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[Sylvain Bauza <sbauza@xxxxxxx>]

Nova doesn't have (yet) in its scope to manage instance allocations and
that really looks like a new feature that should be discussed in a spec.

http://docs.openstack.org/developer/nova/process.html#how-do-i-get-my-
code-merged

** Changed in: nova
       Status: New => Opinion

** Changed in: nova
   Importance: Undecided => Wishlist

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https://bugs.launchpad.net/bugs/1630929

Title:
  Affinity instance slot reservation

Status in OpenStack Compute (nova):
  Opinion

Bug description:
  Currently ServerGroupAffinityFilter schedules all instances of a group
  on the same host, but it doesn't reserve any slots for future
  instances on the host.

  Example:
  max_instances_per_host=10
  quota_server_group_members=5

  When user_1 would spawns 3 instances with affinity server group policy, all the instances will be scheduled on the same host (host_A). 
  If user_2 also spawns instances and they are placed on host_A, quota of max_instances_per_host will be reached, so user_1 can not add 2 new instances to the same server group and error "No valid host found" will be returned.

  My proposition is to add new parameters to nova.conf to configure ServerGroupAffinityFilter:
  - enable_slots_reservation (Boolean)
  - reserved_slots_per_instance (-1 will count difference between max_instances_per_host and quota_server_group_members ; values bigger than 0 will reserve the indicated number of ports per group)

  Nova scheduler checks if on a host there are any instances with
  affinity policy and based on that, it counts available slots.

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