In
IBM System/390
The IBM System/390 is a discontinued mainframe product family implementing the ESA/390, the fifth generation of the System/360 instruction set architecture. The first computers to use the ESA/390 were the Enterprise System/9000 (ES/9000 ...
and
IBM Z
IBM Z is a family name used by IBM for all of its z/Architecture mainframe computers.
In July 2017, with another generation of products, the official family was changed to IBM Z from IBM z Systems; the IBM Z family now includes the newest mod ...
mainframe computer
A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as censuses, industry and consumer statistics, enterpris ...
s, a Coupling Facility or CF is a piece of
computer hardware
Computer hardware includes the physical parts of a computer, such as the computer case, case, central processing unit (CPU), Random-access memory, random access memory (RAM), Computer monitor, monitor, Computer mouse, mouse, Computer keyboard, ...
or
virtual machine
In computing, a virtual machine (VM) is the virtualization/emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized hardw ...
that coordinates multiple processors.
A
Parallel Sysplex
In computing, a Parallel Sysplex is a cluster of IBM mainframes acting together as a single system image with z/OS. Used for disaster recovery, Parallel Sysplex combines data sharing and parallel computing to allow a cluster of up to 32 systems ...
[
] 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 {{Refimprove, date=October 2020
Hardware Management Console (HMC) is a Physical / Virtual appliance used to manage IBM Systems including IBM AS/400, IBM System p, IBM System z, and IBM Power Systems. HMC supports command line (ssh) as well as web ...
(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
In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very l ...
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 IBM enabled the use of an Internal Coupling Facility where the CFCC runs in a logical partition (
LPAR
A logical partition (LPAR) is a subset of a computer's hardware resources, virtualized as a separate computer. In effect, a physical machine can be partitioned into multiple logical partitions, each hosting a separate instance of an operating ...
) 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
z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest:
* O ...
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
Direct memory access (DMA) is a feature of computer systems and allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU).
Without DMA, when the CPU is using programmed input/output, it is t ...
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.
Coupling Facility structures
A CF is used for three purposes:
*Locking information that is shared among all attached systems
*Cache information (such as for a data base) that is shared among all attached systems (or maintaining coherency between local buffer pools in each system).
*Data list information that is shared among all attached systems
These three purposes are catered for by three types of structure:
*Lock
*Cache
*List (and the variant Serialised List)
A structure is a dedicated portion of CF memory. It is said to be connected to by specific CF-exploiting applications on the coupled
z/OS
z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest:
* O ...
systems. A typical Parallel Sysplex contains several structures of each type. Each software exploiter may use several structures of each type. For example, each
Db2 Data Sharing Group uses one Lock structure, one List structure and several cache structures (one for each Group Buffer Pool (GBP)).
Structure duplexing
Structures may be duplexed across different CFs, allowing two copies of the same structure to be kept synchronized. Duplexing is often used as part of an installation's drive to remove single points of failure, with the aim of reducing the incidence and duration of application outages. In the event of the failure of one CF, the other copy of the structure is used to satisfy all requests.
Coupling Facility requests
A request to a CF structure is of one of two kinds:
*Synchronous (sync) requests. When a
z/OS
z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest:
* O ...
system issues a request it waits for the request to complete, actively spinning on one of its own processors. Sync requests are quick but the response time is the same as the coupled system's spinning CPU loss. So Sync requests are relatively expensive in CPU terms – from the coupled system's perspective.
*Asynchronous (async) requests. When a
z/OS
z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest:
* O ...
system issues a request it doesn't wait for the request to complete. Async requests are slower than sync requests (as they have a lower priority in the CF) but don't lead to the coupled system's processor spinning.
Exploiting z/OS applications explicitly issue CF requests as synch or asynch.
Dynamic Request Conversion
In
z/OS
z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest:
* O ...
Release 2, the Dynamic Request Conversion heuristic algorithm was introduced. This uses sampled response times to decide whether to convert Sync requests to Async or not. These decisions are based on such criteria as coupled processor speed. The greater the distance between the coupled
z/OS
z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest:
* O ...
system and the CF the greater the likelihood requests will be converted to Async from Sync.
Async requests are never converted to Sync.
This heuristic algorithm complements a previously-existing algorithm that automatically (but not heuristically) converted requests, based on conditions such as path busy and on request data size. The difference is the new algorithm samples response times dynamically.
CFs are unique to S/390, zSeries and System z mainframes. They are key to Parallel Sysplex technology.
Coupling Facility Levels and Exploiting Software Levels
The CFCC code is released as Levels, usually denoted by their CFLEVEL. For example, CFLEVEL 15 was announced in April 2007. Each level brings new function and sometimes improved performance. In most cases the new function or performance improvement requires a corequisite release of
z/OS
z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest:
* O ...
and perhaps new function in some subsystem (such as
Db2). One such example is Coupling Facility Structure Duplexing. (Sometimes support from the operating system and subsystems is available via
PTFs rather than a full release.)
Notes
References
{{Reflist
IBM mainframe technology