Comment 0 for bug 1746863

I happened upon this while hacking on my WIP CellDatabases fixture patch. Some of the nova/tests/functional/test_server_group.py tests started failing with multiple cells and I found that it's because there's a database query objects.InstanceList.get_by_filters for all instances who are members of the server group, every time the ServerGroup[Anti|]AffinityFilter runs. The query for instances doesn't check all cells, so it fails to return any hosts that group members are currently on.

This makes the ServerGroup[Anti|]AffinityFilter a no-op for multiple cells. Affinity *is* however ultimately checked via the late-affinity check in compute, so affinity is not totally broken for multiple cells.

Aside from that, I would expect the database query to noticeably degrade performance of scheduling if the ServerGroup[Anti|]AffinityFilter is in enabled_filters, for both the single cell and multiple cell cases.

To fix this, I expect we'll need to pre-load RequestSpec.instance_group.hosts before we schedule each instance -- and make sure we query all cells for the instances. I'm not sure what special consideration we might need for multi-create.

This is the code that lazy-loads instance_group.hosts, which in turn calls InstanceGroup.get_hosts, which calls InstanceList.get_by_filters without targeting any cells:

nova/scheduler/filters/affinity_filter.py:

        group_hosts = (spec_obj.instance_group.hosts
                       if spec_obj.instance_group else [])

nova/objects/instance_group.py:

    def obj_load_attr(self, attrname):
        ...
        self.hosts = self.get_hosts()
        self.obj_reset_changes(['hosts'])

    ...

    @base.remotable
    def get_hosts(self, exclude=None):
        """Get a list of hosts for non-deleted instances in the group
        This method allows you to get a list of the hosts where instances in
        this group are currently running. There's also an option to exclude
        certain instance UUIDs from this calculation.
        """
        filter_uuids = self.members
        if exclude:
            filter_uuids = set(filter_uuids) - set(exclude)
        filters = {'uuid': filter_uuids, 'deleted': False}
        instances = objects.InstanceList.get_by_filters(self._context,
                                                        filters=filters)
        return list(set([instance.host for instance in instances
                         if instance.host]))