Basics of RAID

A couple of the recent Tech Tips from www.geeks.com have made mention of RAID, but the level of detail required in those tips didn't shed much light on what RAID actually is. The number of e-mail responses and comments in the Readers Digress section was convincing enough that an introduction to the basics of RAID would be an appropriate Tech Tip, so here it is.

Introduction

The word RAID sounds like it might describe something Marines conduct in Fallujah, or a can of what all roaches fear, but it is simply an acronym that stands for Redundant Array of Independent (or Inexpensive) Disks. Depending on who you talk to, the letter "I" can stand for either independent or inexpensive, but in my opinion independent is more appropriate, and far less subjective.

RAID generally allows data to be written to multiple hard disk drives so that a failure of any one drive in the array does not result in the loss of any data, as well as increasing the system's fault tolerance. I say RAID generally 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 presented with a hardware (or software) failure, as should be experienced when a hard drive fails in one of the redundant configurations of RAID.

The hardware

The basic hardware required to run RAID includes a set of matched hard drives and a RAID controller.

RAID can be run on any type of hard drive, including SCSI, SATA, and ATA. The number of hard drives required is dependent on the particular RAID configuration chosen, as described later. I mention the need for matched hard drives, and although this is not absolutely necessary, it is recommended. Most arrays will only be able to use the capacity of the smallest drive, so if a 250GB Hitachi drive is added to a RAID configuration with an 80GB Hitachi drive, that extra 170GB would probably go to waste (the only time that this doesn't apply is in a RAID configuration called JBOD (Just a Bunch Of Disks); which really "isn't a RAID configuration" but just 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 and transfer rate as the performance of the array would be restricted by the weakest drive used. One more area that should be considered while matching is the type of hard drive. RAID controllers are generally for either SCSI, SATA, or ATA exclusively, although some systems allow RAID arrays to be operated across controllers of different formats.

The RAID controller is where the data cables from the hard drives are connected, and conducts all of the processing of the data, like the typical drive connections found on a motherboard. RAID controllers are available as add on cards, such as this Silicon Image PCI ATA RAID controller, or integrated into 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 nice feature to have if RAID is a consideration. Even for systems without onboard RAID, the relatively low cost of add on cards makes this part of the upgrade relatively pain free.

Another piece of hardware that is not required, but may prove useful in a RAID array is a hot swappable drive bay. It allows a failed hard drive to be removed from a live system by simply unlocking the bay and sliding the drive cage out of the case. A new drive can then be slid in, locked into place, and the system won't skip a beat. This is typically seen on SCSI RAID arrays, but some IDE RAIDS cards will also allow this (such as this product manufactured by Promise Technology: http://www.promise.com/product/product_detail_eng.asp?productId=92&familyId=7).

The software

RAID can be run on any modern operating system provided that the appropriate drivers are available from the RAID controller's manufacturer. A