diff --git a/drivers/md/md.c b/drivers/md/md.c
index 1cd9bfb45e9a0b670eb8a0f1b28f1f316e56a499..9a880239219db5f1e08f2f176ffa0605f0ff22e5 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -1738,6 +1738,11 @@ static void super_1_sync(mddev_t *mddev, mdk_rdev_t *rdev)
 	sb->level = cpu_to_le32(mddev->level);
 	sb->layout = cpu_to_le32(mddev->layout);
 
+	if (test_bit(WriteMostly, &rdev->flags))
+		sb->devflags |= WriteMostly1;
+	else
+		sb->devflags &= ~WriteMostly1;
+
 	if (mddev->bitmap && mddev->bitmap_info.file == NULL) {
 		sb->bitmap_offset = cpu_to_le32((__u32)mddev->bitmap_info.offset);
 		sb->feature_map = cpu_to_le32(MD_FEATURE_BITMAP_OFFSET);