A sysplex is a collection of z/OS® systems that cooperate, using certain hardware and software products, to process work. It is a clustering technology that can provide near-continuous availability.

A conventional large computer system also uses hardware and software products that cooperate to process work. A major difference between a sysplex and a conventional large computer system is the improved growth potential and level of availability in a sysplex. The sysplex increases the number of processing units and z/OS operating systems that can cooperate, which in turn increases the amount of work that can be processed. To facilitate this cooperation, new products were developed and old products were enhanced.

A Parallel Sysplex® is a sysplex that uses multisystem data-sharing technology. It allows direct, concurrent read/write access to shared data from all processing nodes (or servers) in the configuration without impacting performance or data integrity. As many as 32 servers can concurrently cache shared data in local processor memory through hardware-assisted cluster-wide serialization and coherency controls. As a result, work requests that are associated with a single workload, such as business transactions or database queries, can be dynamically distributed for parallel execution on nodes in the sysplex cluster based on available processor capacity.

Figure 1 shows the visible parts of a Parallel Sysplex, namely the hardware. These parts are the key components of Parallel Sysplex as implemented in the hardware architecture.

Figure 1. Parallel Sysplex hardware overview

A Parallel Sysplex relies on one or more coupling facilities (CFs). A coupling facility is a mainframe processor, with memory and special channels, and a built-in operating system. It has no I/O devices, other than the special channels, and the operating system is very small.

A CF functions largely as a fast scratch pad. It is used for three purposes:

• Locking information that is shared among all attached systems
• Cache information (such as for a database) that is shared among all attached systems
• Data list information that is shared among all attached systems

The information in the CF resides in memory and a CF typically has a large memory. A CF can be a separate system or an logical partition (LPAR). Figure 2 illustrates a small Parallel Sysplex with two z/OS images. Again, this whole configuration could be in three LPARs of a single system, in three separate systems, or in a mixed combination.

Figure 2. Parallel Sysplex with two z/OS images

In many ways a Parallel Sysplex system appears as a single large system. It has a single operator interface (which controls all systems). With proper planning and operation (neither of which is trivial), complex workloads can be shared by any or all systems in the Parallel Sysplex, and recovery (by another system in the Parallel Sysplex) can be automatic for many workloads.

Another unique advantage of using Parallel Sysplex technology is the ability to perform hardware and software maintenance and installation in a nondisruptive manner. Through data sharing and dynamic workload management, servers can be dynamically removed from or added to the cluster, allowing installation and maintenance activities to be performed while the remaining systems continue to process work. Furthermore, by adhering to the IBM® software and hardware coexistence policy, software or hardware upgrades— or both— can be introduced one system at a time. This capability allows customers to roll changes through systems at a pace that makes sense for their business.

The ability to perform rolling hardware and software maintenance in a nondisruptive manner allows business to implement critical business function and react to rapid growth without affecting customer availability.