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RAID, an acronym for Redundant Array of Inexpensive Disks or Redundant Array of Independent Disks, is a technology that allows high levels of storage reliability from low-cost and less reliable PC-class disk-drive components, via the technique of arranging the devices into arrays for redundancy. This concept was first defined by David A. Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987 as Redundant array of inexpensive disks. Marketers representing industry RAID manufacturers later reinvented the term to describe a Redundant array of independent disks as a means of dissociating a "low cost" expectation from RAID technology. RAID is now used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. The different schemes/architectures are named by the word RAID followed by a number, as in RAID 0, RAID 1, etc. RAID's various designs involve two key design goals: increase data reliability and/or increase input/output performance. When multiple physical disks are set up to use RAID technology, they are said to be in a RAID array. This array distributes data across multiple disks, but the array is seen by the computer user and operating system as one single disk. RAID can be set up to serve several different purposes. Both hardware and software RAIDs with redundancy may support the use of hot spare drives, a drive physically installed in the array which is inactive until an active drive fails, when the system automatically replaces the failed drive with the spare, rebuilding the array with the spare drive included. This reduces the mean time to recovery (MTTR), though it doesn't eliminate it completely. Subsequent additional failure(s) in the same RAID redundancy group before the array is fully rebuilt can result in loss of the data; rebuilding can take several hours, especially on busy systems. Rapid replacement of failed drives is important as the drives of an array will all have had the same amount of use, and may tend to fail at about the same time rather than randomly. RAID 6 without a spare uses the same number of drives as RAID 5 with a hot spare and protects data against simultaneous failure of up to two drives, but requires a more advanced RAID controller. Further, a hot spare can be shared by multiple RAID sets. [READ THE REST OF THIS ARTICLE]
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