdev_limit, ulong, 0644);
diff --git a/module/zfs/spa.c b/module/zfs/spa.c
index 7fa18cbd1..cf8462cec 100644
--- a/module/zfs/spa.c
+++ b/module/zfs/spa.c
@@ -29,7 +29,7 @@
  * Copyright 2016 Toomas Soome <tsoome@me.com>
  * Copyright (c) 2016 Actifio, Inc. All rights reserved.
  * Copyright 2018 Joyent, Inc.
- * Copyright (c) 2017 Datto Inc.
+ * Copyright (c) 2017, 2019, Datto Inc. All rights reserved.
  * Copyright 2017 Joyent, Inc.
  * Copyright (c) 2017, Intel Corporation.
  */
@@ -6242,9 +6242,9 @@ spa_vdev_attach(spa_t *spa, uint64_t guid, nvlist_t *nvroot, int replacing)
 	 */
 	if (dsl_scan_resilvering(spa_get_dsl(spa)) &&
 	    spa_feature_is_enabled(spa, SPA_FEATURE_RESILVER_DEFER))
-		vdev_set_deferred_resilver(spa, newvd);
+		vdev_defer_resilver(newvd);
 	else
-		dsl_resilver_restart(spa->spa_dsl_pool, dtl_max_txg);
+		dsl_scan_restart_resilver(spa->spa_dsl_pool, dtl_max_txg);
 
 	if (spa->spa_bootfs)
 		spa_event_notify(spa, newvd, NULL, ESC_ZFS_BOOTFS_VDEV_ATTACH);
@@ -7479,7 +7479,7 @@ spa_async_thread(void *arg)
 	if (tasks & SPA_ASYNC_RESILVER &&
 	    (!dsl_scan_resilvering(dp) ||
 	    !spa_feature_is_enabled(dp->dp_spa, SPA_FEATURE_RESILVER_DEFER)))
-		dsl_resilver_restart(dp, 0);
+		dsl_scan_restart_resilver(dp, 0);
 
 	if (tasks & SPA_ASYNC_INITIALIZE_RESTART) {
 		mutex_enter(&spa_namespace_lock);
@@ -7595,6 +7595,12 @@ spa_async_request(spa_t *spa, int task)
 	mutex_exit(&spa->spa_async_lock);
 }
 
+int
+spa_async_tasks(spa_t *spa)
+{
+	return (spa->spa_async_tasks);
+}
+
 /*
  * ==========================================================================
  * SPA syncing routines
diff --git a/module/zfs/vdev.c b/module/zfs/vdev.c
index 952b56581..a68c0dfa7 100644
--- a/module/zfs/vdev.c
+++ b/module/zfs/vdev.c
@@ -27,6 +27,7 @@
  * Copyright 2016 Toomas Soome <tsoome@me.com>
  * Copyright 2017 Joyent, Inc.
  * Copyright (c) 2017, Intel Corporation.
+ * Copyright (c) 2019, Datto Inc. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
@@ -833,7 +834,7 @@ vdev_alloc(spa_t *spa, vdev_t **vdp, nvlist_t *nv, vdev_t *parent, uint_t id,
 		    &vd->vdev_resilver_txg);
 
 		if (nvlist_exists(nv, ZPOOL_CONFIG_RESILVER_DEFER))
-			vdev_set_deferred_resilver(spa, vd);
+			vdev_defer_resilver(vd);
 
 		/*
 		 * In general, when importing a pool we want to ignore the
@@ -1863,18 +1864,12 @@ vdev_open(vdev_t *vd)
 	}
 
 	/*
-	 * If a leaf vdev has a DTL, and seems healthy, then kick off a
-	 * resilver.  But don't do this if we are doing a reopen for a scrub,
-	 * since this would just restart the scrub we are already doing.
+	 * If this is a leaf vdev, assess whether a resilver is needed.
+	 * But don't do this if we are doing a reopen for a scrub, since
+	 * this would just restart the scrub we are already doing.
 	 */
-	if (vd->vdev_ops->vdev_op_leaf && !spa->spa_scrub_reopen &&
-	    vdev_resilver_needed(vd, NULL, NULL)) {
-		if (dsl_scan_resilvering(spa->spa_dsl_pool) &&
-		    spa_feature_is_enabled(spa, SPA_FEATURE_RESILVER_DEFER))
-			vdev_set_deferred_resilver(spa, vd);
-		else
-			spa_async_request(spa, SPA_ASYNC_RESILVER);
-	}
+	if (vd->vdev_ops->vdev_op_leaf && !spa->spa_scrub_reopen)
+		dsl_scan_assess_vdev(spa->spa_dsl_pool, vd);
 
 	return (0);
 }
@@ -3693,14 +3688,11 @@ vdev_clear(spa_t *spa, vdev_t *vd)
 		if (vd != rvd && vdev_writeable(vd->vdev_top))
 			vdev_state_dirty(vd->vdev_top);
 
