diff --git a/sbin/mdmfs/mdmfs.c b/sbin/mdmfs/mdmfs.c
index 6e4a39611a89..2dcf087079e5 100644
--- a/sbin/mdmfs/mdmfs.c
+++ b/sbin/mdmfs/mdmfs.c
@@ -196,6 +196,7 @@ main(int argc, char **argv)
 				usage();
 			mdtype = MD_MALLOC;
 			have_mdtype = true;
+			argappend(&mdconfig_arg, "-o reserve");
 			break;
 		case 'm':
 			argappend(&newfs_arg, "-m %s", optarg);