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Affinity

Goal

  • This guide shows how to distribute instances to different hosts using server groups
  • It is also shown how to force instances on the same host

Prerequisites

Problem

By default, there is no guarantees whether servers will be distributed across different hypervisors. The Nova compute scheduler makes that decision based on available resources.
This can lead to services that are meant to be highly available to share a common host and thus share a single point of failure.
Inversely it might be desired to have two services to be located as close as possible, because they will need high bandwidth between each other.
Both cases are solvable using ServerGroups. That way you can influence the distribution of instances.

Principle

Create a OS::Nova::ServerGroup resource and reference it as scheduler hint in the server definition.

resources:
  anti-affinity_group:
   type: OS::Nova::ServerGroup
   properties:
    name: hosts on separate compute nodes
    policies:
     - anti-affinity

  my_instance:
    type: OS::Nova::Server
    properties:
      user_data_format: RAW
      image: Ubuntu Jammy 22.04 (2022-08-11)
      flavor: m1.small
      name: server app 1
      scheduler_hints:
        group: { get_resource: anti-affinity_group }

Example

Here the ResourceGroups example is extended using ServerGroups for affinity and anti-affinity group.yaml:

heat_template_version: 2014-10-16

#
# you can start this stack using the following command:
# 'openstack stack create -t group.yaml <stack-name>'
#

description: deploys a group of servers with only internal network.

resources:
  same_hosts:
    type: OS::Heat::ResourceGroup
    depends_on: example_subnet
    properties:
      count: 2
      resource_def:
        type: server.yaml
        properties:
          network_id: { get_resource: example_net}
          server_name: server_%index%
          affinity_group: { get_resource: affinity_group }

  separate_hosts:
    type: OS::Heat::ResourceGroup
    depends_on: example_subnet
    properties:
      count: 2
      resource_def:
        type: server.yaml
        properties:
          network_id: { get_resource: example_net}
          server_name: antiserver_%index%
          affinity_group: { get_resource: anti-affinity_group }


  anti-affinity_group:
   type: OS::Nova::ServerGroup
   properties:
    name: hosts on separate compute nodes
    policies:
     - anti-affinity

  affinity_group:
   type: OS::Nova::ServerGroup
   properties:
    name: hosts on one compute-node
    policies:
     - affinity

  example_net:
    type: OS::Neutron::Net
    properties:
      name: example-net

  example_subnet:
    type: OS::Neutron::Subnet
    properties:
      name: example_subnet
      dns_nameservers:
        - 8.8.8.8
        - 8.8.4.4
      network_id: {get_resource: example_net}
      ip_version: 4
      cidr: 10.0.0.0/24
      allocation_pools:
      - {start: 10.0.0.10, end: 10.0.0.250}

The already known server.yaml extended by affinity_group parameter:

heat_template_version: 2014-10-16

description: single server resource used by resource groups.

parameters:
  network_id:
    type: string
  server_name:
    type: string
  affinity_group:
    type: string

resources:
  my_instance:
    type: OS::Nova::Server
    properties:
      user_data_format: RAW
      image: Ubuntu Jammy 22.04 (2022-08-11)
      flavor: m1.small
      name: { get_param: server_name }
      networks:
        - port: { get_resource: example_port }
      scheduler_hints:
        group: { get_param: affinity_group }

  example_port:
    type: OS::Neutron::Port
    properties:
      network_id: { get_param: network_id }

Conclusion

  • increased availability when using anti-affinity policy
  • increased data throughput when using affinity policy

There are two options to check whether the scheduler hints were fully considered. For one you can list the members of a ServerGroup:

openstack server group show "hosts on one compute-node"
+----------+----------------------------------------------------------------------------+
| Field    | Value                                                                      |
+----------+----------------------------------------------------------------------------+
| id       | 0d6a2b32-0cef-419b-9e58-23d0894f9a04                                       |
| members  | c3719d43-a3c4-4359-b2d1-8a4626ccf8d6, cf89eb84-1fb5-4dd8-bb88-2ebac4c64273 |
| name     | hosts on one compute-node                                                  |
| policies | affinity                                                                   |
+----------+----------------------------------------------------------------------------+

Also you can compare the hostIds of the individual instances:

openstack server show server_0 -c name -c hostId
+--------+----------------------------------------------------------+
| Field  | Value                                                    |
+--------+----------------------------------------------------------+
| hostId | eda910fefd0756ea0d88b7c84ba01ddc0f350332e351348a33e0132f |
| name   | server_0                                                 |
+--------+----------------------------------------------------------+
openstack server show server_1 -c name -c hostId
+--------+----------------------------------------------------------+
| Field  | Value                                                    |
+--------+----------------------------------------------------------+
| hostId | eda910fefd0756ea0d88b7c84ba01ddc0f350332e351348a33e0132f |
| name   | server_1                                                 |
+--------+----------------------------------------------------------+

Links/Examples

The templates published on GitHub affinity contain examples for affinity and anti-affinity.