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Near-CDP: Snap and replicate

Near-CDP systems, which do snapshot and replication-based backups, are very efficient because, like CDP systems, they only transfer and store new blocks of data. The changes (or "deltas") can be easily stored on the primary system and replicated to a secondary system for backup. Snapshots must be replicated or backed up to tape because they depend on the primary volume for their data.

The true value of a near-CDP system is demonstrated during operational recovery. Their RPO can be measured in minutes, and their recovery time objective (RTO) is measured by how long it takes you to point the application from the primary storage system to the secondary storage system.

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CDP vs. near-CDP

Just when is "continuous" not really continuous? In the real world the definition is pretty straightforward, but when it comes to continuous data protection (CDP) and near-CDP, things get a little hazy. The Storage Networking Industry Association (SNIA), a leading storage vendors' trade organization, offers its own definition of CDP:

"Continuous data protection (CDP) is a methodology that continuously captures or tracks data modifications and stores changes independent of the primary data, enabling recovery points from any point in the past."

A true CDP system captures every change or new piece of data as soon as it's committed to disk and immediately replicates that change to another system. Near-CDP performs a similar function but does it periodically -- every 15 minutes, 30 minutes, hour, etc. -- so it's truly not "continuous" at all. Because it offers RTOs and RPOs that are very close to what CDP offers, many people refer to it as near-CDP; however, the term "near-CDP" isn't officially recognized by SNIA.

Not all snapshots are the same

If you're using one of the leading backup software products, you should ask the vendor how they accomplish near-CDP functionality. Some of them provide it completely within their product, but most accomplish it by controlling and reporting on the snapshot replication capabilities of a storage or virtualization system.

If you're going to rely entirely on snapshots for historical preservation of data, you need to ensure that the existence of dozens or hundreds of snapshots doesn't negatively impact the performance of your storage system. Therefore, the feasibility of a snapshot-based backup system depends highly on what type of snapshots your storage system provides.

This was first published in January 2011

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