While RAID stands for "Redundant Array of Independent (or Inexpensive) Disks", the RAID concept can be applied to tape as well, producing significant improvements in performance and security. And those performance improvements can mean faster backups and less time that your network is handling heavy backup traffic, especially important if you're backing up over MAN or WAN links.

Like a RAID array of disks, a tape RAID array uses an intelligent controller to write information to multiple devices at any one time. While any of the standard RAID levels can be implemented on tape, the ones that make the most sense are levels 0, 1 and 3. Level 5, which is often used on disk arrays, doesn't work as well with tape because striping information at the byte level onto multiple tapes is more effective than striping it by block because it maximizes parallelism in what is basically a single-user system.

One advantage to tape RAID, like disk RAID, is speed. Tape RAID arrays can back up data at 20 GB an hour or more, depending on system configuration. Another advantage is the ability to mirror backups as they are made, either in their entirety or according to rules laid down by the storage administrator. Mirrored tape backup provides two copies of the backup and opens the door to backing up locally and remotely simultaneously.

Several companies make tape RAID controllers, including

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Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

This was first published in December 2003

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