Pósturaf Fox » Þri 11. Nóv 2003 10:18
Digital Television Glossary
RAID
Redundant array of independent disks. A grouping of standard disk drives together with a RAID controller to create storage that acts as one disk to provide performance beyond that available from individual drives. Primarily designed for operation with computers RAIDs can offer very high capacities, fast data transfer rates and much-increased security of data. The latter is achieved through disk redundancy so that disk errors or failures can be detected and corrected.
A series of RAID configurations is defined by levels and, being designed by computer people, they start counting from zero. Different levels are suited to different applications.
Level 0: No redundancy--benefits only of speed and capacity--generated by combining a number of disks. Also known as "striping."
Level 1 Complete mirror system--two sets of disks both reading and writing the same data. This has the benefits of level 0 plus the security of full redundancy--but at twice the cost. Some performance advantage can be gained in read because only one copy need be read, so two reads can occur simultaneously.
Level 2: An array of nine disks. Each byte is recorded with one bit on each of eight
disks and a parity bit recorded to the ninth. This level is rarely, if ever, used.
Level 3: An array of n+1 disks recording 512 byte sectors on each of the n disks to
create n x 512 "super sectors" + 1 x 512 parity sector on the additional disk which is
used to check the data.
The minimum unit of transfer is a whole superblock. This is most suitable for systems
in which large amounts of sequential data are transferred--such as for audio and
video. For these it is the most efficient RAID level since it is never necessary to
read/modify/write the parity block. It is less suitable for database types of access in
which small amounts of data need to be transferred at random.
Level 4: The same as Level 3 but individual blocks can be transferred. When data is
written it is necessary to read the old data and parity blocks before writing the new
data as well as the updated parity block, which reduces performance.
Level 5: The same as Level 4, but the role of parity the disk is rotated for each
block. In level 4 the parity disk receives excessive load for writes and no load for
reads. In Level 5 the load is balanced across the disks.
Computer Terms Glossary
RAID
1. Short for Redundant Array of Independent (or Inexpensive) Disks, a category of disk drives that employ two or more drives in combination for fault tolerance and performance. RAID disk drives are used frequently on servers but aren't generally necessary for personal computers. There are number of different RAID levels. The three most common are 0, 3, and 5:
Level 0: Provides data stripping (spreading out blocks of each file across multiple disks) but no redundancy. This improves performance but does not deliver fault tolerance.
Level 1: Provides disk mirroring.
Level 3: 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 5: Provides data stripping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance.
2. Reasonably good bug killer.