RAID

RAID (redundant array of independent disks, originally redundant array of inexpensive disks) is a storage technology that combines multiple disk drive components into a logical unit. Data is distributed across the drives in one of several ways called "RAID levels", depending on what level of redundancy and performance (via parallel communication) is required. In October 1986, the IBM S/38 announced "checksum". Checksum was an implementation of RAID-5. The implementation was in the operating system and was software only and had a minimum of 10% overhead. The S/38 "scatter loaded" all data for performance. The downside was the loss of any single disk required a total system restore for all disks. Under checksum, when a disk failed, the system halted and was then shutdown. Under maintenance, the bad disk was replaced and then a parity-bit disk recovery was run. The system was restarted using a recovery procedure similar to the one run after a power failure. While difficult, the recovery from a drive failure was much shorter and easier than without checksum.

RAID is an example of storage virtualization and was first defined by David Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987. Marketers representing industry RAID manufacturers later attempted to reinvent the term to describe a redundant array of independent disks as a means of disassociating 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 physical drives. The physical drives are said to be "in a RAID", however the more common, incorrect parlance is to say that they are "in a RAID array". The array can then be accessed by the operating system as one single drive. The different schemes or architectures are named by the word RAID followed by a number (e.g., RAID 0, RAID 1). Each scheme provides a different balance between three key goals: resiliency, performance, and capacity.