Comment 11 for bug 937694

Reviewed: https://review.openstack.org/12481
Committed: http://github.com/openstack/nova/commit/64f1916d5871344bdd5177993bca709950e0b819
Submitter: Jenkins
Branch: master

commit 64f1916d5871344bdd5177993bca709950e0b819
Author: Pádraig Brady <email address hidden>
Date: Fri Jul 13 18:22:17 2012 +0100

    avoid the buffer cache when copying volumes

    The dd process when clearing volumes, was seen to starve
    the system when writing to an iSCSI SAN connected over GigE.

    So use O_DIRECT within the dd process so that the system
    buffer cache is not impacted, which is generally the
    best thing to do when streaming large amounts of data.

    Also one could drop the I/O priority of the dd process
    by prepending "ionice -c3". That would change the priority
    from "normal" (best effort) to "idle", which means zeroing
    will only proceed when there is no other I/O on the system.
    It was thought best to leave scheduling decisions to the
    system however, rather than specifying them explicitly.

    Fixes bug: 937694
    Change-Id: Ic842d7b83209c41d8ff05075990ed12e6f86283a