RAID DEFINITIONS

HISTORY

Norman Ken Ouchi at IBM was awarded a 1978 U.S. patent 4,092,732 titled "System for recovering data stored in failed memory unit." The claims for this patent describe what would later be termed RAID 5 with full stripe writes. This 1978 patent also mentions that disk mirroring or duplexing (what would later be termed RAID 1) and protection with dedicated parity (that would later be termed RAID 4) were prior art at that time.

The term RAID was first defined by David A. Patterson, Garth A. Gibson and Randy Katz at the University of California, Berkeley in 1987. They studied the possibility of using two or more drives to appear as a single device to the host system and published a paper: "A Case for Redundant Arrays of Inexpensive Disks (RAID)" in June 1988 at the SIGMOD conference. This specification suggested a number of prototype RAID levels, or combinations of drives. Each had theoretical advantages and disadvantages. Over the years, different implementations of the RAID concept have appeared. Most differ substantially from the original idealized RAID levels, but the numbered names have remained. This can be confusing, since one implementation of RAID 5, for example, can differ substantially from another. RAID 3 and RAID 4 are often confused and even used interchangeably.

Their paper formally defined RAID levels 1 through 5 in sections 7 to 11:

• First level RAID: mirrored drives
• Second level RAID: Hamming code for error correction
• Third level RAID: single check disk per group
• Fourth level RAID: independent reads and writes
• Fifth level RAID: spread data/parity over all drives (no single check disk)

References & White Papers

A Case for Redundant Arrays of Inexpensive Disks (RAID) - David Patterson, Garth A. Gibson, Randy Katz - Univ. Calif, Berkeley (1988)