In IBM mainframe computers, a Coupling Facility or CF is a piece of computer hardware which allows multiple processors to access the same data.
A Parallel Sysplex relies on one or more Coupling Facilities (CFs). A coupling facility is a mainframe processor (runs in an own LPAR, with dedicated physical CP, defined thru HMC), with memory and special channels (CF Links), and a specialised operating system called Coupling Facility Control Code (CFCC). It has no I/O devices, other than the CF links. The information in the CF resides entirely in memory as CFCC is not a virtual memory operating system. A CF typically has a large memory - of the order of several gigabytes. In principle any IBM mainframe can serve as a coupling facility. The CF runs no application software.
When originally introduced, the CFCC executed in a separate mainframe unit that was essentially a processor without I/O facilities other than the CF links. Later IBM enabled the use of an Internal Coupling Facility where the CFCC runs in a logical partition ( LPAR ) defined in standard processor complex and communicates over internal links within that processor complex hardware. Links to another processor unit are over copper cables. More than one CF is typically configured in a Sysplex cluster for reliability and availability. Recovery support in the z/OS operating system allows structures to be rebuilt in the alternate CF in the event of a failure.
Supported by CFs, a Sysplex cluster scales very well up to several hundreds of CPUs (in up to 32 members, each with up to 64 CPUs) running transaction and data base applications. Using the CF links, data can be directly exchanged between the CF memory and the memory of the attached systems, using a direct memory access like mechanism, without interrupting a running program. Systems in a Sysplex cluster store CF information in local memory in an area called a bit vector. This enables them to locally query critical state information of other systems in the Sysplex without the need for issuing requests to the CF. The System z Architecture includes 18 special machine instructions and additional hardware features supporting CF operation.
Read more about Coupling Facility: Coupling Facility Structures, Structure Duplexing, Coupling Facility Requests, Coupling Facility Levels and Exploiting Software Levels
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