[Bugs] [Bug 1236271] New: Introduce an ATOMIC_WRITE flag in posix writev

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1236271

            Bug ID: 1236271
           Summary: Introduce an ATOMIC_WRITE flag in posix writev
           Product: GlusterFS
           Version: 3.7.2
         Component: posix
          Keywords: Triaged
          Assignee: bugs at gluster.org
          Reporter: kdhananj at redhat.com
                CC: bugs at gluster.org, gluster-bugs at redhat.com
        Depends On: 1233617



+++ This bug was initially created as a clone of Bug #1233617 +++

Description of problem:

posix_writev performs
a) stat (prestat) on the given fd
b) requested write, and
c) stat (poststat) on the given fd

in a lockless manner. When two or more io-threads try to write to the same file
in parallel, chances are that the stats gathered per write are not truly
reflective of the change (in the number of bytes and blocks) caused by the
individual writes. Sometimes it is useful for translators above (like sharding)
to know the exact change in the number of bytes per write.

The xlator interested in this behavior could pass a flag in xdata to posix,
instructing it to give the precise delta bytes and block count. And posix could
ensure this happens by first holding inode->lock, performing a), b) and c) and
then unlocking the mutex.

Version-Release number of selected component (if applicable):


How reproducible:


Steps to Reproduce:
1.
2.
3.

Actual results:


Expected results:


Additional info:

--- Additional comment from Anand Avati on 2015-06-22 07:41:11 EDT ---

REVIEW: http://review.gluster.org/11345 (storage/posix: Introduce flag
instructing posix to perform prestat, writev and poststat atomically) posted
(#1) for review on master by Krutika Dhananjay (kdhananj at redhat.com)

--- Additional comment from Anand Avati on 2015-06-23 06:30:47 EDT ---

REVIEW: http://review.gluster.org/11345 (storage/posix: Introduce flag
instructing posix to perform prestat, writev and poststat atomically) posted
(#2) for review on master by Krutika Dhananjay (kdhananj at redhat.com)

--- Additional comment from Anand Avati on 2015-06-25 08:07:03 EDT ---

REVIEW: http://review.gluster.org/11345 (storage/posix: Introduce flag
instructing posix to perform prestat, writev and poststat atomically) posted
(#3) for review on master by Krutika Dhananjay (kdhananj at redhat.com)


Referenced Bugs:

https://bugzilla.redhat.com/show_bug.cgi?id=1233617
[Bug 1233617] Introduce an ATOMIC_WRITE flag in posix writev
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