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Replication for DR
A key way users deploy host-level replication software is for DR. Double-Take Software Inc. estimates that approximately 75% of its customers use its replication software for DR or remote-application availability. However, different host-level replication software products, such as Symantec Corp.'s Veritas Replication Exec and Veritas Volume Replicator (VVR), satisfy different data protection requirements. Veritas Replication Exec is a Windows-only product that operates at the file-system level and allows administrators to configure one central DR server to receive replicated data from multiple servers at different sites. Like most Windows products, Veritas Replication Exec supports only asynchronous replication and some data may be lost if you need to switch over to the DR server.
VVR is a block-based product that works with a variety of OSes and permits real-time synchronization of data from source and target servers. It has its own set of restrictions. For instance, admins must have Veritas Storage Foundation installed on both the source and target servers, and convert the volumes to Veritas-managed volumes. Second, all changes to data on the volumes selected for replication must be sent. Finally, the servers must be within 80km of one another for the app on the source server to proceed without waiting on write confirmations from the target server.
Bunker Replication, a feature that uses a combination of synchronous and asynchronous replication within Symantec's VVR 5.0, lets you exceed the 80km limitation. It synchronously replicates data to the bunker site that's within 80km of the primary site while asynchronously replicating data to the server at the more distant DR site. The trick to this approach is that the bunker site houses only the data differential between the primary and DR sites. In this way, if the primary site fails, the bunker site automatically updates the DR site with the data that's normally lost using only asynchronous replication.
The hybrid real-time data replication feature of BakBone Software Inc.'s NetVault:Replicator offers near real-time availability for apps. But it commits writes to the local server without a guarantee that the write was committed to the remote server, asynchronously transmitting the data after the write is complete. Though this technique minimizes app interruption, it's not guaranteed that data on the remote server is consistent with data on the host server and in a recoverable form.
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