How to form a cluster

When forming a LXD cluster, you start with a bootstrap server. This bootstrap server can be an existing LXD server or a newly installed one.

After initializing the bootstrap server, you can join additional servers to the cluster. See Cluster members for more information.

You can form the LXD cluster interactively by providing configuration information during the initialization process or by using preseed files that contain the full configuration.

To quickly and automatically set up a basic LXD cluster, you can use MicroCloud.

Configure the cluster interactively

To form your cluster, you must first run lxd init on the bootstrap server. After that, run it on the other servers that you want to join to the cluster.

When forming a cluster interactively, you answer the questions that lxd init prompts you with to configure the cluster.

Initialize the bootstrap server

To initialize the bootstrap server, run lxd init and answer the questions according to your desired configuration.

You can accept the default values for most questions, but make sure to answer the following questions accordingly:

  • Would you like to use LXD clustering?

    Select yes.

  • What IP address or DNS name should be used to reach this server?

    Make sure to use an IP or DNS address that other servers can reach.

  • Are you joining an existing cluster?

    Select no.

  • Setup password authentication on the cluster?

    Select no to use authentication tokens (recommended) or yes to use a trust password.

Expand to see a full example for lxd init on the bootstrap server
user@host:~$ lxd init
Would you like to use LXD clustering? (yes/no) [default=no]: yesWhat IP address or DNS name should be used to reach this server? [default=192.0.2.101]:Are you joining an existing cluster? (yes/no) [default=no]: noWhat member name should be used to identify this server in the cluster? [default=server1]:Setup password authentication on the cluster? (yes/no) [default=no]: noDo you want to configure a new local storage pool? (yes/no) [default=yes]:Name of the storage backend to use (btrfs, dir, lvm, zfs) [default=zfs]:Create a new ZFS pool? (yes/no) [default=yes]:Would you like to use an existing empty block device (e.g. a disk or partition)? (yes/no) [default=no]:Size in GiB of the new loop device (1GiB minimum) [default=9GiB]:Do you want to configure a new remote storage pool? (yes/no) [default=no]:Would you like to connect to a MAAS server? (yes/no) [default=no]:Would you like to configure LXD to use an existing bridge or host interface? (yes/no) [default=no]:Would you like to create a new Fan overlay network? (yes/no) [default=yes]:What subnet should be used as the Fan underlay? [default=auto]:Would you like stale cached images to be updated automatically? (yes/no) [default=yes]:Would you like a YAML "lxd init" preseed to be printed? (yes/no) [default=no]:

After the initialization process finishes, your first cluster member should be up and available on your network. You can check this with lxc cluster list.

Join additional servers

You can now join further servers to the cluster.

Note

The servers that you add should be newly installed LXD servers. If you are using existing servers, make sure to clear their contents before joining them, because any existing data on them will be lost.

To join a server to the cluster, run lxd init on the cluster. Joining an existing cluster requires root privileges, so make sure to run the command as root or with sudo.

Basically, the initialization process consists of the following steps:

  1. Request to join an existing cluster.

    Answer the first questions that lxd init asks accordingly:

    • Would you like to use LXD clustering?

      Select yes.

    • What IP address or DNS name should be used to reach this server?

      Make sure to use an IP or DNS address that other servers can reach.

    • Are you joining an existing cluster?

      Select yes.

    • Do you have a join token?

      Select yes if you configured the bootstrap server to use authentication tokens (recommended) or no if you configured it to use a trust password.

  2. Authenticate with the cluster.

    There are two alternative methods, depending on which authentication method you choose when configuring the bootstrap server.

    Generate a cluster join token for each new member.

    To do so, run the following command on an existing cluster member (for example, the bootstrap server):

    lxc cluster add <new_member_name>

    This command returns a single-use join token that is valid for a configurable time (see cluster.join_token_expiry). Enter this token when lxd init prompts you for the join token.

    The join token contains the addresses of the existing online members, as well as a single-use secret and the fingerprint of the cluster certificate. This reduces the amount of questions that you must answer during lxd init, because the join token can be used to answer these questions automatically.

  3. Confirm that all local data for the server is lost when joining a cluster.

  4. Configure server-specific settings (see Member configuration for more information).

    You can accept the default values or specify custom values for each server.