-		if (vd->vdev_aux == NULL && !vdev_is_dead(vd)) {
-			if (dsl_scan_resilvering(spa->spa_dsl_pool) &&
-			    spa_feature_is_enabled(spa,
-			    SPA_FEATURE_RESILVER_DEFER))
-				vdev_set_deferred_resilver(spa, vd);
-			else
-				spa_async_request(spa, SPA_ASYNC_RESILVER);
-		}
+		/* If a resilver isn't required, check if vdevs can be culled */
+		if (vd->vdev_aux == NULL && !vdev_is_dead(vd) &&
+		    !dsl_scan_resilvering(spa->spa_dsl_pool) &&
+		    !dsl_scan_resilver_scheduled(spa->spa_dsl_pool))
+			spa_async_request(spa, SPA_ASYNC_RESILVER_DONE);
 
 		spa_event_notify(spa, vd, NULL, ESC_ZFS_VDEV_CLEAR);
 	}
@@ -4693,18 +4685,46 @@ vdev_deadman(vdev_t *vd, char *tag)
 }
 
 void
-vdev_set_deferred_resilver(spa_t *spa, vdev_t *vd)
+vdev_defer_resilver(vdev_t *vd)
 {
-	for (uint64_t i = 0; i < vd->vdev_children; i++)
-		vdev_set_deferred_resilver(spa, vd->vdev_child[i]);
-
-	if (!vd->vdev_ops->vdev_op_leaf || !vdev_writeable(vd) ||
-	    range_tree_is_empty(vd->vdev_dtl[DTL_MISSING])) {
-		return;
-	}
+	ASSERT(vd->vdev_ops->vdev_op_leaf);
 
 	vd->vdev_resilver_deferred = B_TRUE;
-	spa->spa_resilver_deferred = B_TRUE;
+	vd->vdev_spa->spa_resilver_deferred = B_TRUE;
+}
+
+/*
+ * Clears the resilver deferred flag on all leaf devs under vd. Returns
+ * B_TRUE if we have devices that need to be resilvered and are available to
+ * accept resilver I/Os.
+ */
+boolean_t
+vdev_clear_resilver_deferred(vdev_t *vd, dmu_tx_t *tx)
+{
+	boolean_t resilver_needed = B_FALSE;
+	spa_t *spa = vd->vdev_spa;
+
+	for (int c = 0; c < vd->vdev_children; c++) {
+		vdev_t *cvd = vd->vdev_child[c];
+		resilver_needed |= vdev_clear_resilver_deferred(cvd, tx);
+	}
+
+	if (vd == spa->spa_root_vdev &&
+	    spa_feature_is_active(spa, SPA_FEATURE_RESILVER_DEFER)) {
+		spa_feature_decr(spa, SPA_FEATURE_RESILVER_DEFER, tx);
+		vdev_config_dirty(vd);
+		spa->spa_resilver_deferred = B_FALSE;
+		return (resilver_needed);
+	}
+
+	if (!vdev_is_concrete(vd) || vd->vdev_aux ||
+	    !vd->vdev_ops->vdev_op_leaf)
+		return (resilver_needed);
+
+	vd->vdev_resilver_deferred = B_FALSE;
+
+	return (!vdev_is_dead(vd) && !vd->vdev_offline &&
+	    vdev_resilver_needed(vd, NULL, NULL));
 }
 
 /*
diff --git a/tests/runfiles/linux.run b/tests/runfiles/linux.run
index 1361a8b53..ae15cd221 100644
--- a/tests/runfiles/linux.run
+++ b/tests/runfiles/linux.run
@@ -796,6 +796,10 @@ tests = ['reservation_001_pos', 'reservation_002_pos', 'reservation_003_pos',
     'reservation_022_pos']
 tags = ['functional', 'reservation']
 
