Bugs That Are Immune to RAID


By Steve Wildstrom Responding to my Sept. 29 column ("Bless Both of These Backups"), reader Richard Smith writes: "How do you feel about RAID 1, a technology used for backing up computer data? I am thinking about a new computer that includes RAID 1 and would appreciate any comments."

RAID (redundant array of inexpensive drives) is a technology that allows groups of hard drives to work together in various ways. It has been used on servers for some years, but is now being offered on consumer PCs, mostly on relatively high-end desktops. The flavor called RAID 1 performs a sort of automatic backup. It provides insurance against catastrophic failure of a disk drive, but it's not really a substitute for backup.

ERRORS IN, ERRORS OUT. Here's why: RAID 1 takes a pair of drives and mirrors them against each other, so that changes in one are immediately copied to the other -- meaning the two are always identical. If one drive fails, the other keeps on working without any interruption. This is especially important on servers, where the technology greatly increases the system's ability to keep running despite a hardware failure.

The problem comes when a drive doesn't fail, but information on one drive -- user data or program code -- becomes corrupted. Usually, the RAID system does exactly what it is supposed to do: it copies the bad information to the good drive before you can stop it. But then your two drives have matching errors. This objection isn't theoretical -- I've seen it happen, and it can be really ugly.

And even if there's no corruption issue, RAID 1 gives you no way to revert to an older version of a file, or to restore a file that you inadvertently deleted yesterday. What you want is a true backup that can restore the right data to both mirrored disks.

GHOST IN THE MACHINE. Another technique mentioned by several readers is known as image backup. It's also called ghosting, after the most popular software that performs the task: Symantec's (SYMC) Norton Ghost ($70). Ghosting creates a bit-by-bit copy of your hard drive onto another drive. If the primary drive fails, you can just install the backup, power up the PC and go back to work.

Ghosting definitely has its uses -- information technology professionals, for example, regularly use it to set up new standard PCs. It has the same drawback as RAID 1, with the additional disadvantage that that you have to move and install drives to get up and running.

Furthermore, drive failures typically occur on disks that have been in use for a couple of years. And at least on a Windows system, that generally means performance has degraded considerably over time. While restoring from an image backup is speedy, you are likely to get a more satisfactory result by starting with a computer that has been restored to factory condition, then reinstalling applications and reloading data.

Wildstrom is Technology & You columnist for BusinessWeek. You can contact him at techandyou@businessweek.com


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