The word RAID sounds like it might report something Marines conduct in Fallujah, or a can of what all roaches fear, but it is basically an acronym that stands for Redundant Array of Independent (or Cheap) Disks. Depending on who you talk to, the letter "I" can stand for either independent or cheap, but in my view independent is more appropriate, & far less subjective.
RAID usually allows information to be written to multiple hard disk drives so that a failure of any drive in the array does not lead to the loss of any information, as well as increasing the system's fault tolerance. I say RAID usually does this, as there are several RAID configurations that provide different approaches to redundancy, but some RAID configurations are not redundant at all. Fault tolerance refers to a system's ability to continue operating when introduced with a hardware (or application) failure, as ought to be experienced when a hard drive fails in of the redundant configurations of RAID.
The Hardware
The basic hardware necessary to run RAID includes a set of matched hard drives & a RAID controller.
RAID can be run on any type of hard drive, including SCSI, SATA, & ATA. The number of hard drives necessary is dependent on the particular RAID configuration selected, as described later. I mention the necessity for matched hard drives, & although this is not absolutely necessary, it is recommended. Most arrays will only be able to make use of the capacity of the smallest drive, so if a 250GB Hitachi drive is added to a RAID configuration with an 80GB Hitachi drive, that additional 170GB would probably go to waste (the only time that this doesn't apply is in a RAID configuration called JBOD (a Bunch Of Disks); which "isn't a RAID configuration" but a convenient thing that a RAID controller can do - see "Basic RAID Configurations" below for more information). In addition to matching capacities, it is highly recommended that drives match in terms of speed & transfer rate as the performance of the array would be restricted by the weakest drive used. More area that ought to be thought about while matching is the type of hard drive. RAID controllers are usually for SCSI, SATA, or ATA exclusively, although some systems permit RAID arrays to be operated across controllers of different formats.
The RAID controller is where the information cables from the hard drives are connected, & conducts all of the processing of the information, like the typical drive connections found on a motherboard. RAID controllers are obtainable as add on cards, such as this Silicon Picture PCI ATA RAID controller, or integrated in to motherboards, such as the SATA RAID controller found on the Asus K8V SE Deluxe (http://www.geeks.com/details.asp?invtid=K8VSE-DELUXE). Motherboards that include RAID controllers can be operated without the use of RAID, but the integration is a pleasant feature to have if RAID is a consideration. Even for systems without onboard RAID, the comparatively low cost of add on cards makes this part of the upgrade comparatively pain free.
Another piece of hardware that is not necessary, but may show useful in a RAID array is a hot swappable drive bay. It allows a failed hard drive to be removed from a live method by basically unlocking the bay & sliding the drive cage out of the case. A brand new drive can then be slid in, locked in to place, & the method won't skip a beat. This is usually seen on SCSI RAID arrays, but some IDE RAIDS cards will also permit this.
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