+[tests/functional/resilver]
+tests = ['resilver_restart_001']
+tags = ['functional', 'resilver']
+
 [tests/functional/rootpool]
 tests = ['rootpool_002_neg', 'rootpool_003_neg', 'rootpool_007_pos']
 tags = ['functional', 'rootpool']
diff --git a/tests/zfs-tests/tests/functional/Makefile.am b/tests/zfs-tests/tests/functional/Makefile.am
index ac0ba7cf3..a1fe06c16 100644
--- a/tests/zfs-tests/tests/functional/Makefile.am
+++ b/tests/zfs-tests/tests/functional/Makefile.am
@@ -59,6 +59,7 @@ SUBDIRS = \
 	rename_dirs \
 	replacement \
 	reservation \
+	resilver \
 	rootpool \
 	rsend \
 	scrub_mirror \
diff --git a/tests/zfs-tests/tests/functional/resilver/Makefile.am b/tests/zfs-tests/tests/functional/resilver/Makefile.am
new file mode 100644
index 000000000..465d8f3a3
--- /dev/null
+++ b/tests/zfs-tests/tests/functional/resilver/Makefile.am
@@ -0,0 +1,8 @@
+pkgdatadir = $(datadir)/@PACKAGE@/zfs-tests/tests/functional/resilver
+dist_pkgdata_SCRIPTS = \
+	setup.ksh \
+	cleanup.ksh \
+	resilver_restart_001.ksh
+
+dist_pkgdata_DATA = \
+	resilver.cfg
diff --git a/tests/zfs-tests/tests/functional/resilver/cleanup.ksh b/tests/zfs-tests/tests/functional/resilver/cleanup.ksh
new file mode 100755
index 000000000..4dfa81424
--- /dev/null
+++ b/tests/zfs-tests/tests/functional/resilver/cleanup.ksh
@@ -0,0 +1,31 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# The contents of this file are subject to the terms of the
+# Common Development and Distribution License (the "License").
+# You may not use this file except in compliance with the License.
+#
+# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+# or http://www.opensolaris.org/os/licensing.
+# See the License for the specific language governing permissions
+# and limitations under the License.
+#
+# When distributing Covered Code, include this CDDL HEADER in each
+# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+# If applicable, add the following below this CDDL HEADER, with the
+# fields enclosed by brackets "[]" replaced with your own identifying
+# information: Portions Copyright [yyyy] [name of copyright owner]
+#
+# CDDL HEADER END
+
+#
+# Copyright (c) 2019, Datto Inc. All rights reserved.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/resilver/resilver.cfg
+
+verify_runnable "global"
+
+log_pass
diff --git a/tests/zfs-tests/tests/functional/resilver/resilver.cfg b/tests/zfs-tests/tests/functional/resilver/resilver.cfg
new file mode 100644
index 000000000..88dfd24ae
--- /dev/null
+++ b/tests/zfs-tests/tests/functional/resilver/resilver.cfg
@@ -0,0 +1,32 @@
+#
+# CDDL HEADER START
+#
+# The contents of this file are subject to the terms of the
+# Common Development and Distribution License (the "License").
+# You may not use this file except in compliance with the License.
+#
+# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+# or http://www.opensolaris.org/os/licensing.
+# See the License for the specific language governing permissions
+# and limitations under the License.
+#
+# When distributing Covered Code, include this CDDL HEADER in each
+# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+# If applicable, add the following below this CDDL HEADER, with the
+# fields enclosed by brackets "[]" replaced with your own identifying
+# information: Portions Copyright [yyyy] [name of copyright owner]
+#
+# CDDL HEADER END
+
+#
+# Copyright (c) 2019, Datto Inc. All rights reserved.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+verify_runnable "global"
+
+set -A VDEV_FILES $TEST_BASE_DIR/file-{1..4}
