md: separate meta and data devs

Allow the metadata to be on a separate device from the
data.

This doesn't mean the data and metadata will by on separate
physical devices - it simply gives device-mapper and userspace
tools more flexibility.

Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 6 insertions, 0 deletions

diff --git a/drivers/md/md.h b/drivers/md/md.h
index 7e4f358a26a6..eec517ced31a 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -60,6 +60,12 @@ struct mdk_rdev_s
         mddev_t *mddev;                 /* RAID array if running */
         int last_events;                /* IO event timestamp */
 
+        /*
+         * If meta_bdev is non-NULL, it means that a separate device is
+         * being used to store the metadata (superblock/bitmap) which
+         * would otherwise be contained on the same device as the data (bdev).
+         */
+        struct block_device *meta_bdev;
         struct block_device *bdev;      /* block device handle */
 
         struct page     *sb_page;