Mainframe computer





Mainframes (often colloquially referred to as "big iron") are large,
powerful, and expensive computers used mainly by large companies for bulk
data processing (such as bank transaction processing).

The term arose during the early 1970s with the introduction of smaller
computers such as the DEC PDP series, which became known as minicomputers,
so users coined the term "mainframe" to describe larger, earlier types.

Description

Mainframe computers' abilities are not so much defined by their CPU speed as
by their massive internal memory, large, high-capacity external storage,
fast high-throughput I/O, high-quality internal engineering and resulting
proven reliability, and expensive but high-quality technical support. These
machines can and do run successfully for years without interruption, with
repairs taking place whilst they continue to run. Mainframe vendors offer
such services as off-site redundancy—if a machine does break down, the
vendor offers the option to run customers' applications on their own
machines (often without users even noticing the change) whilst repairs go
on. The internal redundancy of these computers can be such that, in at least
one reported case, technicians could move one from one site to another by
disassembling it piece by piece, and reassembling it at the new site, whilst
leaving the machines running. The switchover in this example took place
entirely transparently.

Often, mainframes support thousands of simultaneous users who gain access
through "dumb" terminals.

Some mainframes have the ability to run (or "host") multiple operating
systems and thereby operate not as a single computer but as a number of
"virtual machines". In this role, a single mainframe can replace dozens or
hundreds of smaller PCs, reducing management and administrative costs while
providing greatly improved scalability and reliability. The reliability is
improved because of the hardware redundancy noted above, and the scalability
is achieved because hardware resources can be reallocated among the "virtual
machines" as needed. This is much harder to do with PCs, because adding or
removing hardware resources often requires the machine to be taken offline,
and the hardware limitations are much more restrictive. When running as the
host for many "virtual machines" a mainframe can provide the raw power for
which they have always been valued, but also the flexibility provided by PC
networks.

Currently, IBM mainframes are dominant in the market, with Hitachi, Amdahl,
and Fujitsu also producing machines. Prices start at several hundred
thousand dollars.

History

Several manufacturers produced mainframe computers in the 1960s and 1970s;
in the 'glory days' it was "IBM and the Seven Dwarves": Burroughs, Control
Data, General Electric, Honeywell, NCR, RCA, and Univac. But shrinking
demand and tough competition caused a huge shakeout in the market—RCA
sold out to Univac and GE also left; Honeywell was bought out by Bull,
Univac merged with Sperry to form Sperry/Univac, which was later merged with
Burroughs to form Unisys Corporation in 1986 (so-called "dinosaurs mating").
In 1991, AT&T briefly owned NCR.

Companies found that servers based on lower-cost microcomputer designs could
be deployed at a fraction of the cost and offer local users much greater
control of their own systems, and "dumb terminals" used for interacting with
mainframe systems were gradually replaced by personal computers.
Consequently, demand plummeted and mainframe installations were restricted
mainly to financial institutions with massive data processing requirements.
For a while, there was a consensus among industry analysts that the
mainframe was a dying market, as mainframe platforms were increasingly
replaced by personal computer networks.

This trend started to turn around in the late 1990s as corporations found
new uses for their mainframes, since they can offer web server performance
similar to that of hundreds of smaller machines, but with much lower power
and administration costs.

Another factor currently increasing mainframe use is the development of the
Linux operating system, which is capable of running on many mainframe
systems, either directly or, more commonly, in a virtual machine. This
allows mainframes to take advantage of the software and development
expertise and communities from the PC market.

Comparison with supercomputers

The distinction between supercomputers and mainframes is not a hard and fast
one, but generally one can say that supercomputers focus on problems which
are limited by calculation speed while mainframes focus on problems which
are limited by Input/Output and reliability. As a consequence:

   * Supercomputers typically exploit massive parallelism, often with
     thousands of processors, while mainframes have a single or a small
     number (up to several dozen) of processors.
   * Because of the parallelism visible to the programmer, supercomputers
     are quite complicated to program; in mainframes, the limited
     parallelism (if present) is usually hidden from the programmer.
   * Supercomputers are optimized for complicated computations that take
     place largely in memory, while mainframes are optimized for simple
     computations involving huge amounts of external data accessed from
     databases.
   * Supercomputers tend to cater to science and the military, while
     mainframes tend to target business and civilian government
     applications.