Bug 1339886

In Red Hat OpenStack Platform 9, it is possible to apply soft affinity that allows users to specify a preference for two instances to start on the same Compute node where possible.

Details of the new rules should be added to the documentation. Perhaps a good candidate for "Table 3.4. Scheduling Filters" in the Instances and Images guide?

From the engineering bug: 

"As a tenant I would like to schedule instances on the same host if possible,
so that I can achieve colocation. However if it is not possible to schedule
some instance to the same host then I still want that the subsequent
instances are scheduled together on another host. In this way I can express
a good-to-have relationship between a group of instances.


End User might want to have a less strict affinity rule than what is today
available in server-group API extension. With the proposed good-to-have
affinity rule the End User can request nova to schedule the instance to the
same host if possible. However if it is not possible (e.g. due to resource
limitations) then End User still wants to keep the instances on a small
amount of different host.


This change would extend the existing server-group API extension with a new
policy soft-affinity. When a instance is booted into a group with
soft-affintiy policy the scheduler will use a new weight AffinityWeight to
sort the available hosts according to the number of instances running on them
from the same server-group.

This new soft-affinity policy is mutually exclusive with the other existing
server-group policies."