RAID Mirroring

RAID (Redundant Array of Independent Disks) is a misunderstood technology. Many users consider it to be a real-time backup solution, which it is -- sort of. But relying on RAID mirroring as your sole backup solution is dangerous and can lead to the eternal loss of your data.

What is RAID?

RAID is a set of technologies designed to reduce downtime by providing redundancy and/or to speed up a computer's data access. All RAID configurations use at least two physical hard drives, and distribute data across them in either a "striped" or "mirrored" configuration. The collection of drives is called an "array."

"Striping" (or RAID 0) reduces data access time by saving half of the data to each disk and reassembling it in memory, and offers absolutely no redundancy. If either of the drives (or the data they contain) is damaged, the array will be trashed.

RAID mirroring (or "RAID 1") does provide redundancy by saving the same data simultaneously to two disks. If one drive fails, an alarm is set off, but the computer continues running on the remaining drive. When a replacement drive is installed, the existing good drive is copied over to the new drive, and the arrary is rebuilt. Obviously, when using any sort of RAID, you should have a supply of identical drives on hand to be swapped into the system in the event of a drive failure.

Disadvantages to RAID Mirroring as a Backup Solution

One problem with RAID is that it has a very vulnerable SPOF (Single Point of Failure): the RAID controller. If the controller decides to misbehave in a particularly evil way, it can take down the whole array at the same time. I've personally experienced three incidents in which controller failures on my own leased servers so seriously corrupted the arrays that the arrays had to be restored from backups (which, of course, I had, being the Backup Nut).

Another drawback to RAID is that if the system is damaged due to virus or spyware infection, registry problems, or other systemic problems with the operating system, the problems will be mirrored to both drives in the array in real time. This will leave you with two perfectly healthy hard drives that contain the same messed-up, unusable operating system.

But be of good cheer, brothers and sisters. There is a simple solution to both of these potential problems. Because a RAIDed drive is handled as a logical drive by the system, it can be cloned or imaged just like any other hard drive, glory be. By using a third drive on a different controller (such as an external hard drive), the RAIDed drive can be cloned or imaged periodically; so even if the RAID controller fails or the array becomes infected by evil malware, the last good copy of the drive is preserved and can be used to rebuild the array. Can I get an amen?

A RAID array can be cloned to an external hard drive that is physically compatible to the internal drive (that is, it has the same interface), and the external drive can then be swapped into the machine and used to rebuild the array in the event the internal array fails. (If the problem was caused by the RAID controller, then the controller will have to be replaced first.) The RAID array can also be imaged to an external drive or a mapped drive on another computer on the network using a program like whichever version of the Acronis® Backup & Recovery 10 Family applies to your situation (home computer, business workstation, server, etc.).

I verily recommend using external hard drives or mapped drives as backup media for RAID arrays. If you use a third internal drive to back up the array, the backup drive could be taken down alongside the array it is backing up in certain events (such as a failed controller or power supply). There is less chance of this happening to an external drive (and even less when using a mapped drive).

One thing RAID cannot do, however, is protect your data in the event the physical computer is destroyed or stolen. As with any on-site backup solution, a fire, flood, lightning strike, theft of the computer, or other catastrophic event could leave you with no backup at all. This is where online backup comes in, glory be!

So is RAID worth it?

The Backup Nut considers RAID mirroring to be more of a downtime-prevention strategy than a backup solution, and it's a great solution for downtime reduction. When it works properly (and usually it does), a mirrored RAID array will keep a machine up and running if one of the drives in the array fails, glory be. That's a good thing. But it's not an adequate backup solution by itself.

In other words, RAID can be part of a good backup solution, especially when combined with network document backup and periodic hard drive imaging. But the Backup Nut suggests that you look at RAID mirroring primarily as providing downtime prevention (which really is what it was designed to do, anyway), and network backup and/or hard drive imaging as your real backup solutions.

How to use RAID

In most cases, RAID has to be set up in BIOS before the operating system is installed. You must have either a motherboard that supports RAID, or an add-on RAID controller. The drives must be installed and configured as an array so they work together as one (behold, how good and pleasant it is for hard drives to work together in unity, glory be), and then the operating system is installed to the array.

There is no subjective difference to the user when using a RAIDed system. Both drives show up as one in ordinary use, and the operating system treats them as a single drive. If one drive fails, an alarm will sound, and the remaining drive will continue to run the show.

RAIDed External Storage

Another worthy use for RAID is in external storage devices. Many people use external hard drives for backup or storage, without considering the fact that external hard drives can crash just like internal ones can.

RAIDed external storage enclosures have space for two (or occasionally more) hard drives, which usually can be configured in either a RAID 0 (striped) or RAID 1 (mirrored) configuration. Again, RAID is no guarantee that both drives won't die simultaneously; but if the RAIDed enclosure is being used to back up files that are already on an attached machine, then all three drives (the one in the machine and the two in the enclosure) would have to fall by the wayside together in order for the data to be lost.

If you want to earn the Backup Nut's blessing, dear brothers and sisters, combine a RAIDed external enclosure, hard drive cloning or imaging, and online backup of your critical documents; and verily you will have better backup than 99 percent of ordinary computer users.