Coupling Facility

The S/390 Coupling Facility (CF), the key component of the Parallel Sysplex cluster, enables multisystem coordination and datasharing among OS/390 images in the cluster. It makes highperformance, multisystem data sharing possible, and provides high-speed locking, caching, and message list services between coupling-capable S/390 processors. It is a combination of hardware and licensed internal code. It is connected by Coupling Links, high-bandwidth fiber-optic cables, to OS/390 systems in a Parallel Sysplex. Coupling Links connect point-to-point between the Coupling Facility and all participating S/390 processors. This removes the need for Coupling Facility channels. CFs run Coupling Facility Control Code (CFCC), a special purpose Licensed Internal Code (LIC), to perform coupling operations. CFCC is the “operating system” for the CFs and provides the function that enables the Parallel Sysplex.

"Sizing" a CF involves estimating the number of processors and the amount of storage needed on the CF. It also includes estimating the number of CF links that will be needed. "Tuning" the CF is performed to ensure that sufficient quantities of the various resources needed by exploiters of the coupling technology have been allocated. Observation of the storage allocated and utilized by structures, the response times and request rates, and the processor utilization, is required both to determine bottlenecks and to balance storage, and the number of processors, and the number of CF links to optimally use the coupling technology.

Basic Sysplex support is provided by the cross-system Coupling Facility (XCF) of OS/390. This allows authorized applications (such as IBM transaction, communications and database managers) to efficiently share information with subsystems on other servers in the complex. It ensures that resource serialization and cached buffer integrity are maintained. Serialization is required to control access and updates to data, and to ensure that only one application can change data at a time. Buffer integrity is necessary to ensure that changes are reflected in all memory resident copies of the data. Coupling Facilities provide a common memory for the Parallel Sysplex that is dynamically partitioned into lock, cache and list structures. These structures are typically used to hold status information required for inter-system coherency and provide a serialization mechanism for multiple systems. In addition, the cache structures can be used as buffers for storing shared data with multi-system read/write access.

1. Coupling Facility Configuration Alternatives The major IBM databases (DB2, IMS, VSAM) and transaction managers (CICS, IMS) have extensions that allow them to benefit from datasharing and parallel processing. Parallel Sysplex clusters should be configured with redundant CFs to help ensure that a CF does not become a single point of failure. There are different options available for configuring a functioning Coupling Facility:

 Stand-alone Coupling Facility (e.g. IBM models 9674 or R06) provide a physically isolated and independent CF environment with facilities dedicated to coupling function. For datasharing environments it is recommended that Parallel Sysplex clusters are configured with at least one stand-alone CF for availability reasons. The Stand-alone Coupling Facility is really a regular S/390 SMP. It uses "Coupling Facility Control Code" as ist operating system.  Internal Coupling Facility (ICF) is a dedicated CPU within an S/390 SMP server which can only run Parallel Sysplex coupling code (e.g. CFCC) in a Processor Resource/Systems Manager (PR/SM) CF partition. There may be multiple ICF partitions. ICF partitions are appropriate for test as well as most production environments.  For test purposes a CF may be simulated on a S/390 server

2. Parallel Sysplex Coupling Connectivity The Coupling Facilities and OS/390 images in the Parallel Sysplex communicate over specialized, high-speed connections (links). For availability purposes, it is recommended that there be at least two links connecting each OS/390 image to each CF in a Parallel Sysplex cluster. As processor performance increases, it is important to add links so that link performance does not become a bottleneck. The IBM S/390 coupling connectivity options are:  HiPerLinks are fiber-optic links offering transfer speeds of up to 100MB/sec over distances up to 40 kilometers.

 The Integrated Cluster Bus (ICB), has better performance and reliability characteristics than the HiPerLinks. The ICB uses the S/390 server’s 333 MByte/sec Self Timed Interconnect (STI) bus to perform S/390 coupling communication between S/390 servers and the CF. ICBs provide better coupling efficiency (less CPU overhead associated with coupling systems) when compared to HiPerLinks. The ICB connection is made via a 10 meter cable. Therefore, the distance between the S/390 servers being connected is restricted to seven meters. The ICB is very fast and should be the coupling connectivity of choice when connecting Coupling Facilities and S/390 servers over short distances.

 The Internal Coupling (IC) Channel is a connectivity option that enables efficient communication between a Coupling Facility PR/SM partition and one or more OS/390 logical partitions running on the same S/390 server. The IC channel is a linkless connection (using main memory as a transfer medium and implemented in Licensed Internal Code) and thus does not require any hardware or cables. It provides the fastest Parallel Sysplex connectivity, significantly faster than any external link alternatives, resulting in better coupling efficiency than with external links.

3. Geographically Dispersed Parallel Sysplex The Geographically Dispersed Parallel Sysplex (GDPS) consists of a base or Parallel Sysplex cluster spread across two sites separated by up to 40 kilometers. GDPS provides fast failure recovery, using a single, automated point of control.