md: add ->takeover method to support changing the personality managing an array

Implement this for RAID6 to be able to 'takeover' a RAID5 array.  The
new RAID6 will use a layout which places Q on the last device, and
that device will be missing.
If there are any available spares, one will immediately have Q
recovered onto it.

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

1 files changed, 10 insertions, 0 deletions

diff --git a/drivers/md/md.h b/drivers/md/md.h
index 84b22d67ba14..8034f62a9d28 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -321,6 +321,16 @@ struct mdk_personality
          * others - reserved
          */
         void (*quiesce) (mddev_t *mddev, int state);
+        /* takeover is used to transition an array from one
+         * personality to another.  The new personality must be able
+         * to handle the data in the current layout.
+         * e.g. 2drive raid1 -> 2drive raid5
+         *      ndrive raid5 -> degraded n+1drive raid6 with special layout
+         * If the takeover succeeds, a new 'private' structure is returned.
+         * This needs to be installed and then ->run used to activate the
+         * array.
+         */
+        void *(*takeover) (mddev_t *mddev);
 };