RAID 5

Parity, by definition, is integrity of transmitted data. How is that possible?

To ensure integrity of the transmitted data, n amount of data is put through a simple parity calculation to come up with its parity data. With this parity data, any damaged data can be repaired by using the parity data along with other undamaged data. Confused? Well, here's an example to show you how it is done.

First, let's take a look at the most important element of the parity calculation - the XOR (exclusive OR) logical expression. XOR is used mainly because it has a very interesting effect. Okay, let's say that A = '10' and B = '11'.

Parity = A XOR B = '01'

Now, let's say that A gets corrupted. Here's how we regenerate A from the parity data and B :

A = Parity XOR B = '10'

Amazingly simple, isn't it? You have just witnessed the 'Undo' property of XOR. And this isn't only limited to two operands. Try using as many operands as you like and you will still get the same effect!

So, using XOR allows us to achieve data 'redundancy' by using only an extra hard disk drive. The restoration of data would only require the RAID controller to do some simple calculations to retrieve the original data of the damaged drive.

However, if two or more drives die at the same time, you won't be able to rebuild the array. RAID 5 only provides data recovery for single drive failures. Then again, the probability of having two drives failing at the same time is very low.

There is also another disadvantage of RAID 5. This time, it's related to parity itself. While calculating a single XOR operation is fairly simple, you have to remember that these simple calculations have to be computed each and every time any bit of data is changed on any of the hard disk drive sin the array. This could literally mean that millions of XOR operations have to be computed every second!

That takes a lot of computing power, which is why most RAID 5 implementations use a dedicated processor to handle the parity calculations. You can actually do software RAID 5 but be warned that your system will end up calculating parity more than doing any real work!

Clearly, RAID 5's advantages outweighs its disadvantages. That's why RAID 5 is one of the most popular solutions used by many major corporations all over the world today.

Note : RAID 5 is not available in the Promise FastTrak100 ATA RAID card. Rather, it is available on the higher-end Promise SuperTrack series.

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