Mainframes (often colloquially referred to as "big iron") are powerful computers used mainly by large organizations for critical applications, typically bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.
The term originally referred to the large cabinets that housed the central processing unit and main memory of early computers. Later the term was used to distinguish high-end commercial machines from less powerful units.
Most large-scale computer system architectures were firmly established in the 1960s and most large computers were based on architecture established during that era up until the advent of Web servers in the 1990s. (The first Web server running anywhere outside Switzerland ran on an IBM mainframe at Stanford University as early as 1991. See History of the World Wide Web for details.)
There were several minicomputer operating systems and architectures that arose in the 1970s and 1980s, but minicomputers are generally not considered mainframes. (UNIX arose as a minicomputer operating system; Unix has scaled up over the years to acquire some mainframe characteristics.)
Many defining characteristics of "mainframe" were established in the 1960s, but those characteristics continue to expand and evolve to the present day.
Modern mainframe computers have abilities not so much defined by their single task computational speed (usually defined as MIPS — Millions of Instructions Per Second) as by their redundant internal engineering and resulting high reliability and security, extensive input-output facilities, strict backward compatibility with older software, and high utilization rates to support massive throughput. These machines often run for years without interruption, with repairs and hardware upgrades taking place during normal operation.
Software upgrades are only non-disruptive when using facilities such as IBM's z/OS and Parallel Sysplex, with workload sharing so one system can take over another's application while it is being refreshed. More recently, there are several IBM mainframe installations that have delivered over a decade of continuous business service as of 2007, with hardware upgrades not interrupting service. Mainframes are defined by high availability, one of the main reasons for their longevity, because they are typically used in applications where downtime would be costly or catastrophic. The term Reliability, Availability and Serviceability (RAS) is a defining characteristic of mainframe computers. Proper planning (and implementation) is required to exploit these features.
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