An Introduction to RAID

RAID stands for Redundant Array of Independent Disks. The idea of RAID technology was to use an array of hard disks for either better performance or better security against disk failure. Raid can use 2 or more disks at once to increase data reading and writing speed, It can use 2 or more disks to store the same data so disk failure will not mean that you lose your data, or RAID can be a mixture of both. A RAID Array of disks will appear to an operating system as a single disk as extra storage space is not provided by RAID.

The common RAID functions as mentioned above comes in 3 different levels. These are called RAID 0, RAID 1 and RAID 0+1. This is the terminology you will see when you are buying a motherboard that supports the RAID feature. The real names of these levels are. Stripping, Mirroring and Stripping + Mirroring.

RAID 0 Striping

The Diagram to the left shows the basics of the RAID 0 or striping feature. The idea of RAID 0 is to increase performance. When storing information using the striping feature, the data will be split block by block between the two hard disks. Block one will be send to disk one, block two will be sent to disk two, block three will be sent to disk one and block 4 will be sent to disk 2 and so on. This is much faster than a single disk because when reading the data off the disks the twp of them will be working at the same time to retrieve the same file virtually doubling the speed or retrieval and so virtually halving the time of retrieval. As I mentioned this is a performance setup. Should any one of the disks fail the whole array will become corrupt. Most of the files will be split between disks and so will be rendered useless. If you don't have important data on your computer or you have regular backups of what you do need, then a RAID 0 setup would greatly increase your computers disk performance. To get the best out of this system it is wise to use two disks which are the same make and model. If this is not possible then two of the same size and RPM would be useful but not essential. If two disks of different sizes are used in this system then the logical drive will show as the smallest disk. See drive capacities under RAID at the end of this article.

RAID 1 Mirroring

RAID 1 or mirroring gives added security for your data at the cost of storage space. As with striping this setup uses two hard disk drives to produce a single logical drive. In this instance however the total storage space is only the size of one of the disks (the smallest one). This is because with RAID 1 any data that is written to or read from the hard disk is done on the second hard disk exactly the same. If you save a file to your machine, it will will saved on both disks at the same time. This will however affect system performance with two disks needing to be written to and with the data being the same, its no better in terms of performance unlike the stripping method. However there is always advantages. Mirroring, having the same data on both disks has obvious plus points when it comes to data integrity and security against disk failure. If either disk one or two should fail the other disk will take over as the solitary disk providing and storing data like it did before the failure. Again see the drive capacity section at the end of this article to learn about data redundancy and why the logical drive sizes are what they are with each of the three RAID setups.

RAID 0+1 Striping and Mirroring

RAID 0+1 or Striping + Mirroring as you would imagine is a combination of the above two setups. This setup takes the advantages of both the stripping setup and the mirroring setup. You get the increased performance of splitting the data across multiple drives, however each of these striped drives will have a mirror as well for the data backup and security against failure. The obvious drawback here is the cost involved. The minimum amount of hard disk drives used in this configuration is 4. This puts most home users out of the equation as not only do you need to buy 4 hard disks but the PSU has to cope as well.

Disk Capacities Using the RAID Function

These RAID functions give you varying capacities for your hard disk. To illustrate this we will take an example of a user using only 80Gb hard disks. We will take each of RAID levels mentioned to see what drive capacities you would get out of them.

RAID 0 Striping
With RAID 0 and using the 2 80Gb hard disks you would get the full 160Gb of storage space. Although the data is split between the 2 hard disks. There is no data redundancy (duplicate data). This allows for the full storage space to be used.

RAID 1 Mirroring
When using two 80Gb hard disks with the RAID 1 function you would only receive 80Gb of storage space. Because you are using the two drives to contain the same data, the logical drive will appear as a single 80Gb drive.

RAID 0+1 Striping
In this example we would need to use 4 80Gb drives. RAID 0+1 is a combination of the two above and so storage works out as a combination of the two as well. The logical drive will appear as a single drive, this drives capacity will be 160Gb. The 2 striped drives will be included in the logical drives space, but as above the mirrored drives will appear invisible to the user.