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Mirroring Is Not Backup

Although mirroring and backup involve making copies of the contents of disks, these technologies are fundamentally different. Confusing the two can cause big trouble. Part of the confusion comes from the fact that mirroring, writing data to two separate disk drives in the system at the same time, is a useful adjunct to backup. A backup that can be used for disaster recovery purposes requires a complete image of the disk plus incremental changes.

A mirrored disk lets you restore data that has been lost or damaged due to user error or hardware failure very quickly, often in seconds. On enterprise systems with a limited backup window, a backup can be created by breaking the mirror (no longer writing to two disks simultaneously) and making a backup from the second drive while the main drive continues to run. Once backup is completed, the second disk (the mirror) is resynchronized with the original one.

A complete backup involves not just one copy of the data, but several copies taken at different points in time. Almost always one or more of those incremental backups can be stored off line or remotely.

The problem with mirroring comes when the system's data is corrupted, such as by a virus. Because the data is copied almost immediately, the mirror becomes corrupted at virtually the same time. Because there are no earlier copies available, it may be difficult or impossible to restore the data to an uncorrupted state.

By contrast, a backup image is a point-in-time snapshot of the disk in a known, working state. Recovering individual files or folders from an image requires mounting that image as a virtual disk and simply copying over necessary files. If the disk becomes corrupted, a full restore can bring back that point in time where all of the software was working perfectly.