RAID controller

The RAID controller is best described as a device in which servers and storage intersect. The controller can be internal to the server, in which case it is a card or chip, or external, in which case it is an independent enclosure, such as a NAS (network-attached storage). In either case, the RAID controller manages the physical storage units in a RAID system and delivers them to the server in logical units (e.g., six physical disks may be used to ensure that one drive stays correctly backed up, but the server sees only one drive).

While a RAID controller is almost never purchased separately from the RAID itself, the controller is a vital piece of the puzzle and therefore not as much a commodity purchase as the array.

A RAID (Redundant array of independent (or inexpensive) disks) system is simply a collection of disk drives that employs two or more drives in combination for fault tolerance and performance. RAID drives vary in robustness from Level 0 (data striping without redundancy) to Level 7 (Asynchronous, cached striping with dedicated parity). The most common RAID levels are 0, 1, and 5

   * Level 0: striping without parity (spreading out blocks of each file across multiple disks).
   * Level 1: disk mirroring or duplexing.
   * Level 2: bit-level striping with parity
   * Level 3: byte-level striping with dedicated parity. Same as Level 0, but also reserves one dedicated disk for error correction data. It provides good performance and some level of fault tolerance.
   * Level 4: block-level striping with dedicated parity
   * Level 5: block-level striping with distributed parity
   * Level 6: block-level striping with two sets of distributed parity for extra fault tolerance
   * Level 7: Asynchronous, cached striping with dedicated parity 

References

Storage Basics: Choosing a RAID Controller, May 7, 2004, By Ben Freeman[1]

ORACLE FAQ Glossary of Terms, Nov 12, 2005, Frank Naudé http://www.orafaq.com/glossary/faqglosr.htm