2. PERFORMANCE REPORTING ANALYSIS › 2.8 Sysplex Analysis › 2.8.4 Parallel Sysplex Concepts › 2.8.4.1 Sysplex Functional Overview

2.8.4.1 Sysplex Functional Overview

IBM's System/390 Parallel Sysplex offering represents a
significantly different approach to data processing, and an
exciting step in the continuing evolution of computing.
Hardware and software combine to provide support for full
data sharing among the processors in the sysplex and dynamic
workload balancing across the systems.

With the introduction of Sysplex, there also comes a whole
array of terminology necessary for quantifying the
capabilities and components of the technology.  Below is a
brief definition of some of the major terms:

 o SYSPLEX (SYStem comPLEX):  As the name would indicate,
   this represents a strategy for linking together (coupling)
   multiple MVS images with a standard communication
   interface.  The idea is to ultimately provide a single
   point of control for all processors in the organization.
   At a somewhat lower level, a sysplex can be defined more
   concretely as a set of MVS systems that have been
   initialized with the same sysplex name and are fully
   connected by XCF (see below) signaling paths.

 o Coupling Facility:  Hardware connecting two or more
   System/390 systems, which allows multiple MVS/ESA system
   images to do workload balancing.  The Coupling Facility
   provides support for serialization and buffer management,
   caching for shared data, as well as message queuing to
   ensure the integrity and consistency of data throughout
   the Parallel Sysplex environment.  Communication between
   coupled systems is via very-high-speed fiber optic links
   that can extend up to three kilometers (almost 2 miles).

 o SYSPLEX Timer:  An external hardware timer shared by all
   of the systems in a sysplex, which effectively moves the
   time of day clock control from the individual processors
   to a common external clock capability.  This solves a long
   standing issue in the multisystem environment of
   dealing with cycle synchronization, creating a common time
   stamp across the multiple systems composing the sysplex.

 o XCF (Cross System Coupling Facility):  Not to be confused
   with the Coupling Facility, which is hardware as described
   above, XCF represents multisystem services provided as
   part of the operating system to communicate to authorized
   programs on other systems.  The task of managing multiple
   systems connected in a sysplex is similar to the challenge
   of multitasking in a single system.  The major additional
   requirement is a vehicle for communications between the
   systems, and XCF is that vehicle.

 o Sysplex Couple data set:  MVS requires a DASD data set to
   be shared by all systems in the sysplex.  In this data
   set, MVS stores information related to the sysplex
   systems, XCF groups, and their members.

 o Multisystem Application:  An application program that has
   distributed functions across MVS systems.  GRS (Global
   Resource Serialization) is an example of such an
   application program.  A multisystem application can be an
   installation-defined program, an MVS component or
   subsystem, or a program product.

 o XCF MEMBER:  A specific function, or instance, of a
   multisystem application.  An XCF Member resides on one
   system and can communicate with other members of the same
   group (see below) across the sysplex.

 o XCF GROUP:  A unique name for a set of related XCF Members
   that a multisystem application defines to XCF.
   Communication between group members on different systems
   occurs over the signaling paths that connect the systems;
   on the same system, communication between group members
   occurs through local signaling services.  As an example,
   SYSGRS is the XCF group name for the the global resource
   serialization application.

 o Transport Class:  A means of associating one or more XCF
   groups (based on similar signaling requirements) and then
   assigning them signaling resources (signaling paths and
   message buffers).  A transport class allows you to
   segregate message traffic according to the needs of an
   applications's group, the lengths of its messages, or
   both.

The goal of the Parallel Sysplex is to provide for horizontal
growth of a computing environment while beginning the task
of simplifying the operation of that environment.  Prior to
sysplex, each system application had to handle issues of
communication with other systems in a multisystem
environment.  The introduction of XCF removes from system
applications the redundant communication functions, allowing
those applications to utilize XCF in a high level manner.

So, instead of depending on a communications vehicle like
VTAM for cross-system services, XCF has a direct, fully
configured, connection to all systems in the complex.  The
major users of XCF services are the IBM system functions that
need to communicate with other systems.  Examples include GRS
(Global Resource Serialization) and console support.
Additionally, in support of workload balancing, major
transaction subsystems such as CICS and IMS utilize XCF
services to shift transaction processing dynamically to the
appropriate region, regardless of the system from which the
transaction originated.