+SPARE_VDEV_FILE=$TEST_BASE_DIR/spare-1
+
+VDEV_FILE_SIZE=$(( $SPA_MINDEVSIZE * 2 ))
diff --git a/tests/zfs-tests/tests/functional/resilver/resilver_restart_001.ksh b/tests/zfs-tests/tests/functional/resilver/resilver_restart_001.ksh
new file mode 100755
index 000000000..ad0e1961e
--- /dev/null
+++ b/tests/zfs-tests/tests/functional/resilver/resilver_restart_001.ksh
@@ -0,0 +1,185 @@
+#!/bin/ksh -p
+
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2019, Datto Inc. All rights reserved.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/resilver/resilver.cfg
+
+#
+# DESCRIPTION:
+# Testing resilver restart logic both with and without the deferred resilver
+# feature enabled, verifying that resilver is not restarted when it is
+# unecessary.
+#
+# STRATEGY:
+# 1. Create a pool
+# 2. Create four filesystems with the primary cache disable to force reads
+# 3. Write four files simultaneously, one to each filesystem
+# 4. Do with and without deferred resilvers enabled
+#    a. Replace a vdev with a spare & suspend resilver immediately
+#    b. Verify resilver starts properly
+#    c. Offline / online another vdev to introduce a new DTL range
+#    d. Verify resilver restart restart or defer
+#    e. Inject read errors on vdev that was offlined / onlned
+#    f. Verify that resilver did not restart
+#    g. Unsuspend resilver and wait for it to finish
+#    h. Verify that there are two resilvers and nothing is deferred
+#
+
+function cleanup
+{
+	echo $ORIG_RESILVER_MIN_TIME > $ZFS_PARAMS/zfs_resilver_min_time_ms
+	echo $ORIG_SCAN_SUSPEND_PROGRESS > $ZFS_PARAMS/zfs_scan_suspend_progress
+	log_must zinject -c all
+	destroy_pool $TESTPOOL
+	rm -f ${VDEV_FILES[@]} $SPARE_VDEV_FILE
+}
+
+# Count resilver events in zpool and number of deferred rsilvers on vdevs
+function verify_restarts # <msg> <cnt> <defer>
+{
+	msg=$1
+	cnt=$2
+	defer=$3
+
+	# check the number of resilver start in events log
+	RESILVERS=$(zpool events | grep -c sysevent.fs.zfs.resilver_start)
+	log_note "expected $cnt resilver start(s)$msg, found $RESILVERS"
+	[[ "$RESILVERS" -ne "$cnt" ]] &&
+	    log_fail "expected $cnt resilver start(s)$msg, found $RESILVERS"
+
+	[[ -z "$defer" ]] && return
+
+	# use zdb to find which vdevs have the resilver defer flag
+	VDEV_DEFERS=$(zdb -C $TESTPOOL | \
+	    sed -n -e '/^ *children\[[0-9]\].*$/{h}' \
+	    -e '/ *com.datto:resilver_defer$/{g;p}')
+
+	if [[ "$defer" == "-" ]]
+	then
+		[[ -n $VDEV_DEFERS ]] &&
+		    log_fail "didn't expect any vdevs to have resilver deferred"
+		return
+	fi
+
+	[[ "x${VDEV_DEFERS}x" =~ "x +children[$defer]:x" ]] ||
+	    log_fail "resilver deferred set on unexpected vdev: $VDEV_DEFERS"
+}
+
+log_assert "Check for unnecessary resilver restarts"
+
+ZFS_PARAMS=/sys/module/zfs/parameters
+ORIG_RESILVER_MIN_TIME=$(cat $ZFS_PARAMS/zfs_resilver_min_time_ms)
+ORIG_SCAN_SUSPEND_PROGRESS=$(cat $ZFS_PARAMS/zfs_scan_suspend_progress)
+
+set -A RESTARTS -- '1' '2' '2' '2'
+set -A VDEVS -- '' '' '' ''
+set -A DEFER_RESTARTS -- '1' '1' '1' '2'
+set -A DEFER_VDEVS -- '-' '2' '2' '-'
+
+VDEV_REPLACE="${VDEV_FILES[1]} $SPARE_VDEV_FILE"
+
+log_onexit cleanup
+
+log_must truncate -s $VDEV_FILE_SIZE ${VDEV_FILES[@]} $SPARE_VDEV_FILE
+
+log_must zpool create -f -o feature@resilver_defer=disabled $TESTPOOL \
+    raidz ${VDEV_FILES[@]}
+
+# Create 4 filesystems
+for fs in fs{0..3}
+do
+	log_must zfs create -o primarycache=none -o recordsize=1k $TESTPOOL/$fs
+done
+
+# simultaneously write 16M to each of them
