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Proper disk configuration

Posted on 2002-03-14
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Last Modified: 2010-04-14
I'm installing Win2k Server on a box with (4)36GB drives, connected to a hardware RAID controller.  What is the recommended way to set up these disks?  Is it ok to set all 4 in a RAID5 config, and have the OS part of that?  What are the guidelines for this?
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Question by:tsavage
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Hi TS,

There are no hard and fast guidelines for setting up a RAID 5 configuration. The procedures you implement are really tailored by several factors, available hardware, OS, data to be handled and whether it has a critical nature, need for fail-over support and many other considerations.

RAID 5 requires at least 3 drives and is usually used to combine the speed of striping with the reliability of mirroring. This is done by striping the data across two drives in the array with a user defined stripe size. The 3rd drive in the array, the one not getting striped data, is given a parity bit. A parity bit is generated from the data save operation using an algorithm to produce data to be used to recreate the information stored on both drives that received the striped data to begin with.

The two drives receiving the striped data and the one receiving the parity bit are changed constantly. As an example, if drives 1 and 2 receive striped data on a write and drive 3 receives a parity bit, on the next write drives 2 and 3 will receive the striped data and drive 1 will receive the parity bit. This shifting continues and eliminates the random write performance hit that comes with a single dedicated drive receiving the parity information.

The parity information is calculated on the RAID controller itself, which is where the term "hardware RAID controller" is derived as they require a special chip to create the parity information and determine which drive to send it to.

RAID 5 arrays usually provide a balance between RAID 0 and RAID 1 configurations. Some of the features of striping are in place as well as the features of mirroring. With the parity bit, if information is lost on one of the three drives in the array, it can be rebuilt. Striping is used to break up the data and send it to multiple drives, and its RAID 0 speed aspects are evident.

Data recreation is relatively easy. In a 3 drive RAID 5 array example of what was noted above, using a 64KB stripe size with a 128KB file, first, a parity bit is created for the file that the controller card has received by performing an XOR calculation on the data. Next, the 128KB file is broken into two 64KB files, one of which is sent to drive 1 and the other to drive 2. In the last part of the data-save operation, the parity information that is calculated above is written to the third drive in the array.

Should one of the drives in the array go bad and the 128KB file is lost, the data can be recreated. It does not matter which drive fails: all the data is still available. If the third drive noted above fails, the one that received the parity information for this write, then the original data can be read off of drives 1 and 2 to recreate the parity information. If either drive 1 or drive 2 fails, the parity information stored on drive 3 is used to recreate the data lost on the original drive.

Keep in mind though that RAID 5 is not an all encompassing solution, as it too has its own problems. Due to the parity bit that must be calculated and written to on each drive, there is system resource overhead. This is really evident when changing only one piece of information on one drive in the array. During this process, not only does the information that requires changing require writing but the parity bit must also be recreated. This means that once the data is written, both drives with the stripe blocks on them must be read, a new parity bit be calculated, and then the new parity bit has to be written to the third drive. This problem only increases as additional drives are added to the array.

So, in answer to part of your question, no don't use all 4 drives, use three and use the fourth as a hot swap drive.

Especially important is to insure that you to use only identical drives for a RAID 5 setup. Not only does this ensure speed it also ensures that all of the array's storage capacity is utilized. The size of a RAID 5 array is equal to the size of the smallest drive times the number of drives in the array minus one, as one of the drives always receives a parity bit. RAID 5 provides a good balance between speed and reliability.

Dennis
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