Creating and using a Server Group in Horizon and CLI

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KB0072759

Creating and using a Server Group in Horizon and CLI


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Objective

This documentation explains how to use the Server Groups feature to control the scheduling of a group of instances.

Using Server Groups to provide a mechanism to group instances according to a certain policy.

Prerequisites

To use the OpenStack CLI, make sure you consult the following guides:

Definition

Policies

A Server Group can have one of four different policies: Affinity, Soft Affinity, Anti Affinity or Soft Anti Affinity.

Affinity

A Server Group with the policy of affinity will make sure that all the instances in that group are always placed on the same physical compute node.

Soft Affinity

A Server Group with the policy of soft affinity will try to make sure that all the instances in that group are placed on the same physical compute node, but ultimately will allow it if otherwise not possible.

Anti Affinity

A Server Group with the policy of anti affinity will make sure that the instances in that group are never placed on the same physical compute node.

Soft Anti Affinity

A Server Group with the policy of soft anti affinity will try to make sure that all the instances in that group are not placed on the same physical compute node, but ultimately will allow it if otherwise not possible.

Step 1: Creating Server Groups

  • OpenStack Dashboard
  • CLI

Keystone

  1. Log in to the Horizon interface.
  2. Select the appropriate region from the drop down menu at the top left.
  3. Click on Compute in left tab, then on Server Groups.
  4. Next, cick on + Create Server Group.
  5. In the popup window that appears, select a Name and Policy for your server group.
  6. Click on Submit.

To create a Server Group, use the following command:

openstack server group create \ --policy [affinity|soft-affinity|anti-affinity|soft-anti-affinity] \<server_group_name>

Step 2: Creating an Instance with a Server Group

  • OpenStack Dashboard
  • CLI

Horizon can create, read, update, and delete, as well as obtain a list of Server Groups for a tenant to enable the specification of a group when starting a virtual machine.

  1. Log in to the Horizon interface.
  2. Select the appropriate region from the drop down menu at the top left.
  3. Click on Compute in left tab, then on Instances.
  4. Click on Launch Instance.
  5. In the pop-up window, fill in the required fields to create your instance. In the Server Group field, click on the drop-down arrow next to Available to display the list of available Server Groups. Select a Server Group and it will be moved to Allocated. create server with server group OpenStack
  6. Once done, click on Launch Instance.

To apply a Server Group policy, you must specify the group when creating an instance, as a scheduling hint.
To do that, use the --hint parameter in the following command:

openstack server create --hint group=<server_group_id> [...] <server_name>

If you subsequently launch more instances referencing the same Server Group, the scheduler concentrates or distributes them according to the Server Group's policy.

Modifiying a Server Group

List Server Groups OpenStack

Once created, a Server Group cannot be modified.

In addition, an instance cannot be moved between Server Groups.

In both cases, this is because it would potentially require moving the instance to comply with the Server's Group policy.

Troubleshooting common issues

If you keep creating instances within a Server Group with a policy of anti affinity, you will eventually exceed the total amount of physical compute nodes in the region.

The command will still succeed, but the instance will subsequently fail to be scheduled to a compute node.

Instead, it will assume the ERROR status with the following fault message: _No valid host was found. There are not enough hosts available.

$ openstack server show -c fault -c status <server_id>
+--------+--------------------------------------------------+
| Field  | Value                                            |
+--------+--------------------------------------------------+
| fault  | {'code': 500, 'created': '2025-09-15T11:21:33Z', |
|        | 'message': 'No valid host was found.             |
|        | There are not enough hosts available.'}          |
| status | ERROR                                            |
+--------+--------------------------------------------------+

Server group error OpenStack

Another reason is that OpenStack cannot schedule the instance on a different physical compute node because there is already a server in the Server Group on every node.

The same scheduling error and "fault" message will occur when using a Server Group with a policy of affinity as well, when you create more instances than a physical compute node can host.

However when using a Soft Affinity policy, such as soft affinity or soft anti affinity, the scheduler is allowed to break the Server Group's policy if it is unable to uphold it.

Checking an instance in a Server Group

This means you may want to verify that your instances are on the same or on different physical compute nodes by looking at their hostId value.

We deployed four instances, two named anti_affinity_server with an anti affinity Server Group and two named affinity_server with an affinity Server Group:

openstack server list 
+------------------------+--------+-----------+--------+
| Name                   | Status | Image     | Flavor |
+------------------------+--------+-----------+--------+
| affinity_server-2      | ACTIVE | Debian 13 | d2-4   |
| affinity_server-1      | ACTIVE | Debian 13 | d2-4   |
| anti_affinity_server-1 | ACTIVE | Debian 13 | d2-2   |
| anti_affinity_server-2 | ACTIVE | Debian 13 | d2-2   |
+------------------------+--------+-----------+--------+

You can confirm the scheduler was done following the policy of the Server Group with the following command:

openstack server show -c hostId xSERVER_IDx

With the 4 instances created, we obtain the following output example:

openstack server show -c hostId affinity_server-2
+--------+----------------------------------------------------------+
| Field  | Value                                                    |
+--------+----------------------------------------------------------+
| hostId | c53cff4ce860b94139db559cfeafc08d228b48925ca00f1b05f3a23d |
+--------+----------------------------------------------------------+
openstack server show -c hostId affinity_server-1
+--------+----------------------------------------------------------+
| Field  | Value                                                    |
+--------+----------------------------------------------------------+
| hostId | c53cff4ce860b94139db559cfeafc08d228b48925ca00f1b05f3a23d |
+--------+----------------------------------------------------------+
openstack server show -c hostId anti_affinity_server-1
+--------+----------------------------------------------------------+
| Field  | Value                                                    |
+--------+----------------------------------------------------------+
| hostId | 66e9f9525487cad5cab2da65a51542d5a9bc0ca4bdb9fea7cdce8424 |
+--------+----------------------------------------------------------+
openstack server show -c hostId anti_affinity_server-2
+--------+----------------------------------------------------------+
| Field  | Value                                                    |
+--------+----------------------------------------------------------+
| hostId | 2c2907e312fec85fd92929a5afcc2ca4ac22c09c85480eea95b4fc08 |
+--------+----------------------------------------------------------+

hostId is a unique identifier for each physical compute node. By comparing this value across your various instances, you can ensure that your Server Group policy is being followed or not.

It's not possible to compare hostId between different projects because the values are unique for each project.

Server Group quota

You have to use or specify --os-compute-api-version 2.50 or higher to see server-groups and server-group-members output for a given quota class.

Regarding quota, Server Groups and Server Groups Members are in the Compute Service Quotas.

QuotaDescriptionProperty name
Server GroupsNumber of server groups per project.server_groups
Server Group MembersNumber of servers per server group.server_group_members
openstack quota show --compute <PROJECT>

Here is an output example:

openstack quota show <project> |grep -i server-group
| server-group-members        | 10                               |
| server-groups               | 10                               |

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