RAID: high-performance, reliable secondary storage
ACM Computing Surveys (CSUR)
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RAID (Redundant Array of Inexpensive [or Independent] Disks) is an architectural concept developed to turn relatively slow and inexpensive hard disks into fast, large-capacity, and more reliable storage systems. The RAID concept was introduced by a team of researchers from the University of California at Berkeley in 1987. RAID systems derive their speed from striping data across multiple disks (placing successive pieces of a file on different disks), thus allowing parallel data accesses. Reliability is generally achieved through replication ("mirroring") or by using error detection and correction schemes across the disk array. (Mirroring is a technique that predates RAID, being used, for example, in the IBM AS/400 minicomputer.) There are many levels of RAID, which differ in the way they provide for speed and/or reliability. The original Berkeley work has specified RAID levels 0-5. Slight modifications to these levels have recently resulted in the specification of levels 6 and 7.