Expand to see full examples for lxd init on additional servers
user@host:~$ sudo lxd init
Would you like to use LXD clustering? (yes/no) [default=no]: yesWhat IP address or DNS name should be used to reach this server? [default=192.0.2.102]:Are you joining an existing cluster? (yes/no) [default=no]: yesDo you have a join token? (yes/no/[token]) [default=no]: yesPlease provide join token: eyJzZXJ2ZXJfbmFtZSI6InJwaTAxIiwiZmluZ2VycHJpbnQiOiIyNjZjZmExZDk0ZDZiMjk2Nzk0YjU0YzJlYzdjOTMwNDA5ZjIzNjdmNmM1YjRhZWVjOGM0YjAxYTc2NjU0MjgxIiwiYWRkcmVzc2VzIjpbIjE3Mi4xNy4zMC4xODM6ODQ0MyJdLCJzZWNyZXQiOiJmZGI1OTgyNjgxNTQ2ZGQyNGE2ZGE0Mzg5MTUyOGM1ZGUxNWNmYmQ5M2M3OTU3ODNkNGI5OGU4MTQ4MWMzNmUwIn0=All existing data is lost when joining a cluster, continue? (yes/no) [default=no] yesChoose "size" property for storage pool "local":Choose "source" property for storage pool "local":Choose "zfs.pool_name" property for storage pool "local":Would you like a YAML "lxd init" preseed to be printed? (yes/no) [default=no]:

After the initialization process finishes, your server is added as a new cluster member. You can check this with lxc cluster list.

Configure the cluster through preseed files

To form your cluster, you must first run lxd init on the bootstrap server. After that, run it on the other servers that you want to join to the cluster.

Instead of answering the lxd init questions interactively, you can provide the required information through preseed files. You can feed a file to lxd init with the following command:

cat <preseed-file> | lxd init --preseed

You need a different preseed file for every server.

Initialize the bootstrap server

The required contents of the preseed file depend on whether you want to use authentication tokens (recommended) or a trust password for authentication.

To enable clustering, the preseed file for the bootstrap server must contain the following fields:

config:
  core.https_address: <IP_address_and_port>
cluster:
  server_name: <server_name>
  enabled: true

Here is an example preseed file for the bootstrap server:

config:
  core.https_address: 192.0.2.101:8443
  images.auto_update_interval: 15
storage_pools:
- name: default
  driver: dir
- name: my-pool
  driver: zfs
networks:
- name: lxdbr0
  type: bridge
profiles:
- name: default
  devices:
    root:
      path: /
      pool: my-pool
      type: disk
    eth0:
      name: eth0
      nictype: bridged
      parent: lxdbr0
      type: nic
cluster:
  server_name: server1
  enabled: true

See Preseed YAML file fields for the complete fields of the preseed YAML file.

Join additional servers

The required contents of the preseed files depend on whether you configured the bootstrap server to use authentication tokens (recommended) or a trust password for authentication.

The preseed files for new cluster members require only a cluster section with data and configuration values that are specific to the joining server.

The preseed file for additional servers must include the following fields:

cluster:
  enabled: true
  server_address: <IP_address_of_server>
  cluster_token: <join_token>

Here is an example preseed file for a new cluster member:

cluster:
  enabled: true
  server_address: 192.0.2.102:8443
  cluster_token: eyJzZXJ2ZXJfbmFtZSI6Im5vZGUyIiwiZmluZ2VycHJpbnQiOiJjZjlmNmVhMWIzYjhiNjgxNzQ1YTY1NTY2YjM3ZGUwOTUzNjRmM2MxMDAwMGNjZWQyOTk5NDU5YzY2MGIxNWQ4IiwiYWRkcmVzc2VzIjpbIjE3Mi4xNy4zMC4xODM6ODQ0MyJdLCJzZWNyZXQiOiIxNGJmY2EzMDhkOTNhY2E3MGJmYThkMzE0NWM4NWY3YmE0ZmU1YmYyNmJiNDhmMmUwNzhhOGZhMDczZDc0YTFiIn0=
  member_config:
  - entity: storage-pool
    name: default
    key: source
    value: ""
  - entity: storage-pool
    name: my-pool
    key: source
    value: ""
  - entity: storage-pool
    name: my-pool
    key: driver
    value: "zfs"

See Preseed YAML file fields for the complete fields of the preseed YAML file.

Use MicroCloud

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Instead of setting up your LXD cluster manually, you can use MicroCloud to get a fully highly available LXD cluster with OVN and with Ceph storage up and running.

To install the required snaps, run the following command:

snap install lxd microceph microovn microcloud

Then start the bootstrapping process with the following command:

microcloud init

During the initialization process, MicroCloud detects the other servers, sets up OVN networking and prompts you to add disks to Ceph.

When the initialization is complete, you’ll have an OVN cluster, a Ceph cluster and a LXD cluster, and LXD itself will have been configured with both networking and storage suitable for use in a cluster.

See the MicroCloud documentation for more information.