How to manage a cluster

After your cluster is formed, use lxc cluster list to see a list of its members and their status:

user@host:~$ lxc cluster list

+---------+----------------------------+------------------+--------------+----------------+-------------+--------+-------------------+| NAME    |            URL             |      ROLES       | ARCHITECTURE | FAILURE DOMAIN | DESCRIPTION | STATE  |      MESSAGE      |+---------+----------------------------+------------------+--------------+----------------+-------------+--------+-------------------+| server1 | https://192.0.2.101:8443   | database-leader  | x86_64       | default        |             | ONLINE | Fully operational ||         |                            | database         |              |                |             |        |                   |+---------+----------------------------+------------------+--------------+----------------+-------------+--------+-------------------+| server2 | https://192.0.2.102:8443   | database-standby | aarch64      | default        |             | ONLINE | Fully operational |+---------+----------------------------+------------------+--------------+----------------+-------------+--------+-------------------+| server3 | https://192.0.2.103:8443   | database-standby | aarch64      | default        |             | ONLINE | Fully operational |+---------+----------------------------+------------------+--------------+----------------+-------------+--------+-------------------+

To see more detailed information about an individual cluster member, run the following command:

lxc cluster show <member_name>

Configure your cluster

To configure your cluster, use lxc config. For example:

lxc config set cluster.max_voters 5

Keep in mind that some server configuration options are global and others are local. You can configure the global options on any cluster member, and the changes are propagated to the other cluster members through the distributed database. The local options are set only on the server where you configure them (or alternatively on the server that you target with --target).

In addition to the server configuration, there are a few cluster configurations that are specific to each cluster member. See Cluster member configuration for all available configurations.

To set these configuration options, use lxc cluster set or lxc cluster edit. For example:

lxc cluster set server1 scheduler.instance manual

Assign member roles

To add or remove a member role for a cluster member, use the lxc cluster role command. For example:

lxc cluster role add server1 event-hub

Note

You can add or remove only those roles that are not assigned automatically by LXD.

Edit the cluster member configuration

To edit all properties of a cluster member, including the member-specific configuration, the member roles, the failure domain and the cluster groups, use the lxc cluster edit command.

Evacuate and restore cluster members

There are scenarios where you might need to empty a given cluster member of all its instances (for example, for routine maintenance like applying system updates that require a reboot, or to perform hardware changes).

To do so, use the lxc cluster evacuate command. This command migrates all instances on the given server, moving them to other cluster members. The evacuated cluster member is then transitioned to an “evacuated” state, which prevents the creation of any instances on it.

You can control how each instance is moved through the cluster.evacuate instance configuration key. Instances are shut down cleanly, respecting the boot.host_shutdown_timeout configuration key.

When the evacuated server is available again, use the lxc cluster restore command to move the server back into a normal running state. This command also moves the evacuated instances back from the servers that were temporarily holding them.

Automatic evacuation

If you set the cluster.healing_threshold configuration to a non-zero value, instances are automatically evacuated if a cluster member goes offline.

When the evacuated server is available again, you must manually restore it.

Delete cluster members

To cleanly delete a member from the cluster, use the following command:

lxc cluster remove <member_name>

You can only cleanly delete members that are online and that don’t have any instances located on them.

Deal with offline cluster members

If a cluster member goes permanently offline, you can force-remove it from the cluster. Make sure to do so as soon as you discover that you cannot recover the member. If you keep an offline member in your cluster, you might encounter issues when upgrading your cluster to a newer version.

To force-remove a cluster member, enter the following command on one of the cluster members that is still online:

lxc cluster remove --force <member_name>

Caution

Force-removing a cluster member will leave the member’s database in an inconsistent state (for example, the storage pool on the member will not be removed). As a result, it will not be possible to re-initialize LXD later, and the server must be fully reinstalled.

Upgrade cluster members

To upgrade a cluster, you must upgrade all of its members. All members must be upgraded to the same version of LXD.

Caution

Do not attempt to upgrade your cluster if any of its members are offline. Offline members cannot be upgraded, and your cluster will end up in a blocked state.

Also note that if you are using the snap, upgrades might happen automatically, so to prevent any issues you should always recover or remove offline members immediately.

To upgrade a single member, simply upgrade the LXD package on the host and restart the LXD daemon. For example, if you are using the snap then refresh to the latest version and cohort in the current channel (also reloads LXD):

sudo snap refresh lxd --cohort="+"

If the new version of the daemon has database schema or API changes, the upgraded member might transition into a “blocked” state. In this case, the member does not serve any LXD API requests (which means that lxc commands don’t work on that member anymore), but any running instances will continue to run.

This happens if there are other cluster members that have not been upgraded and are therefore running an older version. Run lxc cluster list on a cluster member that is not blocked to see if any members are blocked.

As you proceed upgrading the rest of the cluster members, they will all transition to the “blocked” state. When you upgrade the last member, the blocked members will notice that all servers are now up-to-date, and the blocked members become operational again.

Update the cluster certificate

In a LXD cluster, the API on all servers responds with the same shared certificate, which is usually a standard self-signed certificate with an expiry set to ten years.

The certificate is stored at /var/snap/lxd/common/lxd/cluster.crt (if you use the snap) or /var/lib/lxd/cluster.crt (otherwise) and is the same on all cluster members.

You can replace the standard certificate with another one. To do so, use the lxc cluster update-certificate command. This command replaces the certificate on all servers in your cluster.