Redundant array of independent disks (RAID) is a method of storing duplicate data on two or more hard drives. It is used for data backup, fault tolerance, to improve throughput, increase storage functions and to enhance performance. RAID is attained by combining two or more hard drives and a RAID controller into a logical unit. The OS sees RAID as a single logical hard drive called a RAID array. There are different levels of RAID, each distributing data across the hard drives with their own attributes and features. Originally, there were five levels, but RAID has advanced to several levels with numerous nonstandard levels and nested levels. The levels are numbered RAID 0, RAID 1, RAID 2, etc. They are standardized by the storage networking industry association and are defined in the common RAID disk data format (DDF) standard data structure.
RAID was first patented by IBM in 1978. In 1987 a team of electrical engineers and computer science specialists from the University of Berkley in California defined RAID levels 1 through 5. Their work was published by the Association for Computing Machinery's Special Interest Group on Management of Data in 1988. It was called a case of redundant arrays of inexpensive disks (RAID). The objective was to combine multiple inexpensive devices into an array, which featured more storage, dependability and faster processing. Later, RAID marketers eliminated the term "inexpensive" so there was not a low cost association by consumers and changed the term to “Independent.” RAID is mostly used for data protection allowing a continuation of two data copies, one in each drive. It is often used in high end servers and some small workstations. When RAID duplicates data, a physical disc is in RAID array. The RAID array is read by the OS as one single disc instead of multiple discs. The RAID objective for each disc is to provide better input/output (I/O) operations and enhanced data reliability. RAID levels can be individually defined or have nonstandard levels, as well as nested levels combining two or more basic levels of RAID.
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