[Bugs] [Bug 1233617] New: Introduce an ATOMIC_WRITE flag in posix writev

bugzilla at redhat.com bugzilla at redhat.com
Fri Jun 19 09:48:38 UTC 2015


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

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.

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