Characteristics of a sysplex

A sysplex can include the following software and hardware:

z/OS®
These products include the cross-system coupling facility (XCF) component, which enables authorized programs in a sysplex to communicate with programs on the same MVS™ system or other MVS systems; and the global resource serialization component, which serializes sysplex resources.

z/OS provides support for the coupling facility, which provides high speed access to shared data across applications and subsystems running on different MVS systems.
Signaling paths between MVS systems
There must be at least two operational signaling paths (one inbound and one outbound path) between each of the MVS systems in the sysplex. The signaling paths can be defined through:
- Coupling facility list structures
- ESCON® or FICON® channels operating in CTC mode
- 3088 Multisystem Channel Communication Unit
Sysplex couple data set
MVS requires a DASD data set (and an alternate data set is recommended for availability) to be shared by all systems in the sysplex. On the sysplex couple data set, MVS stores information related to the sysplex, systems, XCF groups, and their members. An XCF group is the set of related members that a multisystem application defines to XCF. A multisystem application can be an installation-defined program, an MVS component or subsystem, or a program product.

A sysplex couple data set is always required for a multisystem sysplex and for most single-system sysplexes. However, you can define a single system sysplex that does not require a sysplex couple data set.
Common time reference
When the sysplex consists of multiple MVS systems running on two or more processors, MVS requires that the processors be connected to connected to the same time source to synchronize TOD clocks across CPCs. This can be done by using a common 9037 Sysplex Timer or, starting with the z890/z990 servers, through STP. Starting with the z114/z196 servers, connections to the 9037 Sysplex Timer are no longer supported. MVS uses the Sysplex Timer to synchronize TOD clocks across systems.

For a multisystem sysplex defined on a single processor (under PR/SM™ or VM) the SIMETRID parameter in the CLOCKxx parmlib member must specify the simulated Sysplex Timer identifier to synchronize timings for the MVS systems. When a sysplex with LPARs resides on two or more physical CECs, it requires ETRMODE YES or STPMODE YES to be defined in the CLOCKxx parmlib member.

To manage certain aspects of your sysplex, you can install one or more additional couple data sets, for:

Coupling facility resource management (CFRM), to define how the system is to manage coupling facility resources.
Sysplex failure management (SFM), to define how the system is to manage system and signaling connectivity failures and PR/SM reconfiguration actions.
Workload management (WLM), to define service goals for workloads.
Automatic restart management (ARM), to define how to process restarts for started tasks and batch jobs that have registered with automatic restart management.
System logger (LOGR), to define, update, or delete structure or log stream definitions.
z/OS UNIX System Services (z/OS UNIX), to contain file information when using a shared file system in a sysplex.

Figure 1 shows a sysplex of two z/OS systems, SYSA and SYSB, where both systems are connected to a Sysplex Timer and a coupling facility and can access the sysplex couple data set and the CFRM couple data set. The sysplex might also be configured so the systems can access one or more couple data sets, such as ARM (automatic restart management), LOGR, SFM, WLM, or z/OS UNIX.

(For diagrams of additional configurations, see Adding MVS systems to a sysplex and Sysplex configurations.)

Figure 1. Sysplex with Coupling Facility and Couple Data Sets