Feature

Continuous data protection; it's back!

Ezine

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Finally, many people viewed CDP as the Star Trek of the backup industry -- a great idea before its time. Star Trek, maybe not fully understood when it first aired, was canceled after three seasons. Similarly, many people thought CDP was a solution looking for a problem, and most shops could meet their backup and recovery requirements without completely changing the way they did backups, which was required with continuous data protection.

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What is near-CDP?

When continuous data protection (CDP) products first appeared, they created quite a buzz, and marketing departments love buzz. But there were other companies with products that continuously protected data, and they wanted to use the CDP moniker, too.

CDP vendors like Kashya Inc. and Revivio Inc. objected, saying that snapshots weren't CDP. They also noted that snapshots can only recover to a particular point in time, while continuous data protection can recover to any point in time. Hence the term near-CDP was coined, allowing snapshot-based vendors to steal some of the CDP buzz.

But years later, the term near-CDP is still not in the Storage Networking Industry Association (SNIA) lexicon. Purists say you're either continuous or you're not, but others think it's still the best term we have to describe snapshots coupled with replication.

Near-CDP systems have more in common with CDP than with traditional backup. CDP and near-CDP systems transfer only changed blocks to the backup system. There are no repeated full backups, and if only a few bytes change in a file, only a few bytes are sent to the recovery system. They also transfer the changed blocks to the recovery systems throughout the day, rather than in a large batch process at night. And both CDP and near-CDP systems provide instantaneous recovery and can offer recovery points from a few seconds to an hour, depending on implementation.

The only important difference between CDP and near-CDP is the ability of continuous data protection to offer a recovery point objective (RPO) of zero (or almost zero), and it doesn't require the creation of application-aware snapshots up front. However, most CDP users create snapshots anyway and recover to those snapshots, preferring a known stable point in time to a more recent recovery point that will require a crash recovery process. So, maybe the CDP vs. near-CDP debate is a lot of arguing over nothing.

This was first published in August 2010

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