[PATCH] md: optimise reconstruction when re-adding a recently failed drive.

When an array is degraded, bit in the intent-bitmap are never cleared.  So if
a recently failed drive is re-added, we only need to reconstruct the block
that are still reflected in the bitmap.

This patch adds support for this re-adding.

Signed-off-by: Neil Brown <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/raid/md_k.h

@@ -183,6 +183,10 @@ struct mdk_rdev_s
 
 	int desc_nr;			/* descriptor index in the superblock */
 	int raid_disk;			/* role of device in array */
+	int saved_raid_disk;		/* role that device used to have in the
+					 * array and could again if we did a partial
+					 * resync from the bitmap
+					 */
 
 	atomic_t	nr_pending;	/* number of pending requests.
 					 * only maintained for arrays that