openstack/specs/affinity-antiaffinity-of-instances-in-a-groups.rst

This task can be divided into two separate parts. First is a change of behavior of host aggregates, to be able to include ironic nodes. Second, involves changes to the scheduler to support spreading instances across various groups.

Questions

There are several ways that aggregate (anti)affinity could be defined expressed.

1. Pass through hints, both information about server group and condition for server placement against host aggregates:

   openstack aggregate-create rack1
   openstack aggregate-create rack2
   openstack aggregate-set-metadata rack1 rack=1
   openstack aggregate-set-metadata rack2 rack=1
   openstack aggregate-add-host rack1 <UUID of Ironic node A>
   openstack aggregate-add-host rack2 <UUID of Ironic node B>
   openstack server-group-create not-same-rack anti-affinity
   openstack server create --image=IMAGE_ID --flavor=1 \
       --hint group=not-same-rack --hint property=rack

2. Implement own policies to set on instance groups, like rack_antiaffinity, pz_antiaffinity etc, and than during boot time just pass a server group name. For instance:

   openstack aggregate-create rack1
   openstack aggregate-create rack2
   openstack aggregate-set-metadata rack1 rack=1
   openstack aggregate-set-metadata rack2 rack=1
   openstack aggregate-add-host rack1 <UUID of Ironic node A>
   openstack aggregate-add-host rack2 <UUID of Ironic node B>
   openstack server-group-create not-same-rack rack-anti-affinity
   openstack server create --image=IMAGE_ID --flavor=1 \
       --hint group=not-same-rack

3. Hardcode policy into the server group name, but this requires from users, to be very strict about naming of server groups:

   openstack aggregate-create rack1
   openstack aggregate-create rack2
   openstack aggregate-set-metadata rack1 rack=1
   openstack aggregate-set-metadata rack2 rack=1
   openstack aggregate-add-host rack1 <UUID of Ironic node A>
   openstack aggregate-add-host rack2 <UUID of Ironic node B>
   openstack server-group-create rack-<unique-name> anti-affinity
   openstack server create --image=IMAGE_ID --flavor=1 \
       --hint group=rack-<unique-name>

Which solution should we take?