cluster/rebalance: Fix hardlink migration failures

A brief about how hardlink migration works:
  - Different hardlinks (to the same file) may hash to different bricks,
but their cached subvol will be same. Rebalance picks up the first hardlink,
calculates it's  hash(call it TARGET) and set the hashed subvolume as an xattr
on the data file.
  - Now all the hardlinks those come after this will fetch that xattr and will
create linkto files on TARGET (all linkto files for the hardlinks will be hardlink
to each other on TARGET).
  - When number of hardlinks on source is equal to the number of hardlinks on
TARGET, the data migration will happen.

RACE:1
  Since rebalance is multi-threaded, the first lookup (which decides where the TARGET
subvol should be), can be called by two hardlink migration parallely and they may end
up creating linkto files on two different TARGET subvols. Hence, hardlinks won't be
migrated.

Fix: Rely on the xattr response of lookup inside gf_defrag_handle_hardlink since it
is executed under synclock.

RACE:2
  The linkto files on TARGET can be created by other clients also if they are doing
lookup on the hardlinks.  Consider a scenario where you have 100 hardlinks.  When
rebalance is migrating 99th hardlink, as a result of continuous lookups from other
client, linkcount on TARGET is equal to source linkcount. Rebalance will migrate data
on the 99th hardlink itself. On 100th hardlink migration, hardlink will have TARGET as
cached subvolume. If it's hash is also the same, then a migration will be triggered from
TARGET to TARGET leading to data loss.

Fix: Make sure before the final data migration, source is not same as destination.

RACE:3
  Since a hardlink can be migrating to a non-hashed subvolume, a lookup from other
client or even the rebalance it self, might delete the linkto file on TARGET leading
to hardlinks never getting migrated.

This will be addressed in a different patch in future.

Change-Id: If0f6852f0e662384ee3875a2ac9d19ac4a6cea98
BUG: 1469964
Signed-off-by: Susant Palai <spalai@redhat.com>
Reviewed-on: https://review.gluster.org/17755
Smoke: Gluster Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: N Balachandran <nbalacha@redhat.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/xlators/cluster/dht/src/dht-common.h b/xlators/cluster/dht/src/dht-common.h
index d6ca0448b09..60b0909a1e1 100644
--- a/xlators/cluster/dht/src/dht-common.h
+++ b/xlators/cluster/dht/src/dht-common.h
@@ -1191,8 +1191,7 @@ void*
 gf_defrag_start (void *this);
 
 int32_t
-gf_defrag_handle_hardlink (xlator_t *this, loc_t *loc, dict_t  *xattrs,
-                           struct iatt *stbuf, int *fop_errno);
+gf_defrag_handle_hardlink (xlator_t *this, loc_t *loc, int *fop_errno);
 int
 dht_migrate_file (xlator_t *this, loc_t *loc, xlator_t *from, xlator_t *to,
                  int flag, int *fop_errno);