RAID

RAID is an acronym for Redundant Array of Independent Disks or Redundant Array of Inexpensive Disks.

Purposes
RAID is a technique by which an array of smaller less-expensive disks can provide a cost-effective alternative to a single high-performance and high-capacity disk by splitting the bandwidth and storage space among them. This can be used for a range of purposes:
 * Mirroring: duplication of data between disks as a form of backup through redundancy.
 * Duplexing: duplication of of the drive and controller of matching devices for mirroring purposes.
 * Deferred: data cached in memory can be written to the first drive of the array that becomes available.
 * Hot swapping: a failed drive in a mirrored array can be replaced while the other is still operating.
 * Hot sparing: a spare drive can be initialized into an array when another fails.
 * Synchronization: drives can be synchronized so that one stream of data is written to all at once.

Versions

 * RAID 0: Data is split (striped) and interleaved between two or more drives of similar performance, to improve write speeds. However, failure of one of them will cause the RAID to fail due to lack of redundancy.
 * RAID 1: Identical data is written simultaneously to two or more drives. There is no performance advantage, but rendundancy ensures the preservation of data in case one of the drives fails.
 * RAID 2: Data is striped (like RAID 0), but data is recorded another drive to improve reliability. However, there is a performance hit, and this method has gone out of use as ECC hardware has become commonplace among storage devices.
 * RAID 3: Data is striped among multiple drives (like a combination of RAID 0 and 1) to improve reliability through redundancy while maintaining performance. However, this form of RAID can still fail if multiple drives happen to fail simultaneously.
 * RAID 4: Similar to RAID 3, but with the addition of another drive to store parity data like RAID 2 for improved reliability. Data lost from any lost drive can be reconstructed with the parity drive. However, failure of the RAID controller can still be catastrophic.
 * RAID 5: Data is interleaved among drives with checksum and parity information is recorded on all of them. Used by with a minimum of 3 drives and improved performance as the number of drives increases.
 * RAID 6: An extension to RAID 5 which provides for a structured file system to log the data.
 * RAID 10: An expensive alternative to RAID 1 in which to data writes are segmented to improve performance while maintaining reliability.
 * RAID 53: An expensive alternative in which RAID 0 arrays are segmented as RAID 3 arrays. Performance is improved, but drives are required to be synchronized.