Considerations

Both the XCF and CFS groups must be considered when planning sysplex coupled data sets. Specifically for the XCF group of VTAM®, the MAXMSG parameter for PATHIN and PATHOUT should be adequately tuned to minimize the number of no buffer conditions. Inadequate tuning of MAXMSG can lead to unexpected XCF outages (INOPs). See z/OS MVS Setting Up a Sysplex, for additional information.

The following information applies to all VTAM and TCP/IP functions that require a coupling facility structure in an MVS™ sysplex. If you plan to use the generic resource function, the multinode persistent session function, the sysplexports function, or the Sysplex Wide Security Associations function, a coupling facility structure must exist for each function.

An MVS coupling facility provides the hardware and software that supports high-speed shared storage across multiple MVS systems and provides multiple copies or images of the MVS operating system to process work concurrently.
There must be an active coupling facility resource management (CFRM) policy for the sysplex environment.
A CFRM policy defines how an installation will manage its coupling facility resources, which can be shared among multiple subsystems. For more information about how to create, activate, and make changes to the CFRM policy, see z/OS MVS Setting Up a Sysplex.

The CFRM policy information describes the coupling facilities that are used in the sysplex and specifies the requirements for structure size and allocation within each coupling facility.
Each coupling facility structure to be used must be defined in the active CFRM policy for the sysplex. If subplexing is used, the structure names specified in the CFRM policy must include the subplex suffixes. You might need to define (using the CFRM policy) more than one structure of each type as being in the sysplex. See Sample CFRM coding for an example of CFRM coding.
When defining the coupling facility structure in a CFRM policy, specify the amount of coupling facility storage which should be allocated on behalf of this structure. For information about determining the amount of storage necessary, see Determining the size of the coupling facility structure.
All VTAM and TCP/IP functions using a coupling facility are designed to maintain data integrity when any single type of failure occurs, but do not reliably handle concurrent failures of different types. For instance, if a VTAM node and the coupling facility structure fail concurrently, data is lost. Thus, it is advisable to isolate the coupling facilities from the VTAM nodes (for example, they should not share the same power supply).
In the same way, multiple coupling facilities in the same configuration should be isolated from each other. If more than one coupling facility is present in a configuration, a new version of the structure can be created if the coupling facility containing the active version of the structure fails. Also, if a link failure to the structure occurs, the structure might be moved to a new coupling facility where it could be accessed by all VTAM nodes in the configuration.