Coupling Facility
Hardware or LPAR that allows coordination among multiple IBM System/390 and IBM Z mainframes / From Wikipedia, the free encyclopedia
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In IBM System/390 and IBM Z mainframe computers, a Coupling Facility or CF is a piece of computer hardware or virtual machine that coordinates multiple processors.
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A Parallel Sysplex[1][2][3] 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 through Hardware Management Console (HMC)), with memory and special channels (CF Links), and a specialized 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 tens of gigabytes. The CF runs no application software.
When originally introduced, the CFCC executed in a separate 9674 mainframe unit that was essentially a processor without I/O facilities other than the CF links. Later[lower-alpha 1] 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. Internal links are simulated, whereas links to another processor unit are over copper or optical fiber 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 190 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.