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RAID controller

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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 disk array or NAS (network-attached storage) server. In either case, the RAID controller manages the physical storage units in a RAID system and presents them to the server in logical units (e.g., multiple physical disks may be presented as a single logical volume, perhaps using some of the capacity of the disks to provide data protection via a redundancy scheme).

External disk arrays are usually purchased as an integrated subsystem of RAID controllers, disk drives, power supplies, and management software. RAID adapters for use internal to a PC or server are often sold as separate commodities to which commodity disk drives are attached.

A RAID (Redundant array of independent (or inexpensive) disks) system is 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

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