Comment 12 for bug 1452823

Reviewed: https://review.openstack.org/184827
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=03562c362cfa217ec794dd98f197f623c1ffe45d
Submitter: Jenkins
Branch: stable/kilo

commit 03562c362cfa217ec794dd98f197f623c1ffe45d
Author: John Griffith <email address hidden>
Date: Mon May 11 13:55:41 2015 -0600

    Check volume_backend in retype

    The retype command will always attempt a call
    to the driver.retype method. In *most* cases this
    will hit the default impl which returns False because
    most drivers don't implement any retype (most, a few do).

    The problem is that the drivers that do implement this in
    most cases will iterate through the settings and just make
    the changes that are valid and ignore the rest, and then
    return True. I think this is "ok" for the drivers to do,
    drivers should be allowed to be somewhat dumb WRT Cinder
    state management and placement info. If we give them an
    invalid command (which we're doing here) then it's on us
    higher up the chain IMO.

    The result is that for example if you're trying to retype
    from backend-a to backend-b and backend-a implements retype
    it can return True telling the manager that the volume was
    succesfully retyped, even when it wasn't.

    There's a lot of confusion around this bug, YES the
    filter scheduler is used to determine if the retype is
    valid and to what host. That's not the issue, the issue
    is that regardless of the source and destination host settings
    that are provided from the filter-scheduler, we always make the
    call to the driver, introducing the opportunity for a false
    success status being reported back.

    This patch adds a very simple check between the source and
    destination host settings as provided by the scheduler and in
    the case that the two are "different"(not including pool designations)
    we skip calling the driver.retype method altogether and fall through
    to the migrate process.

    This introduces a new hosts_are_equivalent method in
    cinder.volume.utils

    Conflicts:
     cinder/volume/utils.py

    Closes-Bug: #1452823
    (cherry picked from commit bbb83aff023c90cfc186459603320ad3c622a40b)
    Change-Id: Idfd7dfa2284fcea66cf23c4273efda61ee8f7eba