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[Matt Riedemann <mriedem.os@xxxxxxxxx>]

The inefficient query goes back to Icehouse when the
InstanceGroup.get_hosts() method was introduced:

https://review.opendev.org/#/c/77786/

So we can at least backport the #1 fix above to stable/ocata.

** Also affects: nova/rocky
   Importance: Undecided
       Status: New

** Also affects: nova/ocata
   Importance: Undecided
       Status: New

** Also affects: nova/stein
   Importance: Undecided
       Status: New

** Also affects: nova/queens
   Importance: Undecided
       Status: New

** Also affects: nova/pike
   Importance: Undecided
       Status: New

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Engineering Team, which is subscribed to OpenStack Compute (nova).
https://bugs.launchpad.net/bugs/1830234

Title:
  InstanceGroup.get_hosts() uses inefficient DB queries

Status in OpenStack Compute (nova):
  Triaged
Status in OpenStack Compute (nova) ocata series:
  New
Status in OpenStack Compute (nova) pike series:
  New
Status in OpenStack Compute (nova) queens series:
  New
Status in OpenStack Compute (nova) rocky series:
  New
Status in OpenStack Compute (nova) stein series:
  New

Bug description:
  The InstanceGroup.get_hosts() method is pretty inefficient when
  pulling instances out of the database here:

  https://github.com/openstack/nova/blob/c7e9e667426a6d88d396a59cb40d30763a3265f9/nova/objects/instance_group.py#L500

  because that by default is going to join on the following tables:

  https://github.com/openstack/nova/blob/c7e9e667426a6d88d396a59cb40d30763a3265f9/nova/db/sqlalchemy/api.py#L2098

      if columns_to_join is None:
          columns_to_join_new = ['info_cache', 'security_groups']
          manual_joins = ['metadata', 'system_metadata']

  And then just turn around and only use the instance.host value:

  https://github.com/openstack/nova/blob/c7e9e667426a6d88d396a59cb40d30763a3265f9/nova/objects/instance_group.py#L502

  return list(set([instance.host for instance in instances
                   if instance.host]))

  We should be:

  1. Avoiding those unnecessary joins by passing expected_attrs=[] which
  is a simple backportable fix.

  2. Write a new DB API method which would get the set of distinct
  instances.host values for the list of instance uuids where the
  instances.host value is not None, so I think:

  SELECT host FROM instances WHERE host IS NOT NULL AND deleted == 0 AND
  uuid IN ($instance_uuids) GROUP BY instances.host;

  That way we let the DB query do the work and we don't have to load up
  all of the additional instances fields we don't care about.

  The DB API optimization would likely need to be a remotable method
  because InstanceGroup.get_hosts() is calling in the late affinity
  check ComputeManager._validate_instance_group_policy() method in the
  nova-compute service (and because InstanceGroup.get_hosts() is
  remotable itself).

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