Learning Guide

Near-CDP in backup and recovery explained

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By W. Curtis Preston

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

Continuous data protection vendors like Kashya Inc. and Revivio Inc. objected, saying that snapshots weren't continuous data protection. 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 continuous data protection 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.

W. Curtis Preston is an executive editor in TechTarget's Storage Media Group and an independent backup expert.

This article was previously published in Storage magazine.

This was first published in October 2010

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