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We currently have a 2 node DRBD cluster running which we would like to add a third node to. I have so far been unable to find any resources on whether it is possible to add a third node to an existing cluster, only how to set up a cluster with three nodes initially.

My question is, is it possible to add a 3rd node and how would I go about doing it?

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SvW nailed it, but to spell out the steps - it's pretty much like getting DRBD underneath your plain old partitions, but now the existing DRBD resource is the backing device. Assuming that the physical backing device (likely your LV) cannot be enlarged, you will need to

  1. shrink your filesystem
  2. add the stacked resource (create metadata for it)
    • it uses internal metadata that is inside the orginal DRBD
  3. make sure that on your active node, both the original and stacked DRBDs are primary and you only ever mount the stacked resource from that point on
    • if you mount the original, lower DRBD device, you are circumventing the stacked resource and corrupt the third node's data
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Yes, it is possible if you stack the DRBD resources, meaning that you use a replicated device as physical device for another replication pair. There are a number of considerations for this though, so it might not be what you want to achieve.

It's even in the docs: http://www.drbd.org/users-guide/s-three-nodes.html

Also note that DRBD 9.0 (currently in testing) will add multi-node-redundancy out of the box (http://www.drbd.org/users-guide-9.0/s-multi-node.html).

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