Planning the PLEXCFG system parameter

The PLEXCFG system parameter restricts the type of sysplex configuration into which the system is allowed to IPL. For example, if you specify a multisystem sysplex (PLEXCFG=MULTISYSTEM), then you cannot specify GRS=NONE.

For System Logger setup for the PLEXCFG parameter, see Sysplex requirement.

On PLEXCFG, specify one or more of the following:

PLEXCFG=MULTISYSTEM

Indicates that the system is to be part of a sysplex consisting of one or more MVS™ systems that reside on one or more processors. The same sysplex couple data sets must be used by all systems.

You must specify a COUPLExx parmlib member that identifies the same sysplex couple data sets for all systems in the sysplex (on the COUPLE statement) and signaling paths, if applicable, between systems (on the PATHIN and PATHOUT statements). You must also specify, in the CLOCKxx parmlib member, the time synchronization using ETRMODE, STPMODE, or SIMETRID.

Use MULTISYSTEM when you plan to IPL two or more MVS systems into a multisystem sysplex and exploit full XCF coupling services. GRS=NONE is not valid with PLEXCFG=MULTISYSTEM.

PLEXCFG=XCFLOCAL

Indicates that the system is to be a single, standalone MVS system that is not a member of a sysplex and cannot use couple data sets. The COUPLExx parmlib member cannot specify a sysplex couple data set, and, therefore, other couple data sets cannot be used. Thus, functions, such as WLM, that require a couple data set are not available.

In XCF-local mode, XCF does not provide signaling services between MVS systems. However, multisystem applications can create groups and members, and messages can flow between group members on this system. If signaling paths are specified, they are tested for their operational ability, but they are not used.

Use XCF-local mode for a system that is independent of other systems. In XCF-local mode, XCF services (except permanent status recording) are available on the system and you can do maintenance, such as formatting a couple data set or changing the COUPLExx parmlib member. The default COUPLE00 parmlib member is intended to be used to IPL the system in XCF-local mode, as is the specification of COUPLE=**.

PLEXCFG=MONOPLEX

Indicates that the system is to be a single-system sysplex that must use a sysplex couple data set. Additional couple data sets, such as those that contain policy information, can also be used. XCF coupling services are available on the system, and multisystem applications can create groups and members. Messages can flow between members on this system (but not between this system and other MVS systems) via XCF signaling services. If signaling paths are specified, they are not used.

You must specify a COUPLExx parmlib member that gives the system access to a sysplex couple data set to be used only by this system. When a system IPLs into a single-system sysplex, no other system is allowed to join the sysplex.

Use MONOPLEX when you want only one system in the sysplex (for example, to test multisystem applications on one system) or when you want to use a function, such as WLM, that require a couple data set.

PLEXCFG=ANY

Indicates that the system can be part of any valid sysplex configuration. Specifying ANY is logically equivalent to specifying XCFLOCAL, MONOPLEX, or MULTISYSTEM. ANY is the default. If the system is ever to be part of a single-system sysplex or a multisystem sysplex, you must specify a COUPLExx parmlib member that gives the system access to a couple dataset. Note that the default COUPLE00 parmlib member does not include a PCOUPLE statement, and therefore is intended to be used only to bring up the system in XCF-local mode. Specifying COUPLE=** will also allow the system to be brought up in XCF-local mode. See the COUPLE parameter of IEASYSxx in z/OS MVS Initialization and Tuning Reference additional information about COUPLE=**.

When PLEXCFG=ANY is specified, MVS will initialize and establish a PLEXCFG “mode” of XCFLOCAL, MONOPLEX, or MULTISYSTEM based on parmlib settings in COUPLExx and CLOCKxx as well as the availability of sysplex resources. For example:

If you specify PLEXCFG=ANY and a sysplex couple data set is not specified in COUPLExx, or if COUPLE=** is specified, the MVS system will be initialized in XCF-local mode.
If you specify PLEXCFG=ANY and a sysplex couple data set is specified in COUPLExx but the data set is not available, the MVS system can be initialized in XCF-local mode either by switching to a COUPLExx member that does not specify sysplex couple data sets or by specifying COUPLE=**.
If you specify PLEXCFG=ANY and a sysplex couple data set is specified and available via COUPLExx, and a sysplex timer is specified (ETRMODE YES in CLOCKxx) but ETR signals are not present, the MVS system can be initialized in MONOPLEX mode. The system is also considered to be in ETR-LOCAL mode. If ETR signals are restored and the CPC TOD and the ETR TOD are resynchronized (thus establishing ETR mode), the MVS system will be in a sysplex that is MULTISYSTEM-capable.
If you specify PLEXCFG=ANY and a sysplex couple data set is specified and available via COUPLExx and a sysplex time reference is specified (CLOCKxx specifies either SIMETRID or ETRMODE YES) and available, the MVS system will be in a system that is MULTISYSTEM-capable.

PLEXCFG=ANY is the least restrictive of the PLEXCFG parameters and can provide flexibility during system initialization that can be useful when a sysplex couple data set or Sysplex Timer are not available. The first goal, however, should be to provide an adequate level of resource redundancy to help prevent availability problems, and then specify the desired configuration for the PLEXCFG parameter.

When using PLEXCFG=ANY, ensure that systems requiring a multisystem configuration do not run concurrently with each other in XCF-local mode. For example, if system “A” and system “B” normally run in a multisystem sysplex using global resource serialization to serialize access to a shared common data base, you would not want to bring up “A” and “B” in XCF-local mode using two different global resource serialization complexes. To do so would introduce data integrity exposures to the shared common data base.

Programming note: To prevent the operator from overriding the PLEXCFG parameter, use OPI=NO on the PLEXCFG keyword in the IEASYSxx parmlib member. (For example, PLEXCFG=MULTISYSTEM,OPI=NO). Note, however, that IBM® recommends that OPI=YES be used in case PLEXCFG=XCFLOCAL needs to be specified to IPL the system in XCF-local mode to format the sysplex couple data set.