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[OpenStack Infra <1727558@xxxxxxxxxxxxxxxxxx>]

Reviewed:  https://review.openstack.org/514339
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=24e79bcbf7790d1f4fea2cbdf066599cc746c2dc
Submitter: Zuul
Branch:    master

commit 24e79bcbf7790d1f4fea2cbdf066599cc746c2dc
Author: Kashyap Chamarthy <kchamart@xxxxxxxxxx>
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 <kchamart@xxxxxxxxxx>
        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 <melwittt@xxxxxxxxx>
    
    Closes-Bug: #1727558
    
    Change-Id: I7370cc2942a6c8c51ab5355b50a9e5666cca042e


** Changed in: nova
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/1727558

Title:
  libvirt driver ignores 'disk_cachemodes' configuration setting

Status in OpenStack Compute (nova):
  Fix Released
Status in OpenStack Compute (nova) newton series:
  In Progress
Status in OpenStack Compute (nova) ocata series:
  In Progress
Status in OpenStack Compute (nova) pike series:
  In Progress

Bug description:
  The libvirt driver is ignoring the 'disk_cachemodes' configuration
  setting and is always setting "cache='none'" in the device xml.

  For example, with a setting in nova.conf of
  "disk_cachemodes=network=writeback":

  Expected result:

  # virsh dumpxml <instance> | grep cache
          <driver name='qemu' type='raw' cache='writeback' discard='unmap'/>
          <driver name='qemu' type='raw' cache='writeback' discard='unmap'/>
          <driver name='qemu' type='raw' cache='writeback' discard='unmap'/>
          <driver name='qemu' type='raw' cache='writeback' discard='unmap'/>

  Actual result:

  # virsh dumpxml <instance> | grep cache
          <driver name='qemu' type='raw' cache='none' discard='unmap'/>
          <driver name='qemu' type='raw' cache='none' discard='unmap'/>
          <driver name='qemu' type='raw' cache='none' discard='unmap'/>
          <driver name='qemu' type='raw' cache='none' discard='unmap'/>

  This is a regression in pike [1] that was also backported to ocata and
  newton.

  [1] https://review.openstack.org/#/c/485752

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