RAID

RAID (originally an acronym for 'redundant array of inexpensive disks'; later redefined to stand for 'redundant array of independent disks') is the name for a family of schemes to increase the reliability, performance or size of disk mass storage by using a number of drives in parallel.

The basic ideas pre-date the original definition of the term (below): early UNIX disk device drivers had split a single virtual drive across multiple physical drivers ('striping'); and mirrored disks (for reliability) had been used by Tandem.

Types

Different RAID schemes are referred to by labels of the form 'RAID n', where 'n' is a digit; all produce a single virtual drive. RAID 2-6 all use an ECC, and are able to recover from damage to a single physical drive. In RAID 2-4, the ECC information is stored on a single other drive; in RAID 5 and 6, it is stored on multiple drives.

The main ones are:

• RAID 0 - a virtual drive is striped (split) across multiple physical drives; provides increased performance only
• RAID 1 - a virtual drive is mirrored across multiple physical drives; provides increased reliability only
• RAID 2 - a virtual drive is striped across multiple physical drives, a bit at a time
• RAID 3 - a virtual drive is striped across multiple physical drives, a byte at a time
• RAID 4 - a virtual drive is striped across multiple physical drives, a block at a time
• RAID 5 - a virtual drive is striped across multiple physical drives, with distributed ECC
• RAID 6 - a virtual drive is striped across multiple physical drives, with replicated, distributed ECC

External links

• A Case for Redundant Arrays of Inexpensive Disks (RAID)
• RAID: A Personal Recollection of How Storage Became a System
• rk.4 - 'striping' in UNIX V6