Three datacentre (3DC) multi-target with delta is an improvement on 3DC multi-target that provides on-going protection even in the event of a failure at the primary site.
A replication from the production site to the local secondary site is configured using Universal Replicator. A replication from the production site to the remote secondary site is configured using Global-Active Device or TrueCopy. Additionally, a suspended, asynchronous Universal Replicator (delta-UR) replication is established between the local and remote secondary sites. The local and remote secondary sites will be near identical once pairing with the primary site is complete. Differences that appear between the secondary sites over time, due to a number of factors, are tracked by the suspended delta-UR replication.
Failure of the local or remote secondary site is handled in the same way as for 3DC multi-target, in that the primary site remains protected by the surviving secondary site.
In the event that the production site fails, the local secondary site can take over. The local secondary site is then rapidly brought under the protection of the remote secondary site by resuming the suspended delta-UR replication. Because only the deltas between the local and remote secondary sites need to be resynchronized, this pairing takes only a short time to achieve, meaning that the local site only remains unprotected for a brief period. Without the pre-existing, suspended delta UR between the local and remote secondary sites, it could take hours or even days to establish a replication between the secondary sites, leaving the local secondary site vulnerable for an extended period while this takes place.