Redundant Arrays of Inexpensive Disks (RAID)
RAID (redundant array of independent discs) is a concept in storage subsystems that can deliver higher levels of protection against down-time and data loss than conventional disc drives. RAID refers to a drive architecture designed to safeguard critical data through redundancy. In theory, RAID arrays composed of conventional discs can function for hundreds or even thousands of years without losing data because of a disc failure. RAID also offers other benefits: it can improve input/output performance, make servicing simpler and quicker and allow users to fine-tune the drive system to match the needs of specific applications.
The RAID concept was first articulated by a group of Berkeley re-searchers in 1987. Their work defined five levels of RAID : five ways of distributing data across an array of discs so that the failure of a single disc does not cause data loss. Of the original designs, three have proven commercially attractive: Level 1, Level 3 and Level 5. Though not strictly RAID, an additional level, Level 0, is commonly included among RAID designs because of its similarity to the original designs.
Level 0 distributes data across discs by striping-a technique for storing consecutive chunks of data on different discs. Striping improves I/O throughput but provides no more data protection than conventional drives. Level 1 achieves data redundancy through mirroring-keeping identical data on two separate discs. Mirroring offers excellent reliability and some improvement in I/O throughput but is relatively inefficient in its use of total disc capacity.
Level 3 and Level 5 combine striping with parity codes to allow recovery of data if a disc fails. Level 3 and Level 5 both use disc capacity more efficiently than Level 1. There is more to RAID than redundancy. RAID contributes to automatic load balancing, avoiding hot discs where 80 percent of the I/O requests target 20 percent of the disc capacity. The right choice of RAID level can speed up data transfers or handle more I/O requests per second. Level-3 drives process large amounts of data quickly by transferring data from all discs in parallel, shortening transfer times for applications such as image processing or CAD that read in large, sequential files. Level-5 arrays can handle large numbers of I/O requests concurrently, so they are a good match for applications that make many small requests.