After a node outage
After an outage of a node DRBD® automatically resynchronizes the temporarily unavailable node to the latest version of the data, in the background, without interfering with the service running. Of course this also works if the role of the surviving node was changed while the peer was down.
In case a complete power outage takes both nodes down, DRBD will detect which of the nodes was down longer, and will do the resynchronization in the right direction.
After an outage of the replication network
Restoring service after the temporary failure of the replication network is just a typical example of how the automatic recovery mechanism just described works. DRBD will reestablish the connection and do the necessary resynchronization automatically.
After an outage of a storage subsystem
DRBD can mask the failure of a disk on the active node, i.e., the service can continue to run there, without needing to failover the service. If the disk can be replaced without shutting down the machine, it can be reattached to DRBD. DRBD resynchronizes the data as needed to the replacement disk.
After an outage of all network links
DRBD supports you with various automatic and manual recovery options in the event of split brain.
Split brain is a situation where, due to the temporary failure of all network links between cluster nodes, and possibly due to intervention by cluster management software or human error, both nodes switched to the primary role while disconnected. This is a potentially harmful state, as it implies that modifications to the data might have been made on either node, without having been replicated to the peer. Thus, it is likely in this situation that two diverging sets of data have been created that cannot be merged.