diff --git a/module/zfs/vdev_queue.c b/module/zfs/vdev_queue.c
index 4ed62f9632..c4e80d517f 100644
--- a/module/zfs/vdev_queue.c
+++ b/module/zfs/vdev_queue.c
@@ -24,7 +24,7 @@
  */
 
 /*
- * Copyright (c) 2012, 2014 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2017 by Delphix. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
@@ -545,7 +545,7 @@ vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
 
 	/*
 	 * Walk backwards through sufficiently contiguous I/Os
-	 * recording the last non-option I/O.
+	 * recording the last non-optional I/O.
 	 */
 	while ((dio = AVL_PREV(t, first)) != NULL &&
 	    (dio->io_flags & ZIO_FLAG_AGG_INHERIT) == flags &&
@@ -567,10 +567,14 @@ vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
 
 	/*
 	 * Walk forward through sufficiently contiguous I/Os.
+	 * The aggregation limit does not apply to optional i/os, so that
+	 * we can issue contiguous writes even if they are larger than the
+	 * aggregation limit.
 	 */
 	while ((dio = AVL_NEXT(t, last)) != NULL &&
 	    (dio->io_flags & ZIO_FLAG_AGG_INHERIT) == flags &&
-	    IO_SPAN(first, dio) <= zfs_vdev_aggregation_limit &&
+	    (IO_SPAN(first, dio) <= zfs_vdev_aggregation_limit ||
+	    (dio->io_flags & ZIO_FLAG_OPTIONAL)) &&
 	    IO_GAP(last, dio) <= maxgap) {
 		last = dio;
 		if (!(last->io_flags & ZIO_FLAG_OPTIONAL))
@@ -605,6 +609,7 @@ vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
 		dio = AVL_NEXT(t, last);
 		dio->io_flags &= ~ZIO_FLAG_OPTIONAL;
 	} else {
+		/* do not include the optional i/o */
 		while (last != mandatory && last != first) {
 			ASSERT(last->io_flags & ZIO_FLAG_OPTIONAL);
 			last = AVL_PREV(t, last);
@@ -616,7 +621,6 @@ vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
 		return (NULL);
 
 	size = IO_SPAN(first, last);
-	ASSERT3U(size, <=, zfs_vdev_aggregation_limit);
 
 	buf = zio_buf_alloc_flags(size, KM_NOSLEEP);
 	if (buf == NULL)