Comment 3 for bug 1692775

Reviewed: https://review.openstack.org/507510
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=af0f00bc52f79d9395adfe0575b0dbe353e18bbe
Submitter: Zuul
Branch: master

commit af0f00bc52f79d9395adfe0575b0dbe353e18bbe
Author: Gorka Eguileor <email address hidden>
Date: Wed Sep 13 19:46:17 2017 +0200

    Run backup compression on native thread

    Backup data compression is a CPU bound operation that will not yield to
    other greenthreads, so given enough simultaneous backup operations they
    will lead to other threads' starvation.

    This is really problematic for DB connections, since starvation will
    lead to connections getting dropped with errors such as "Lost connection
    to MySQL server during query".

    Detailed information on why these connections get dropped can be found
    in comment "[31 Aug 2007 9:21] Magnus Blåudd" on this MySQL bug [1].

    These DB issues may result in backups unnecessary ending in an "error"
    state.

    This patch fixes this by moving the compression to a native thread so
    the cooperative multitasking in Cinder Backup can continue switching
    threads.

    [1] https://bugs.mysql.com/bug.php?id=28359

    Closes-Bug: #1692775
    Closes-Bug: #1719580
    Change-Id: I1946dc0ad9cb7a68072a39816fa9fa224c2eb6a5