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The Introduction of RAID A-1
User’s Manual
Appendix A. The Introduction of RAID
What is RAID?
RAID (Redundant Array of Inexpensive/Independent Disks) technology was developed to
offer a combination of outstanding data availability, excellent performance, and high
capacity that one single disk drive can not match. A RAID array is defined as two or more
disks grouped together to appear as one single device to the host system, which can tolerate
the failure of a drive without losing data, and which can operate independently from each
other.
To manage MTBF (Mean Time Between Failures) and prevent any single drive failure
causing data loss within an array, UC Berkeley scientists proposed five types of redundant
array architectures, defining them as RAID levels 1 through 5. Each RAID level has its own
strengths and weaknesses, and is well suited for certain types of applications and computing
environments. RAID 1, RAID 3 and RAID 5 of these five types are commonly used. RAID
2 and RAID 4 do not offer any significant advantages over these other types. RAID 3 is
designed for single-user or data-intensive environments, such as imaging or data acquisition
that access extremely large sequential records. This leaves RAID 1 and RAID 5 as the RAID
levels applicable for networked and transaction processing-based environments utilizing
NetWare, Windows NT, Unix, and OS/2.
In addition to these five redundant array architectures, it has become popular to refer to a
non-redundant array of disk drives as RAID 0 array.
Why RAID?
Data security is a very important issue for system administrators. They have to adopt
efficient methods of data protection to guard against potential losses due to drive failures.
Tape-based backups are used to be one solution for data security, but this method is
becoming a task more difficult. The demand to store increasingly large software
applications will impel disk capacities to exceed 10GBs by the end of 1997. Slow,
cumbersome tape backup solutions lose their effectiveness for servers and workstations.
RAID technology is another solution for data security. There are a number of factors
responsible for the growing adoption of arrays for critical network storage. Because today’s
applications create larger files, the need for network storage has increased proportionately.
To accommodate expanding storage requirements, users are adding disk drives --- raising
the probability of drive failures. In addition, the development of CPU speed has exceeded
data transfer rates to storage media, causing I/O bottlenecks for networking applications.