[Bugs] [Bug 1440051] Application VMs with their disk images on sharded-replica 3 volume are unable to boot after performing rebalance

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1440051



--- Comment #2 from Worker Ant <bugzilla-bot at gluster.org> ---
COMMIT: https://review.gluster.org/17010 committed in master by Pranith Kumar
Karampuri (pkarampu at redhat.com) 
------
commit 99c8c0b03a3368d81756440ab48091e1f2430a5f
Author: Krutika Dhananjay <kdhananj at redhat.com>
Date:   Thu Apr 6 18:10:41 2017 +0530

    features/shard: Fix vm corruption upon fix-layout

    shard's writev implementation, as part of identifying
    presence of participant shards that aren't in memory,
    first sends an MKNOD on these shards, and upon EEXIST error,
    looks up the shards before proceeding with the writes.

    The VM corruption was caused when the following happened:
    1. DHT had n subvolumes initially.
    2. Upon add-brick + fix-layout, the layout of .shard changed
       although the existing shards under it were yet to be migrated
       to their new hashed subvolumes.
    3. During this time, there were writes on the VM falling in regions
       of the file whose corresponding shards were already existing under
       .shard.
    4. Sharding xl sent MKNOD on these shards, now creating them in their
       new hashed subvolumes although there already exist shard blocks for
       this region with valid data.
    5. All subsequent writes were wound on these newly created copies.

    The net outcome is that both copies of the shard didn't have the correct
    data. This caused the affected VMs to be unbootable.

    FIX:
    For want of better alternatives in DHT, the fix changes shard fops to do
    a LOOKUP before the MKNOD and upon EEXIST error, perform another lookup.

    Change-Id: I8a2e97d91ba3275fbc7174a008c7234fa5295d36
    BUG: 1440051
    RCA'd-by: Raghavendra Gowdappa <rgowdapp at redhat.com>
    Reported-by: Mahdi Adnan <mahdi.adnan at outlook.com>
    Signed-off-by: Krutika Dhananjay <kdhananj at redhat.com>
    Reviewed-on: https://review.gluster.org/17010
    Smoke: Gluster Build System <jenkins at build.gluster.org>
    Reviewed-by: Pranith Kumar Karampuri <pkarampu at redhat.com>
    NetBSD-regression: NetBSD Build System <jenkins at build.gluster.org>
    CentOS-regression: Gluster Build System <jenkins at build.gluster.org>

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