A sysplex is IBM's systems complex (the word sysplex comes from the first part of the word system and the last part of the word complex), introduced in 1990 as a platform for the MVS/ESA operating system for IBM mainframe servers. An enhanced version, Parallel Sysplex, was subsequently introduced for the newer operating system, OS/390. The sysplex consists of the multiple computers (the systems) that make up the complex. A sysplex is designed to be a solution for business needs involving any or all of the following: parallel processing; online transaction processing (OLTP); very high transaction volumes; very numerous small work units - online transactions, for example (or large work units that can be broken up into multiple small work units); or applications running simultaneously on separate systems that must be able to update to a single database without compromising data integrity.
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According to IBM, the Parallel Sysplex is the end result of IBM large systems' developments over the years, from the single system uniprocessor, to tightly-coupled multiprocessors, to loosely-coupled configurations, to the sysplex, and finally to the Parallel Sysplex. A single system uniprocessor consists of a single central processor complex (CPC) - which consists of a single central processor (CP) and all associated system hardware and software, controlled by a single copy of the operating system. Tightly coupled multiprocessors consist of a number of CPs added to a CPC that share central storage and a single copy of the operating system. Work is assigned to an available CP by the operating system and can be rerouted to another if the first CP fails. A loosely coupled configuration has multiple CPCs (which may be tightly coupled multiprocessors) with separate storage areas, managed by more than one copy of the operating system and connected by channel-to-channel communications.
A sysplex is similar to a loosely coupled configuration, but differs in that it has a standard communication mechanism (the cross-system coupling facility, or XCF) for MVS system applications that enables communication between application programs on one or multiple computers. The sysplex is made up of a number of CPCs that collaborate, through specialized hardware and software, to process a work load. This is what a large computer system does in general; a sysplex, through XCF, increases the number of processing units and operating systems that can be connected.
The Parallel Sysplex, IBM's latest method of configuration for CPCs, is a clustering architecture that has improved communication capabilities and supports more connected CPCs and more copies of the operating system. There are several areas of improvement over the base sysplex. The Parallel Sysplex Coupling Facility is a new processor that stores crucial system information, usually configured on a separate device. Use of the coupling facility increases the capacity for data sharing among systems and subsystems. Because it is used through both systems and subsystems, it also ensures data integrity and consistency throughout the sysplex. Another feature of the new technology is the Workload Manager (WLM), part of OS/390 that is in each system in a Parallel Sysplex configuration. WLM manages resources more responsively than the earlier schedule-based methods through dynamic workload balancing and prioritization according to user-set criteria. The data-sharing capability enables simultaneous, multiple-system access to data.