RAID Basics
 

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RAID which stands for Redundant Array of Inexpensive Disks (as named by the inventor [1] ), or alternatively Redundant Array of Independent Disks (a less relative name, and thus now the generally accepted one [2] ) is a technology that employs the simultaneous use of two or more hard disk drives to achieve greater levels of performance, reliability, and/or larger data volume sizes.

The phrase "RAID" is an general term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. RAID's various designs all involve two key design goals: increased data reliability and increased input/output performance. When several physical disks are set up to use RAID technology, they are said to be in a RAID array. This array distributes data across several disks, but the array is seen by the computer user and operating system as one single disk. The most common RAID types are outlined below:

RAID 0 (striped disks) distributes data across several disks in a way which gives improved speed and full capacity, but all data on all disks will be lost if any one disk fails.

RAID 1 (mirrored disks) uses two (possibly more) disks which each store the same data, so that data is not lost as long as one disk survives. Total capacity of the array is just the capacity of a single disk. The failure of one drive, in the event of a hardware or software malfunction, does not increase the chance of a failure nor decrease the reliability of the remaining drives (second, third, etc).

RAID 5 (striped disks with parity) combines three or more disks in a way that protects data against loss of any one disk; the storage capacity of the array is reduced by one disk. The less common RAID 6 can recover from the loss of two disks.

 

  • 1 "A Case for Redundant Arrays of Inexpensive Disks" - Patterson, Gibson, Katz; University of California 1988
  • 2 "Because the term inexpensive is relative and can be misleading, the proper meaning of the acronym is now generally accepted as Redunant Array of Independent Disks" - Chapter 7, page 302, The Essentials of Computer Organization and Architecture; Linda Null and Julia Lobur; ISBN 0-76370444-X