Affinity and Anti-Affinity with Server Groups

Last changed: 2025-03-14

A Server Group is a collection of instances, and can be used as a mechanism for indicating the location of instances on hypervisors in relation to other instances. They allow you to indicate whether instances should run on the same hypervisor (Affinity) or different hypervisors (Anti-Affinity). Affinity is advantageous if you wish to minimise network latency, while anti-affinity can improve fault-tolerance and load distribution.

See also:

• Upstream Documentation

Policies and rules

Server groups can be configured with a policy and rules. There are currently four policies supported:

affinity - AVOID THIS

Restricts instances belonging to the server group to the same hypervisor.

Warning

NREC does not allow the affinity policy The affinity policy interferes with the migration of instances during hypervisor maintenance. While NREC technically allows this policy, the NREC team will delete any server groups with affinity policy whenever they are encountered.

anti-affinity

Restricts instances belonging to the server group to separate hypervisors.

soft-affinity

Attempts to restrict instances belonging to the server group to the same hypervisor. Where it is not possible to schedule all instances on one hypervisor, they will be scheduled together on as few hypervisors as possible.

soft-anti-affinity

Attempts to restrict instances belonging to the server group to separate hypervisors. Where it is not possible to schedule all instances to separate hypervisors, they will be scheduled on as many separate hypervisors as possible.

There is currently one rule supported:

max_server_per_host

Indicates the max number of instances that can be scheduled to any given hypervisor when using the anti-affinity policy. This rule is not compatible with other policies, and can only be specified when creating the server group using OpenStack CLI.

Usage

Important

Server Groups and memberships are immutable Once a server group has been created, it cannot be modified. Also, an instance can not move between server groups.

Creating a server group “my-server-group” with policy soft-anti-affinity:

1. Navigate to Compute → Server Groups, and click on Create Server Group:

   

2. Name the server group, select the policy, and click Submit:

   

3. The server group is now created:

   

4. When launching an instance, you may select your server group:

   

Doing the same with CLI

openstack server group create --policy soft-anti-affinity my-server-group

An existing server group can be used when creating a server. Example:

openstack server create --hint group=my-server-group ... <NAME>