Encyclopedia of Computer Science and Technology

RRAID (redundant array of inexpensive disks)Computer storage is relatively cheap today (see hard disk),but having continued access to data in the event of hardwarefailure is essential to any enterprise. RAID, or redundantarray of inexpensive disks, is a way to turn plentifulstorage into higher reliability and/or speed of access. RAIDworks by turning a group of drives into a single logicalStriping spreads data across several disk drives so that a singlehead movement on each drive can fetch a large amount of data.Mirroring duplicates each sector of data on a second disk drive,ensuring that if one drive fails the data can still be retrieved. Combinationsof both techniques are often used, trading space for reliability(or vice versa).unit; the operating system need not deal with this internalorganization, but simply reads or writes data as usual.To improve reliability, data can be mirrored, or copied totwo or more disks. While the data obviously takes up morespace, the advantage is that the data remains intact andrecoverable if any one drive fails. Further reliability can beachieved by storing redundant data (such as parity bits orHamming codes), to diagnose and fix some disk problems(see error correction and fault tolerance).To achieve greater speed of data access, data can be“striped,” where a file is broken into pieces, with each piecestored on a sector on a different drive. Thus instead of thehead of a single drive having to jump around to multiplesectors to read the data, the heads on all the drives cansimultaneously read many parts of the file, which are thenassembled into the proper order.Levels and CompromisesBy combining mirroring, error correction, and/or striping,different “levels” of RAID can be implemented to suit differentneeds. There are various trade-offs: Striping can increaseaccess speed, but uses more storage space and, by increasingthe number of disks, also increases the chance that one willfail. Implementing error correction can make failure recoverable,but slows data access down because data has to beread from more than one location and compared.The most commonly used RAID levels are:• RAID 0—striping data across disks, higher speed butno error correction; failure of any disk can make dataunrecoverable398