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116. A RAID Primer

By Andrew D. Wright

RAID. When most people see that, they'll think of bug spray.

Instead it's a powerful way of coping with a couple of major shortcomings of modern computer hardware. RAID stands for Redundant Array of Independent Drives.

Modern computer hard drives are marvels of engineering but they are also the slowest part of the computer. Hard disk platters have to spin up, read/write heads have to wait for the data to come round on the next rotation to access it. To a CPU running billions of operations a second it's like sending parcels by clipper ship to the Antipodes.

One answer to this is to stripe data across two (or more) hard drives. A computer can be pulling pieces of information from different drives at the same time, greatly reducing the time needed to access it.

This is what is called RAID 0. RAID 0 can increase disk access speeds by dramatic margins, from ten to fifty per cent or more, depending on the particular disk setup. All data is broken up into pieces that are split up over the hard drives in the RAID 0 array.

The main disadvantage to RAID 0 is if one of the drives in the array were to fail, all data in the array would be lost, since the data written to the surviving drive(s) would be incomplete and corrupted.

Which brings us to the second main shortcoming of hard drives: they are prone to break down.

RAID 1 seeks to make up for this by mirroring all data equally across two (or more) hard drives. In a RAID 1 array, should one hard drive fail, the computer can operate with the remaining drive(s). A new drive can be put into the array and the computer can still be used while the data from the rest of the array is rebuilt on the new mirror drive without losing any.

A RAID 1 array will run at pretty much the same speed as a single drive by itself would. Reading from the two disks can be a smidge faster but writing all changes to two disks instead of one will be a tiny bit slower. The performance difference is so small it is not likely to be noticed.

There are a number of other types of RAID array, some of which allow striping of data as well as mirroring of data and require specialized RAID controllers.

A striped RAID 0 array with its very fast data access would be useful for hard-core gamers and people working with very large files doing tasks such as creating graphics and video editing. The risk of losing data to a drive failure could be reduced by running nightly backups copying the RAID array data onto slower non-RAID drives.

Any office environment where data integrity and speed of recovery in the event of a drive crash is important should look hard at RAID 1. Accountants, managers, anyone who would like to be able to survive a hard drive crash and use their data without any downtime while repairs to the array are underway can do this with a RAID 1 array.

Next column: The easy guide to setting up RAID arrays. Now you know what they are, how do you set them up? What's the difference between software and hardware RAID?


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Originally published 9 September 2007

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