The Probert Encyclopaedia of Science & Technology

RAID

RAID (Redundant Array of Independent Disks) is a method in computing in which data stored on a hard disk is transparently spread across several disks rather than a single disk.
RAID provides three potential advantages over using simple disks: enhanced speed, increased storage capacity using a single 'virtual' disk rather than several separate disks and a lessened impact of a disk failure. The actual benefits provided depend upon the implementation employed.
The data is stored using techniques such as RAID Level 0 in which the data is stored across two or more disks which appear as a single disk to the user (a technique known as striping), this method providing faster disk access at the risk of losing all data if any one disk drive in the array fails. RAID level 0 is typically used in desktop and workstation computers where high performance is required for non-critical data; RAID level 1 in which the data is stored across two or more disks each holding a copy of the other disk or disks, known as mirroring. In this case, should one disk fail the data can be retrieved from one of the other copies. RAID level 1 provides faster read access to the disks, but at the expense of requiring additional disk drives and slower write access; and RAID level 5 in which data is stored across two or more disks for performance, and also recovery information is written to another disk so that in the event that one disk fails, the data which was stored on the failed disk can be recreated onto a replacement disk at the expense of slower disk writes. RAID level 5 is typically used in file and application servers. RAID level 10 uses four disk drives to create a combination of striping and mirroring, one pair of disks stripe the data across them, the other two disks mirror the first two to provide a backup copy in case of a disk failure. RAID level 10 is typically used in database servers which require both high performance and reliability.
RAID is implemented either by a hardware system, or by software, such as the Intel Matrix Storage Manager. A hardware RAID system comprises a collection of disk which plug into a host computer through a single cable and appear to the operating system as a single disk drive. A software RAID system employs a program that operates on the host computer to present the disks as a single volume. Software RAID controllers are cheaper than hardware controllers, and with the advent of faster processors became more viable.
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