redundancy
1.
The provision of multiple interchangeable components to perform a single function in order to cope with
failures and errors. Redundancy normally applies primarily to
hardware. For example, one might install two or even three
computers to do the same job. There are several ways these
could be used. They could all be active all the time thus
giving extra performance through {parallel processing} as well
as extra availability; one could be active and the others
simply monitoring its activity so as to be ready to take over
if it failed ("warm standby"); the "spares" could be kept
turned off and only switched on when needed ("cold standby").
Another common form of hardware redundancy is {disk
mirroring}.
Redundancy can also be used to detect and recover from errors,
either in hardware or software. A well known example of this
is the {cyclic redundancy check} which adds redundant data to
a block in order to detect corruption during storage or
transmission. If the cost of errors is high enough, e.g. in a
{safety-critical system}, redundancy may be used in both
hardware AND software with three separate computers programmed
by three separate teams and some system to check that they all
produce the same answer, or some kind of majority voting
system.
2. The proportion of a message's gross
information content that can be eliminated without losing
essential information.
Technically, redundancy is one minus the ratio of the actual
uncertainty to the maximum uncertainty. This is the fraction
of the structure of the message which is determined not by the
choice of the sender, but rather by the accepted statistical
rules governing the choice of the symbols in question.
[Shannon and Weaver, 1948, p. l3]
[Better explanation?]
(1995-05-09)