2 files changed, 8 insertions, 1 deletions

diff --git a/sbin/mdconfig/mdconfig.8 b/sbin/mdconfig/mdconfig.8
index 0b97ddf1c1206..4b2f8e246861e 100644
--- a/sbin/mdconfig/mdconfig.8
+++ b/sbin/mdconfig/mdconfig.8
@@ -141,6 +141,9 @@ other devices.
 .It Fl o Oo Cm no Oc Ns Ar option
 Set or reset options.
 .Bl -tag -width indent
+.It Oo Cm no Oc Ns Cm async
+For vnode backed devices:  avoid IO_SYNC for increased performance but
+at the risk of deadlocking the entire kernel.
 .It Oo Cm no Oc Ns Cm reserve
 Allocate and reserve all needed storage from the start, rather than as needed.
 .It Oo Cm no Oc Ns Cm cluster
diff --git a/sbin/mdconfig/mdconfig.c b/sbin/mdconfig/mdconfig.c
index 6296da7992686..b23e903831aba 100644
--- a/sbin/mdconfig/mdconfig.c
+++ b/sbin/mdconfig/mdconfig.c
@@ -117,7 +117,11 @@ main(int argc, char **argv)
 		case 'o':
 			if (cmdline != 2)
 				usage();
-			if (!strcmp(optarg, "cluster"))
+			if (!strcmp(optarg, "async"))
+				mdio.md_options |= MD_ASYNC;
+			else if (!strcmp(optarg, "noasync"))
+				mdio.md_options &= ~MD_ASYNC;
+			else if (!strcmp(optarg, "cluster"))
 				mdio.md_options |= MD_CLUSTER;
 			else if (!strcmp(optarg, "nocluster"))
 				mdio.md_options &= ~MD_CLUSTER;