This is my sixth excerpt from my The 7 Deadly Sins of Backup and Recovery whitepaper.

The idea of replicated disk storage originated in the fail-safe systems of 20 years ago.  In this technology, two disks or disk arrays simultaneously stored exactly the same data, such that if one had a failure, no data was lost.

Along the way, it occurred to disaster recovery professionals that replicated disk systems could be used for off-site storage by inserting a WAN between the two disk arrays. Today, replication is typically used by SANs and “data mirror” products.  Replication uses block-level communications to constantly synchronize data between two different locations.

The advantages to this method are near real-time data protection and ease of configuration and maintenance.  Once established, a replication environment requires little ongoing attention.

The disadvantages are that the synchronization occurs at a very low level, below the ability to see data and file system errors or corruption.  This means that the inevitable errors (some of which are potentially fatal to a restoration) that occur naturally over time will be immediately replicated to the off-site location…and the hoped-for protection is thereby immediately lost.

Replication is also highly resource-intensive.  These systems create a substantial burden on the local CPU since most replication technologies utilize a type of RAID engine to capture the block-level changes.

This sort of replication is also extremely bandwidth-intensive.  As every data change for every transaction is moved across the network to the second location, this approach can impose a huge burden on a corporation’s network infrastructure.

Stay tuned until next time when I discuss a better approach to off-site data storagevaulting. Vaulting uses a less real-time technology but allows a level of file system integrity not found in a replication environment.