Comment 32 for bug 1789654

Reviewed: https://review.openstack.org/613271
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=705284e18c3a9f2c93ecb15bd1bfee5d7710b684
Submitter: Zuul
Branch: stable/queens

commit 705284e18c3a9f2c93ecb15bd1bfee5d7710b684
Author: Matt Riedemann <email address hidden>
Date: Thu Aug 30 17:57:24 2018 -0400

    Don't persist zero allocation ratios in ResourceTracker

    The ComputeNode object itself has a facade which provides the
    actual default values for cpu, disk and ram allocation ratios
    when the config option values are left at the default (0.0).

    When we initially create a ComputeNode in the ResourceTracker,
    the *_allocation_ratio values in the object are going to be
    unset, and then _copy_resources, called from _init_compute_node,
    will set the values to the config option values, again, defaulted
    to 0.0, but the ComputeNode object, after calling create() or
    save(), will change those *on the object* to the non-0.0 we
    actually know and love (16.0 for cpu, 1.5 for ram, and 1.0 for disk).

    During the update_available_resource periodic, we'll again go
    through _init_compute_node and _copy_resources in the ResourceTracker
    which will set the configured values (default of 0.0) onto the
    ComputeNode object, which makes the _resource_change method, called
    from _update, return True and trigger a ComputeNode.save() call
    from the _update method. At that point we're *persisting* the 0.0
    allocation ratios in the database, even though ComputeNode.save
    will change them to their non-0.0 default values *on the object*
    because of the _from_db_object call at the end of ComputeNode.save.

    So even if the ComputeNode object allocation ratio values are the
    non-0.0 defaults, we'll *always* update the database on every
    periodic even if nothing else changed in inventory.

    This change modifies the _copy_resource method to only update the
    ComputeNode fields if the configured ratios are not the 0.0 default.

    Change-Id: I43a23a3290db0c835fed01b8d6a38962dc61adce
    Related-Bug: #1789654
    (cherry picked from commit 2588af87c862cfd02d860f6b860381e907b279ff)
    (cherry picked from commit 01265b98c4cd2b1377e891a06ce748fc6f8f3425)