diff --git a/sys/geom/geom_io.c b/sys/geom/geom_io.c
--- a/sys/geom/geom_io.c
+++ b/sys/geom/geom_io.c
@@ -137,12 +137,17 @@
 	return (bp);
 }
 
+static int reserved_new_bios = 65536;
+
 struct bio *
 g_new_bio(void)
 {
 	struct bio *bp;
 
-	bp = uma_zalloc(biozone, M_NOWAIT | M_ZERO);
+	bp = uma_zalloc(biozone, M_NOWAIT | M_ZERO |
+	    (reserved_new_bios > 0 ? M_USE_RESERVE : 0));
+	if (__predict_false(NULL == bp))
+		printf("g_new_bio(): failed to allocate\n");
 #ifdef KTR
 	if (KTR_GEOM_ENABLED) {
 		struct stack st;
@@ -193,7 +198,8 @@
 {
 	struct bio *bp2;
 
-	bp2 = uma_zalloc(biozone, M_NOWAIT | M_ZERO);
+	bp2 = uma_zalloc(biozone, M_NOWAIT | M_ZERO |
+	    (reserved_new_bios > 0 ? M_USE_RESERVE : 0));
 	if (bp2 != NULL) {
 		bp2->bio_parent = bp;
 		bp2->bio_cmd = bp->bio_cmd;
@@ -279,6 +285,14 @@
 	    NULL, NULL,
 	    NULL, NULL,
 	    0, 0);
+	/*
+	 * XXX the reservation of a uma(9) zone cannot be altered if it is serving
+	 * any items.
+	 */
+	if (reserved_new_bios > 0) {
+		uma_prealloc(biozone, reserved_new_bios);
+		uma_zone_reserve(biozone, reserved_new_bios);
+	}
 }
 
 int
@@ -734,6 +748,9 @@
 SYSCTL_INT(_kern_geom, OID_AUTO, inflight_transient_maps, CTLFLAG_RD,
     &inflight_transient_maps, 0,
     "Current count of the active transient maps");
+SYSCTL_INT(_kern_geom, OID_AUTO, reserved_new_bios, CTLFLAG_RDTUN,
+    &reserved_new_bios, 0,
+    "Number of reserved new bios for non-blocking allocation");
 
 static int
 g_io_transient_map_bio(struct bio *bp)