Comment 15 for bug 1878024

Reviewed: https://review.opendev.org/729523
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=968981b5853724a8225cfc16b04ea83b4f485a9a
Submitter: Zuul
Branch: stable/ussuri

commit 968981b5853724a8225cfc16b04ea83b4f485a9a
Author: Balazs Gibizer <email address hidden>
Date: Fri May 15 15:00:20 2020 +0200

    Reserve DISK_GB resource for the image cache

    If the nova compute image cache is on the same disk as the instance
    directory then the images in the cache will consume the same disk
    resource as the nova instances which can lead to disk overallocation.

    This patch adds a new config option
    [workarounds]/reserve_disk_resource_for_image_cache . If it is set the
    libvirt driver will reserve DISK_GB resources in placement for the
    actual size of the image cache.

    This is a low complexity solution with known limitations:
    * Reservation is updated long after the image is downloaded
    * This code allows the reservation to push the provider into negative
    available resource if the reservation + allocations exceed the total
    inventory.

    Change-Id: If874f018ea996587e178219569c2903c2ee923cf
    Closes-Bug: #1878024
    (cherry picked from commit 89fe504abfbd41a9c37f9b544c0ce98b23b45799)