When adding nodes to your Proxmox cluster, make sure no VM or container is running on the node. When you add a node to the cluster, it will inherit the cluster configuration, which will overwrite all current local configurations. If you have an important VM or container running, you can create a backup, remove it, and import it later after joining the cluster.
To join a node to the cluster, you need to copy some information from the first node where you created the cluster. To do that, navigate to Datacenter > Cluster and then click the Join Information button, as shown in the screenshot.
Click the Copy Information button to copy the cluster join information to your clipboard.
Now, connect the other Proxmox node that you want to join to the cluster. Navigate to Datacenter > Cluster, and click the Join Cluster button, as shown below:
In the Information box, paste the join information, type the root password of the first node, and click the Join button.
The second node will now be added to the Proxmox cluster. As soon as the node is added to the cluster, the server certificate is changed, so you need to reload the page and log in again to the Proxmox VE web interface. The screenshot below shows that our proxmox-lab cluster now has two nodes: pmox-host1 and pmox-host2.
The information about cluster nodes is listed under the Cluster option. In addition, any shared storage that is attached to the first node is automatically attached to the new node.
Once you have a Proxmox cluster up and running, you can start migrating your VMs or containers when needed, as shown in the screenshot below:
The migration is pretty fast, particularly when you're using shared storage (e.g., NAS or Ceph), so your VM will experience little to no downtime at all. The cool feature you get with a Proxmox cluster is that you can still migrate your VM (or container), even if you're using local storage. Of course, it takes longer to migrate, but it works.