When using three-way replication, DRBD adds a third node to an existing 2-node cluster and replicates data to that node, where it can be used for backup and disaster recovery purposes.

Three-way replication works by adding another, stacked DRBD resource on top of the existing resource holding your production data, as seen in this illustration:

Figure 2.1. DRBD resource stacking

The stacked resource is replicated using asynchronous replication (DRBD protocol A), whereas the production data would usually make use of synchronous replication (DRBD protocol C).

Three-way replication can be used permanently, where the third node is continously updated with data from the production cluster. Alternatively, it may also be employed on demand, where the production cluster is normally disconnected from the backup site, and site-to-site synchronization is performed on a regular basis, for example by running a nightly cron job.