+set -A DATAPATHS /$TESTPOOL/fs{0..3}/dat.0
+log_note "Writing data files"
+for path in ${DATAPATHS[@]}
+do
+	dd if=/dev/urandom of=$path bs=1M count=16 > /dev/null 2>&1 &
+done
+wait
+
+# Test without and with deferred resilve feature enabled
+for test in "without" "with"
+do
+	log_note "Testing $test deferred resilvers"
+
+	if [[ $test == "with" ]]
+	then
+		log_must zpool set feature@resilver_defer=enabled $TESTPOOL
+		RESTARTS=( "${DEFER_RESTARTS[@]}" )
+		VDEVS=( "${DEFER_VDEVS[@]}" )
+		VDEV_REPLACE="$SPARE_VDEV_FILE ${VDEV_FILES[1]}"
+	fi
+
+	# clear the events
+	log_must zpool events -c
+
+	# limit scanning time
+	echo 50 > $ZFS_PARAMS/zfs_resilver_min_time_ms
+
+	# initiate a resilver and suspend the scan as soon as possible
+	log_must zpool replace $TESTPOOL $VDEV_REPLACE
+	echo 1 > $ZFS_PARAMS/zfs_scan_suspend_progress
+
+	# there should only be 1 resilver start
+	verify_restarts '' "${RESTARTS[0]}" "${VDEVS[0]}"
+
+	# offline then online a vdev to introduce a new DTL range after current
+	# scan, which should restart (or defer) the resilver
+	log_must zpool offline $TESTPOOL ${VDEV_FILES[2]}
+	log_must zpool sync $TESTPOOL
+	log_must zpool online $TESTPOOL ${VDEV_FILES[2]}
+	log_must zpool sync $TESTPOOL
+
+	# there should now be 2 resilver starts w/o defer, 1 with defer
+	verify_restarts ' after offline/online' "${RESTARTS[1]}" "${VDEVS[1]}"
+
+	# inject read io errors on vdev and verify resilver does not restart
+	log_must zinject -a -d ${VDEV_FILES[2]} -e io -T read -f 0.25 $TESTPOOL
+	log_must cat ${DATAPATHS[1]} > /dev/null
+	log_must zinject -c all
+
+	# there should still be 2 resilver starts w/o defer, 1 with defer
+	verify_restarts ' after zinject' "${RESTARTS[2]}" "${VDEVS[2]}"
+
+	# unsuspend resilver
+	echo 0 > $ZFS_PARAMS/zfs_scan_suspend_progress
+	echo 3000 > $ZFS_PARAMS/zfs_resilver_min_time_ms
+
+	# wait for resilver to finish
+	for iter in {0..59}
+	do
+		is_pool_resilvered $TESTPOOL && break
+		sleep 1
+	done
+	is_pool_resilvered $TESTPOOL ||
+	    log_fail "resilver timed out"
+
+	# wait for a few txg's to see if a resilver happens
+	log_must zpool sync $TESTPOOL
+
+	# there should now be 2 resilver starts
+	verify_restarts ' after resilver' "${RESTARTS[3]}" "${VDEVS[3]}"
+done
+
+log_pass "Resilver did not restart unnecessarily"
diff --git a/tests/zfs-tests/tests/functional/resilver/setup.ksh b/tests/zfs-tests/tests/functional/resilver/setup.ksh
new file mode 100755
index 000000000..4dfa81424
--- /dev/null
+++ b/tests/zfs-tests/tests/functional/resilver/setup.ksh
@@ -0,0 +1,31 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# The contents of this file are subject to the terms of the
+# Common Development and Distribution License (the "License").
+# You may not use this file except in compliance with the License.
+#
+# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+# or http://www.opensolaris.org/os/licensing.
+# See the License for the specific language governing permissions
+# and limitations under the License.
+#
+# When distributing Covered Code, include this CDDL HEADER in each
+# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+# If applicable, add the following below this CDDL HEADER, with the
+# fields enclosed by brackets "[]" replaced with your own identifying
+# information: Portions Copyright [yyyy] [name of copyright owner]
+#
+# CDDL HEADER END
+
+#
+# Copyright (c) 2019, Datto Inc. All rights reserved.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/resilver/resilver.cfg
+
+verify_runnable "global"
+
+log_pass