Red Hat Bugzilla – Bug 1469971
cluster/dht: Fix hardlink migration failures
Last modified: 2017-09-21 01:02:13 EDT
+++ This bug was initially created as a clone of Bug #1469964 +++
Description of problem:
There are few races in remove-brick hardlink migration code path detailed below.
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.
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.
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.
This is reproducible intermittently. Since this is related to hardlink migration, this happens only with remove-brick process.
--- Additional comment from Worker Ant on 2017-07-12 12:44:13 MVT ---
REVIEW: https://review.gluster.org/17755 (cluster/rebalance: Fix hardlink migration failures) posted (#1) for review on master by Susant Palai (email@example.com)
upstream patch : https://review.gluster.org/#/c/17755/
On glusterfs version 3.8.4-36.el7rhgs.x86_64, followed the steps mentioned in Comment 10 for 10 times and could not hit this issue.
Moving this BZ to Verified.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.