Disks are often among the least reliable components of the computer
systems, yet the failure of a disk can result in the unrecoverable loss of vital
business data, or at the very least a need to restore from tape with
consequent delays.
•Cost
It is cheaper to provide a given storage capacity and a given performance
level with several small disks connected together than with a single disk.
There is nothing unusual about connecting several disks to a computer to
increase the amount of storage. Mainframes and minicomputers have always
had banks of disks. It becomes a disk array when several disks are connected
and accessed by the disk controller in a predetermined pattern designed to
optimize performance and/or reliability.
Disk arrays seem to have been invented independently by a variety of groups,
but it was the Computer Architecture Group at the University of California,
Berkeley who invented the term RAID. RAID stands for

Redundant Array of

Inexpensive Disks

and provides a method of classifying the different ways of
using multiple disks to increase availability and performance.
1.6.6 RAID Classifications
The original RAID classification described five levels of RAID (RAID-1 through 5).
RAID-0 (data-striping) and RAID-1 Enhanced (data stripe mirroring) have been
added since the original levels were defined. RAID-0 is not a pure RAID type,
since it does not provide any redundancy.
Different designs of arrays perform optimally in different environments. The two
main environments are those where high transfer rates are very important, and
those where a high I/O rate is needed, that is, applications requesting short
length random records.
Table 6 shows the RAID array classifications, and is followed by brief
descriptions of their designs and capabilities.
Table 6. RAID Classifications
RAID Level Description
RAID-0 Block Interleave Data Striping without Parity
RAID-1 Disk Mirroring/Duplexing
RAID-1 (Enhanced) Data Stripe Mirroring
RAID-2 Bit Interleave Data Striping with Hamming Code
RAID-3 Bit Interleave Data Striping with Parity Disk
RAID-4 Block Interleave Data Striping with one Parity Disk
RAID-5 Block Interleave Data Striping with Skewed Parity
Chapter 1. IBM PC Server Technologies 23