nova/doc/source/user/server-groups.rst

Server Groups

Server groups provide a mechanism for indicating the locality of servers relative to other servers. They allow you to indicate whether servers should run on the same host (affinity) or different hosts (anti-affinity). Affinity is advantageous if you wish to minimise network latency, while anti-affinity can improve fault-tolerance and load distribution.

Note

Server groups are useful for separating or grouping workloads but should not generally be relied on to provide HA. Instead, consider using availability zones. Unlike server groups, availability zones can only be configured by admins but they are often used to model failure domains, particularly in larger deployments. For more information, refer to /user/availability-zones.

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

affinity

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

anti-affinity

Restricts instances belonging to the server group to separate hosts.

soft-affinity

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

Note

Requires API microversion 2.15 or later.

soft-anti-affinity

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

Note

Requires API microversion 2.15 or later.

There is currently one rule supported:

max_server_per_host

Indicates the max number of instances that can be scheduled to any given host when using the anti-affinity policy. This rule is not compatible with other policies.

Note

Requires API microversion 2.64 or later.

Usage

Server groups can be configured and used by end-users. For example:

$ openstack --os-compute-api-version 2.64 server group create \
    --policy POLICY --rule RULE NAME

Once a server group has been created, you can use it when creating a server. This is achieved using the --hint option. For example:

$ openstack server create \
    --hint group=SERVER_GROUP_UUID ... NAME

Once created, a server group cannot be modified. In addition, a server cannot move between server groups. In both cases, this is because doing so would require potentially moving the server to satisfy the server group policy.