Comment 6 for bug 1727558

Reviewed: https://review.openstack.org/515543
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=ca03fe87371d8c13fc2895f8ff3e7e1ca88cfe79
Submitter: Zuul
Branch: stable/newton

commit ca03fe87371d8c13fc2895f8ff3e7e1ca88cfe79
Author: Kashyap Chamarthy <email address hidden>
Date: Mon Oct 23 16:27:01 2017 +0200

    libvirt: Don't disregard cache mode for instance boot disks

    One of the things this commit:

        commit 14c38ac0f253036da79f9d07aedf7dfd5778fde8
        Author: Kashyap Chamarthy <email address hidden>
        Date: Thu Jul 20 19:01:23 2017 +0200

            libvirt: Post-migration, set cache value for Cinder volume(s)

        [...]

    did was to supposedly remove "duplicate" calls to _set_cache_mode().

    But that came back to bite us.

    Now, while the Cinder volumes are taken care of w.r.t handling its cache
    value during migration, but the above referred commit (14c38ac) seemed
    to introduce a regression because it disregards the 'disk_cachemodes'
    Nova config parameter altogether for boot disks -- i.e. even though if
    a user set the cache mode to be 'writeback', it's ignored and
    instead 'none' is set unconditionally.

    Add the _set_cache_mode() calls back in _get_guest_storage_config().

    Co-Authored-By: melanie witt <email address hidden>

    Closes-Bug: #1727558

     Conflicts:
     nova/virt/libvirt/driver.py

    NOTE(melwitt): The conflict is from a helper function
    _get_scsi_controller in ocata that doesn't exist in newton.

    Change-Id: I7370cc2942a6c8c51ab5355b50a9e5666cca042e
    (cherry picked from commit 24e79bcbf7790d1f4fea2cbdf066599cc746c2dc)
    (cherry picked from commit 60d6e87cac10ff1f95a028c6176e768214ec8b77)
    (cherry picked from commit fc10b54f25023d7e780b110928bda3a19e4c03f0)