Redundant Array of Inexpensive Disks.

("Inexpensive" is sometimes replaced with "Independent", but the former term is the one that was used when the term "RAID" was first coined by the researchers at the University of California at Berkeley, who first investigated the use of multiple-drive arrays in 1987.)

RAID is a method of combining 2 or more drives to provide either redundancy, or higher speed. Compare with JBOD - Just a Bunch of Disks.

There are many different types of RAID, each with it's own tradeoffs and advantages. These are called RAID Levels. See RAID-0 and RAID-1 for examples.

The largest tradeoff of using RAID is cost, but cost of data recovery is often higher, so RAID is typically used in most servers and in business environments where data protection is more important than making full use of hard drive capacities.

RAID can be implemented either as Software RAID or Hardware RAID.

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