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
