A somewhat dated term still used to refer to very large capacity computers, and to distinguish them from the smaller computers now widely available. However, the distinction between mainframe and other computers is not always clear. The term minicomputer, for example, is sometimes used to refer to computers which do not fall into the category of either microcomputer or mainframe computer. Minicomputers are normally used for specific operations, such as the control of a chemical process, or to serve a small department in a manufacturing company.
For other uses, see Mainframe.Mainframes (often colloquially referred to as Big Iron) are computers used mainly by large organizations for critical applications, typically bulk data processing such as census, industry/consumer statistics, ERP, and financial transaction processing. Today, this term almost exclusively refers to IBM zSeries mainframes - descendants of the System/360.
Description
Modern mainframe computers have abilities not so much defined by their performance capabilities as by their redundant internal engineering and resulting high reliability and security, and strict backward compatibility for older software. The term Reliability, Availability and Serviceability (RAS) is a defining characteristic of mainframe computers.
Mainframes often support thousands of simultaneous users who gain access through specialized terminals or from smaller computers emulating terminals. Early mainframes either supported this timesharing mode or operated in batch mode where users had no direct access to the computing service; Nowadays mainframes support access via any user interface, including the Web.
Characteristics
Nearly all 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 even hundreds of smaller servers, reducing management and administrative costs while providing greatly improved scalability and reliability.
Mainframes can add system capacity nondisruptively and granularly. Modern mainframes, notably the IBM zSeries and System z9 servers, offer two levels of virtualization: logical partitions (LPARs, via the PR/SM facility), virtual machines (via the z/VM operating system), and have native hardware support for Linux and Java.
Mainframe return on investment (ROI), like any other computing platform, is dependent on its ability to scale, support mixed workloads, reduce labor costs, deliver uninterrupted service for critical business applications, and several other risk-adjusted cost factors. Hewlett-Packard also continues to manufacture its own mainframe, the NonStop system originally created by Tandem.
Market
As of early 2006, IBM mainframes dominate the mainframe market at well over 90% market share, however IBM is not the only vendor. Unisys manufactures ClearPath mainframes, based on earlier Sperry and Burroughs product lines, and a recent survey suggests its customers are loyal. Hewlett-Packard sells its unique NonStop systems, which it acquired with Tandem Computers, and Groupe Bull's DPS mainframes are available in Europe.
History
Several manufacturers produced mainframe computers from the late 1950s through the 1970s. IBM's dominance grew out of their 700/7000 series and, later, the development of the 360 series mainframes. The latter architecture has continued to evolve into their current zSeries/z9 mainframes which, along with the then Burroughs and now Unisys MCP-based mainframes, are among the few mainframe architectures still extant that dates from this early period. Terminals used for interacting with mainframe systems were gradually replaced by personal computers. Consequently, demand plummeted and new mainframe installations were restricted mainly to financial services and government. In the early 1990s, there was a consensus among industry analysts that the mainframe was a dying market as mainframe platforms were increasingly replaced by personal computer networks.
That trend started to turn around in the late 1990s as corporations found new uses for their existing mainframes. The growth of e-business also dramatically increased the number of back-end transactions processed by mainframe software as well as the size and throughput of databases. Another factor currently increasing mainframe use is the development of the Linux operating system, which can run on many mainframe systems, typically in virtual machines. Linux allows users to take advantage of the open source software combined with mainframe hardware RAS.
Mainframes vs. supercomputers
The distinction between supercomputers and mainframes is not a hard and fast one, but supercomputers generally focus on problems which are limited by calculation speed while mainframes focus on problems which are limited by input/output and reliability. Supercomputers typically expose it to the programmer in complex manners, while mainframes typically use it to run multiple tasks. One result of this difference is that adding processors to a mainframe often speeds up the entire workload transparently. Consequently, most supercomputers are one-off designs, whereas mainframes typically form part of a manufacturer's standard model lineup. Mainframes tend to have numerous ancillary service processors assisting their main central processors (for cryptographic support, I/O handling, monitoring, memory handling, etc.) so that the actual "processor count" is much higher than would otherwise be obvious.
There has been some blurring of the term "mainframe", with some PC and server vendors refer to their systems as "mainframes" or "mainframe-like".
Statistics
85% of all mainframe programs are written in the COBOL programming language 7% are written in Assembly language, C or C++ 5% are written in PL/I 3% are written in Java and other languagesJava use is increasing rapidly as of 2004, and these figures are likely significantly out-of-date, especially on newer mainframe systems.
90% of IBM's zSeries mainframes have its CICS transaction processing software installed. In the early 1990s the media and many business and computing analysts predicted the death of the mainframe. The quality of service offered by mainframes mean they are often the preferred technology for many business critical applications. As of 2004, IBM claimed over 200 new (21st century) mainframe customers — customers that had never previously owned a mainframe.Speed and performance
The CPU speed of mainframes has historically been measured in millions of instructions per second(MIPS). MIPS have been used as an easy comparative rating of the speed and capacity of mainframes. The smallest System z9 IBM mainframes today run at about 25 MIPS and the largest about 17,801 MIPS. The complex CPU architectures of modern mainframes have reduced the relevance of MIPS ratings to the actual number of instructions executed. IBM has long published a set of LSPR (Large System Performance Reference) ratio tables for mainframes that take into account different types of workloads, and are a more representative measurement, however these comparisons are not available for non-IBM systems.
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