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RAID for Home Computers?

RAID stands for Redundant Array of Inexpensive Disks. RAID 1 is a particular configuration of RAID, also known as "mirroring". The basic idea is this: make your computer two copies of everything to two separate hard disks, such that if one hard disk dies, the computer can keep running.

Disk mirroring is not at all uncommon on servers. Since they likely serve more than one user, they likely inconvenience more than one user if their hard disk crashes. Thus, the extra expense of a redundant disk (and possibly an extra hardware card to perform the dual I/O) can be amortized. But RAID support is now appearing with increasing frequency built into motherboards, so it makes sense to ask: should a home computer have a disk mirror so that it can easily survive a disk failure?

Likelihood of Disk Failure

To buy or build a home computer with disk mirroring costs extra money -- for the extra disk, and possibly also for a RAID controller (or a fancier motherboard that contains a RAID controller). The main payback is security against a disk failure, so deciding whether the extra cost is worth it required assessing how likely such a failure us. The short answer is, "nobody knows", but the longer answer is more interesting, if more complex.

A variety of sources peg the general failure rate of hard disks at about 1% per year. But you don't care about the average -- you care about your personal odds of losing a hard disk. In which case, it's a good idea to look at what encourages hard disk failure.

Recall that hard disks have a bathtub-shaped failure curve, meaning that they are actually more likely to fail very early on (so-called "infant mortality") than after they have survived several months of use. That means that disk mirroring may be a better investment in a brand-new machine than in one that's a year old.

Another important thing about hard disk failure is that it's often heat-related. In fact, one Hitachi manager has been quoted as saying that cutting the ambient temperature by ten degrees Centigrade (about 18 degrees Farenheit) can double the lifetime of a hard drive!

The Cost of RAID

The cost of RAID is dropping, which is a big part of what makes the idea of using disk mirroring in a typical Windows home computer a more interesting question. That drop in cost does not really come from software -- although Microsoft supplies the ability to implement disk mirroring in some server versions of its operating system, you don't get it in Windows XP (either Home or Pro). That means, the typical Windows XP home computer needs some hardware (a RAID controller) to make Windows think it has only one disk, when it is in fact writing to two different disks at once.

It's tempting to say that the cost of RAID is dropping because the cost of disk drives are dropping, but that's not quite true. The cost of disk space per megabyte has been dropping at a respectable rate for years, but the price of the smallest new hard drive has not been dropping much at all. For example, as I write this (September 2004), a modest-sized 40GB Seagate drive retails for around $70. Now, you can shop hard and play the rebate game to get quite a bit more disk space per gigabyte, but what you can't do is buy a brand-new 10GB drive for $20. I can't tell you for sure how much disk space you'll be able to buy next year, but I predict a brand-new disk will cost you more than $50. The hard disk game is in adding more gigabytes to the drive, not dropping the price of the disk itself.

If you ignore the price of a monitor, which can vary wildly depending on your needs, then I can build a quite tolerable basic computer for under $300. That means the hard disk can easily be more than one-fourth the cost of the computer (minus monitor). Therefore, for the price-sensitive, the cost of having two disk drives instead of one is not trivial.

That leaves the cost of the RAID controller, and that's where motherboard manufacturers come on. Built-in RAID controllers are becoming increasingly common on motherboards, and that's going to push more people to consider doing disk mirroring, even on a modest home computer.

Recommendation

My recommendation is that you first figure out what the dollar cost of this extra disk protection is (not sure? guesstimate it will add about $100 to the cost of your system), and then consider the following table of pros and cons of paying for disk mirroring in a home computer.

ProHigh degree of convenient protection against a single disk failure.
ProMay slightly speed up disk reads. If you pay for a RAID controller that lets every disk have its own channel, you may get as much as 30% better speeds.
ConWill likely slow down disk writes.
ProInfant mortality makes disk failure somewhat more likely in brand-new disks, making the RAID protection more valuable.
ConMay inspire a false sense of confidence that leads to avoiding good backup hygiene. Disk mirroring is no protection against viruses, accidental deletion of files, and other forms of logical (as opposed to physical) loss of data on the disk.
ProIf operating in a hotter environment, disk failure may be more likely, increasing the value of disk mirroring protection.
ConIf you just add a second disk with no additional cooling, you may raise the ambient temperature inside the case, increasing the chances of disk failure.

To boil it all down to a trite formula: if the money difference is trivial, then get RAID. If the money difference is significant for you, then consider just trying to add some extra cooling to the single disk. It may not prevent most forms of infant mortality in brand new disks, but it's a very good bet for longer life overall. If you're building a new single-disk computer, another fair bet is to go with the disk that comes with the longest warranty (which indicates the manufacturer's betting the disk won't fail).

One final note about hard disk failure. If your computer is nearly seven years old, it's time to start eyeballing that hard drive as something that's likely to fail. Seven years of life is really about all hard disk designers are shooting for in home computer hard disks. Be very sure you're doing regular and thorough backups if you're determined to live with a disk that old.

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