Comment 7 for bug 1770434

I think I was able to reproduce the problem in a fresh single node devstack [1].

The GroupAntiAffinityFilter printed the following when scheduled the second instance:

Group anti affinity: check if ubuntu not in [] {{(pid=25524) host_passes /opt/stack/nova/nova/scheduler/filters/affinity_filter.py:105}}

This means that the instance_group.hosts field is empty during the second scheduling towards the same host the first instance is already running on [2].

It felt like I'm booting the two instances two quickly and hitting a race. So I waited couple of minutes and tried a 3rd instance. But that also placed to the same (and only) compute host.

[1] http://paste.openstack.org/show/725339/
[2] https://github.com/openstack/nova/blob/fcf406003bcde2b60a515f694f2640e27a9d65fd/nova/scheduler/filters/affinity_filter.py#L101-L105