Understanding Continuous Data Protection

Replication--saving changes on a regular or even near-constant basis to files on a separate local or remote array, using products such as Veritas Replication Exec and Computer Associates BrightStor High Availability--has become standard. If you lose a disk, replication will get you up and running quickly; and in the event your data center burns down, you'll have an up-to-the-minute backup if you replicated to a remote site. But replication has some drawbacks. If someone overwrites a file or a virus infects your data center, replication systems will copy these unwanted changes along with everything else, and your backup copy will be corrupted too. And if you want to restore a file to a certain state, replication is useless. It holds the file in its current state.

Continuous Data Protection Vendors Click to Enlarge

Snapshots have been growing in popularity, with disk-array vendors EMC and Adaptec leading the charge. Snapshots are regular, incremental backups to disk. Some snapshot implementations copy the entire area being backed up each time, though most newer ones copy only the changed data. A single disk can hold many snapshots, so you can choose the newest, uncorrupted version for your restore. But with a snapshot, as with tape, restores occur at the file or volume level. Also, a valid snapshot of the data before corruption may not be available when you need it, and after you restore a file to precorrupt status, you lose the changes made since the last snapshot was taken.

CDP to the Rescue

For a total storage solution, we must retain information about every change to a file over its life and be able to restore any version of that file. That's the promise of continuous data protection. Toward that end, the Storage Networking Industry Association has put together a working group to develop its Data Protection Initiative, a set of standards for CDP and other disk-to-disk technologies.