RAID Systems

 RAID is a subsystem storage concept designed for the purpose of offering higher levels of protection from data loss that can occur from any down time caused by malfunctions compared to the protection offered by conventional disk drives. RAID arrays composed of conventional discs can function for hundreds or even thousands of years without losing data because of a disk failure. RAID technology improved input/output performance, make servicing easier and faster and allow users to fine-tune the drive system to match the needs of specific applications. Initially used with servers, desktop PCs are increasingly being retrofitted by adding a RAID controller and extra IDE or SCSI disks. Newer motherboards often have RAID controllers.
Mirroring or parity achieves fault tolerance. Mirroring is 100% duplication of the data on two drives (RAID 1). Parity is used to calculate the data in two drives and store the results on a third (RAID 3 or 5). After a failed drive is replaced, the RAID controller automatically rebuilds the lost data from the other two. RAID systems may have a spare drive (hot spare) ready and waiting to be the replacement for a drive that fails. Different levels of RAID are:

RAID 0 ? Speed: Level 0 is disk striping only, which interleaves data across multiple disks for better performance. It does not provide safeguards against failure. RAID 0 is widely used in gaming machines for higher speed.
RAID 1 - Fault Tolerance: Uses disk mirroring, which provides 100% duplication of data. Offers highest reliability, but doubles storage cost. RAID 1 is widely used in business applications.
RAID 2 ? Speed: Bits (rather than bytes or groups of bytes) are interleaved across multiple disks. The Connection Machine used this technique, but this is a rare method.
RAID 3 - Speed and Fault Tolerance: Data are striped across three or more drives. Used to achieve the highest data transfer, because all drives operate in parallel. Parity bits are stored on separate, dedicated drives.
RAID 4 - Speed and Fault Tolerance: Similar to Level 3, but manages disks independently rather than in unison. Not often used.
RAID 5 - Speed and Fault Tolerance: Data are striped across three or more drives for performance, and parity bits are used for fault tolerance. The parity bits from two drives are stored on a third drive. RAID 5 is widely used on servers to provide speed and fault tolerance.
RAID 6 - Speed and Fault Tolerance: Highest reliability, but not widely used. Similar to RAID 5, but performs two different parity computations or the same computation on overlapping subsets of the data.
RAID 10 - Speed and Fault Tolerance: A combination of RAID 1 and RAID 0 combined. Raid 0 is used for performance, and RAID 1 is used for fault tolerance.

(From Computer Desktop Encyclopedia
C 1998 The Computer Language )