IBM Parallel Sysplex

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In computing, a Parallel Sysplex is a cluster of IBM mainframes acting together in a single system image, usually with z/OS. A Parallel Sysplex combines data sharing (same disk under control of sharing structures in a Coupling Facility) and parallel computing to allow a cluster of up to 32 computers to share a workload for high performance and high availability. Parallel Sysplex provides horizontal scaling.

The forerunner to Parallel Sysplex was Virtual Coupling, a technique which allowed up to 12 IBM 390 systems to execute jobs in parallel. The true Parallel Sysplex was introduced with then-new mainframe models in April 1994^[1]. IBM continues to improve and enhance Parallel Sysplex.

Major components of a Parallel Sysplex include:

Coupling Facility (CF or ICF) hardware, allowing multiple processors to share, cache, update, and balance data access;
Sysplex Timers or Server Time Protocol to synchronize the clocks of all member systems;
High speed, high quality, redundant cabling;
Software (operating system services and, usually, middleware such as DB2).

The Coupling Facility may be either a dedicated external system (a small mainframe, such as a System z9 BC, specially configured with only coupling facility processors) or integral processors on the mainframes themselves configured as ICFs (Internal Coupling Facilities). Both are popular — there are minor technical advantages and disadvantages between external CF and ICF installations. For achieving highest availability without unnecessary spending, IBM generally recommends combining a single external CF with an ICF. However, with z/OS 1.8 and higher an ICF-only implementation is quite viable.

A Parallel Sysplex has at least two CFs or ICFs for redundancy (or at least one of each). Every mainframe participating in a Parallel Sysplex does not need an ICF or its own external CF — mainframes merely attach, via cables, to the external CFs or ICFs.

Server Time Protocol (STP) replaced the Sysplex Timers beginning in 2005 for newer mainframe models. While the Sysplex Timers are physically separate machines, STP is an integral feature within the mainframe itself. With STP and ICFs it is now possible (and common) to construct a complete Parallel Sysplex installation solely with two mainframes and cabling. Moreover, a single mainframe can contain the internal equivalent of a complete physical Parallel Sysplex, so there is no technical requirement to have a multi-frame Parallel Sysplex at least for application testing and development purposes.

Parallel Sysplex enables another major capability: online maintenance. For example, with Parallel Sysplex it is possible (and common) to configure DB2 for z/OS to deliver continuous business service, permitting whole version upgrades and patches while DB2 continues to run and serve users.