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The Winn L. Rosch Hardware Bible, 6th Edition
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Home » Storage
Everything you need to know about RAID
Author: Gabriel Torres 88,823 views
Type: Tutorials Last Updated: November 20, 2004
Page: 1 of 4
Basics: Data Stripping

More attentive users may have noticed that for some time several new models of motherboard have been turning up on the market with a new feature called RAID, meaning Redundant Array of Independent Disks. But what does this mean in practical terms and how can this feature be of use to normal users?

The RAID system consists of a set of two or more hard disks, with two basic purposes: provide a faster disk system (i. e., speed up disk data loading) through a technique called data stripping - or RAID 0 - and/or set up a safer disk system by means of a technique known as mirroring or RAID 1. These two techniques may be used either separately or jointly.

Lets have a look at data stripping first. Picture a computer equipped with two equal hard disks. In a run of the mill machine – without RAID – each disk is accessed independently from the other. With data stripping, the pair of hard disks will make up a single set, leading the computer to believe that it is dealing with a single, larger disk. If each disk has a 20 GB capacity, the computer will then believe that it is fitted with a single 40 GB disk. Upon storing a file on disk, the RAID system will split it between the pair of hard disks, writing half the file on one and the other half on the other. This takes place in a user-transparent fashion.

But what is the advantage of all this? Let us assume that you are working with a 200 KB file. In a conventional disk system, this file will have to be entirely written on a single disk, using the single existing communication channel. With data splitting, this file will be split into two 100 KB files, each written on one of the disks at the same time. Well, seeing that a 100 KB file takes half the time to be written than a 200-KB file, the disk access speed will be doubled!

To give you a clearer idea, imagine you are working on a really big file, for instance, 100 MB – quite big for normal users, but in professional audio and video editing, files this size are relatively common. If your hard disk (and motherboard) uses ATA-100 standard, it means that it theoretical transfers data at 100MB/s. We stress theoretically because this rate is lower in practice. So in our theoretical example, it will take 1 second to transfer (write or read) our file. Now, if we are using a RAID 0 system on our computer, i. e., two equal hard disks with data splitting and assuming that they are ATA-100 disks, the said file will be split into two 50 MB files and, according, it will only take 0.5 sec to write (or read) each disk. As the access rate to each disk has fallen to half the time (0.5 sec), it follows that the performance has doubled!

But the RAID system is not restricted to using two hard disks. In principle, we can set up as many disks as we want. Following the same example, if we use four equal disks instead of two, the computer will believe that all four are a single disk and will automatically split the file into four parts, multiplying fourfold the file's R/W rate. In the example, the 100 MB file will be split into four 25 MB each file and, according, it will be written at a mere 0.25 sec if we assume that ATA-100 disks are being used.

It goes without saying that the more disks we use, the more expensive our system will be. But with applications handling extensive files, such as professional audio and video editing, the system becomes really attractive seeing that the machine file R/W rate is much faster.

We stress that all this splitting takes place out of sight, and the user is not aware that his file has been split into pieces.

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