What is "RAID" and why is "RAID 0" such a bad, bad idea?

"RAID" stands for "Redundant Array of Inexpensive Disks", and refers to several different ways to use multiple hard drives to either increase performance, reliability or both. RAID is typically transparent to you or the operating system, other than the fact that there is a dedicated piece of hardware, known as a RAID controller, that manages the distribution of data across the drives.

Various standard configurations of RAID drives, typically referred to as "RAID arrays", have been labeled with numbers to differentiate the differing arrangements.

RAID 0 uses two (or more) drives to increase performance.

The approach is this: when you write data to the logical (say, "C:") drive, the data is interweaved in some way such that half of it is written to one physical drive, and half on the other. Effectively, this doubles the speed of your hard disk, since the computer can essentially be writing to both at the same time, instead of writing the same amount of data in two steps to a single drive.

Sounds like a nifty approach, right? You get the same amount of space as you would with two individual drives, but you get it twice as fast.

The problem is that you get twice as much of something else as well: chance of failure and catastrophic data loss.

If one drive has, say, a 1 in 1,000 chance of failure, then two drives have a 2 in 1,000 chance.

Now, if you have two separate drives you still have that same 2 in 1,000 chance of one of them failing - but all you lose is what's on that one drive that fails. In a RAID 0 configuration, you've doubled your chances of failure, and when failure happens, all of your data on both drives is lost.

Think of it this way: if we take a simple sentence like:

RAID 0 configurations are a bad idea.

and sliced it in half such that every other letter was "on alternate drives", this might be what appears on each drive:

Drive 1: R I     o f g r t o s a e a b d i e .
Drive 2:  A D 0 c n i u a i n   r     a   d a

Together they're easy (and quick!) to combine into the original sentence. Take away either one and the other is so much gibberish.

RAID 0 is valuable as a speed enhancement only if you're willing to take double the risk of failure. That means either that it's OK if the failure happens and you lose the data, or you're conscientious about making sure that it's backed up regularly and thoroughly.

I'm not really sure why I'm seeing this configuration more and more, but it does concern me that people are taking it on without understanding the risks.

What's frustrating, in part, is that once you have a RAID controller anyway, with the addition of a third (inexpensive) drive you can get all that speed as well as fault tolerance using different RAID configuration.

Using RAID 5, a third drive contains "parity" information - kind of the "C" in an "A + B = C" type of equation - lose any of A, B or C and it can be recalculated from the other two. In the world of hard disk drives, that means a drive can fail completely and your system continues to operate while the disk is replaced. RAID controllers often include "hot swap" ability, so that could even be literal: the drive could be replaced without stopping or restarting the system.

That's just one example - RAID as a concept actually includes several different arrangements that you can use to balance performance, fault tolerance and risk.

Just make sure that if you're using RAID 0, you know what you're getting into.