This article will show you how you can create DRBD metadata while leaving room for additional nodes to be added at a later time.
If you have a cluster that is currently two nodes, but you know you will add a third node later, you'll need to either leave some additional room at the end of the block device and then recreate the entire cluster's metadata on all devices (requiring full syncs), or you can create the future node's bitmap slot in DRBD's metadata when you first create the metadata on the original two nodes:
drbdadm create-md --max-peers=3 res0
This will allow you to add the third node without having to touch, or resize any disks, or recreate metadata (full resync) on the original peers.
Edited 2021/12/14 - DJV