Index: head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zio.h
===================================================================
--- head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zio.h
+++ head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zio.h
@@ -567,6 +567,7 @@
 extern void zio_add_child(zio_t *pio, zio_t *cio);
 
 extern void *zio_buf_alloc(size_t size);
+extern void *zio_buf_alloc_nowait(size_t size);
 extern void zio_buf_free(void *buf, size_t size);
 extern void *zio_data_buf_alloc(size_t size);
 extern void zio_data_buf_free(void *buf, size_t size);
Index: head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_queue.c
===================================================================
--- head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_queue.c
+++ head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_queue.c
@@ -647,6 +647,7 @@
 vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
 {
 	zio_t *first, *last, *aio, *dio, *mandatory, *nio;
+	void *abuf;
 	uint64_t maxgap = 0;
 	uint64_t size;
 	boolean_t stretch;
@@ -755,8 +756,12 @@
 	size = IO_SPAN(first, last);
 	ASSERT3U(size, <=, zfs_vdev_aggregation_limit);
 
+	abuf = zio_buf_alloc_nowait(size);
+	if (abuf == NULL)
+		return (NULL);
+
 	aio = zio_vdev_delegated_io(first->io_vd, first->io_offset,
-	    zio_buf_alloc(size), size, first->io_type, zio->io_priority,
+	    abuf, size, first->io_type, zio->io_priority,
 	    flags | ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_QUEUE,
 	    vdev_queue_agg_io_done, NULL);
 	aio->io_timestamp = first->io_timestamp;
Index: head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c
===================================================================
--- head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c
+++ head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c
@@ -272,18 +272,33 @@
  * useful to inspect ZFS metadata, but if possible, we should avoid keeping
  * excess / transient data in-core during a crashdump.
  */
-void *
-zio_buf_alloc(size_t size)
+static void *
+zio_buf_alloc_impl(size_t size, boolean_t canwait)
 {
 	size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;
 	int flags = zio_exclude_metadata ? KM_NODEBUG : 0;
 
 	VERIFY3U(c, <, SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);
 
-	if (zio_use_uma)
-		return (kmem_cache_alloc(zio_buf_cache[c], KM_PUSHPAGE));
-	else
-		return (kmem_alloc(size, KM_SLEEP|flags));
+	if (zio_use_uma) {
+		return (kmem_cache_alloc(zio_buf_cache[c],
+		    canwait ? KM_PUSHPAGE : KM_NOSLEEP));
+	} else {
+		return (kmem_alloc(size,
+		    (canwait ? KM_SLEEP : KM_NOSLEEP) | flags));
+	}
+}
+
+void *
+zio_buf_alloc(size_t size)
+{
+	return (zio_buf_alloc_impl(size, B_TRUE));
+}
+
+void *
+zio_buf_alloc_nowait(size_t size)
+{
+	return (zio_buf_alloc_impl(size, B_FALSE));
 }
 
 /*