md: allow a partially recovered device to be hot-added to an array.

When adding a new device into an array it is normally important to
clear any stale data from ->recovery_offset else the new device may
not be recovered properly.

However when re-adding a device which is known to be nearly in-sync,
this is not needed and can be detrimental.  The (bitmap-based)
resync will still happen, and further recovery is only needed from
where-ever it was already up to.

So if save_raid_disk is set, signifying a re-add, don't clear
->recovery_offset.

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

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 539f08885e7f..757e388308a8 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -7736,7 +7736,8 @@ static int remove_and_add_spares(struct mddev *mddev,
                        !test_bit(Bitmap_sync, &rdev->flags)))
                         continue;
 
-                rdev->recovery_offset = 0;
+                if (rdev->saved_raid_disk < 0)
+                        rdev->recovery_offset = 0;
                 if (mddev->pers->
                     hot_add_disk(mddev, rdev) == 0) {
                         if (sysfs_link_rdev(mddev, rdev))