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What is a RAID?

Posted on May 12, 2004 by Richard Millard

What is a RAID?

This is probably the most frequently asked question I get these days. Everyone has a general sense that they're good somehow, but a real vague idea of what they do. I wanted to quickly cover what each RAID array is, and explain the differences between the them.

Short for Redundant Array of Independent (or Inexpensive) Disks, a RAID is a category of disk drives that uses two or more drives in combination for fault tolerance and increased performance. RAID disk drives are used frequently on servers but aren't generally necessary for personal computers.

There are several different types (or levels) of RAID arrays, the most common being 0, 1, and 5.

RAID 0, Data Striping

Most operating systems include support for operating multiple hard drives. However, the system load is often unbalaced, writing a lot of information to one drive, and very little to the other. The general principle behind RAID 0 is to balance the load between all hard drives in a system, and keep them all running as quickly as possible.

I've made an analogy to a the painter. Imagine that you're painting a portrait with your right hand. It takes you 4 hours, and by the time you're done, you've got a tired right hand, and an unused left hand. However, if you had used 2 brushes, and each hand painted half the portrait, you'd have finished in half the time, both hands would have done an equal amount of work, and all your friends would envy your coordination! This is the general principle of data striping.

By writing and reading from both hard drives at the same time, you decrease the amount of time you've got to wait for data access. The most obvious drawback of RAID-0 striping is that both drives contain half the data for each file on your system. Should either of the drives fail, you'll lose all of your data. RAID-0 is increased performance at the risk of potential data loss.

RAID 1, Data Mirroring

A RAID 1 array is pretty simple in concept. Basically, both hard drives are totally identical duplicates of each other. This is a very inexpensive method of data redundancy, only requiring two hard drives.

Generally, this means faster reading time from the hard drives (can read from two disks), but slower writing times, as you have to write to both hard drives. If you're looking for fault protection, this is a good RAID option.


RAID 5 is where things start to get more complicated. In a simple way, it combines increased performance you'd see in RAID 0, with the more reliable disk protection you'd want from RAID 1.

To avoid getting into complicated descriptions of data parity, in general, the concept is at the system distributes information across the hard drives in such a way that should a hard drive ever fail, the system can rebuild the lost data. Because you're running multiple striped drives, you'll see an increase in performance as well.

In general, RAID 5:

Highest Read data rate
Medium Write data rate
Usually requires special hardware

The drawback is that you basically sacrifice a disks worth of space in order to use a RAID 5. If you were using 3x 100gb hard drives, for example, you'd only have 200gb of usable space.


There's a whole bunch of other outdated, or rarely used RAID levels, but those are the main 3.

Hopefully this offered a bit of insight into what a RAID array is, and how it can benefit your computer performance. Please feel free to comment below with any corrections / suggestions / questions.