diff --git a/userspace/marsadm b/userspace/marsadm
index cdf829fb..2c679afb 100755
--- a/userspace/marsadm
+++ b/userspace/marsadm
@@ -863,8 +863,12 @@ sub create_res {
   my $size = 0;
   if (-b $dev) {
     ldie "block device '$dev' must be an absolute path starting with '/'\n" unless $dev =~ m/^\//;
-    use Fcntl 'SEEK_END', 'O_RDWR', 'O_EXCL';
-    sysopen(TEST, $dev, O_RDWR|O_EXCL) or ldie "cannot open device '$dev' for exclusive rw access\n";
+    use Fcntl 'SEEK_END', 'O_RDONLY', 'O_RDWR', 'O_EXCL';
+    my $flags = O_RDWR | O_EXCL;
+    if (!$force) {
+      $flags = O_RDONLY;
+    }
+    sysopen(TEST, $dev, $flags) or ldie "cannot open device '$dev' for exclusive rw access\n";
     $size = sysseek(TEST, 0, SEEK_END);
     close(TEST);
     lprint "block device '$dev': determined size = $size bytes\n";