In computing, a redundant array of independent disks, also known as redundant array of inexpensive disks (commonly abbreviated RAID) is a system which uses multiple hard drives to share or replicate data among the drives. Depending on the version chosen, the benefit of RAID is one or more of increased data integrity, fault-tolerance, throughput or capacity compared to single drives. In its original implementations (in which it was an abbreviation for "redundant array of inexpensive disks"), its key advantage was the ability to combine multiple low-cost devices using older technology into an array that offered greater capacity, reliability, speed, or a combination of these things, than was affordably available in a single device using the newest technology.
At the very simplest level, RAID combines multiple hard drives into a single logical unit. Thus, instead of seeing several different hard drives, the operating system sees only one. RAID is typically used on server computers, and is usually (but not necessarily) implemented with identically-sized disk drives. With decreases in hard drive prices and wider availability of RAID options built into motherboard chipsets, RAID is also being found and offered as an option in more advanced user computers. This is especially true in computers dedicated to storage-intensive tasks, such as video and audio editing.
The original RAID specification suggested a number of prototype "RAID levels", or combinations of disks. Each had theoretical advantages and disadvantages. Over the years, different implementations of the RAID concept have appeared. Most differ substantially from the original idealized RAID levels, but the numbered names have remained. This can be confusing, since one implementation of RAID 5, for example, can differ substantially from another. RAID 3 and RAID 4 are often confused and even used interchangeably.
The very definition of RAID has been argued over the years. The use of the term redundant leads many to split hairs over whether RAID 0 is a "real" RAID type. Similarly, the change from inexpensive to independent confuses many as to the intended purpose of RAID. There are even some single-disk implementations of the RAID concept. For the purpose of this article, we will say that any system which employs the basic RAID concepts to combine physical disk space for purposes of reliability, capacity, or performance is a RAID system.
Norman Ken Ouchi at IBM was awarded U.S. Patent 4,092,732 titled "System for recovering data stored in failed memory unit" in 1978 and the claims for this patent describe what would later be termed RAID 5 with full stripe writes. This 1978 patent also mentions that disk mirroring or duplexing (what would later be termed RAID 1) and protection with dedicated parity (what would later be termed RAID 4) were prior art at that time.
In 1988, RAID levels 1 through 5 were formally defined by David A. Patterson, Garth A. Gibson and Randy H. Katz in the paper, "A Case for Redundant Arrays of Inexpensive Disks (RAID)". This was published in the SIGMOD Conference 1988: pp 109-116. The term "RAID" started with this paper.
It was a particularly ground-breaking work in that the concepts are both novel and "obvious" in retrospect once they had been described. This paper spawned the entire disk array industry.
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