(Redundant Array of Inexpensive Disks) In the late 1980s, this is what RAID stood for , being compared to "single large expensive disks" (SLEDs), which were used at the time. As hard disks became cheaper, the RAID Advisory Board changed "inexpensive" to "independent."
By definition, RAID is a hard disk subsystem that is used to increase performance and/or provide fault tolerance, i.e. enable a hard disk drive to work in conjunction with others, and have a built-in failsafe to ensure that if one of the hard disks fails, the storage system can manage the failure and continue to operate. RAID is a set of two or more ordinary hard disks and a specialised disk controller that contains the RAID functionality. Developed initially for servers and stand-alone disk storage systems, RAID is increasingly popular in desktop PCs and is more commonly used in corporate organisations where data is often at its most critical. RAID can also be implemented via software only, but with less performance, especially when rebuilding data after a failure.
RAID improves performance by disk striping, which interleaves bytes or groups of bytes across multiple drives, so more than one disk is reading and writing simultaneously. Fault tolerance is achieved by mirroring or parity. Mirroring is 100% duplication of the data on two drives (RAID 1).
Parity is used (RAID 3 and RAID 5) to calculate the data in two drives and store the results on a third in the following manner: a bit from drive 1 is XOR'd with a bit from drive 2, and the result bit is stored on drive 3. After the failed drive is replaced, the RAID controller automatically rebuilds the lost data.
RAID systems may be built using a spare drive (hot spare or hot swap) ready and waiting to be the replacement for a drive that fails.
RAID systems come in all sizes from desktop units to floor-standing models. RAID can be added to any desktop PC by adding a RAID controller board and additional IDE, SATA, PATA or SCSI disks. Increasingly, RAID is built on the motherboard. Stand-alone RAID storage units may also include large amounts of cache as well as redundant power supplies.