Mainframe has three major clustering techniques: Basic Shared DASD, CTC rings, and Parallel Sysplex. Parallel Sysplex – is analogous in concept to a UNIX cluster – allow the customer to operate multiple copies of the operating system as a single system. This allows systems to be added or removed as needed, while applications continue to run.
Basic Shared DASD
As the name suggests, this configuration allows multiple z/OS images to share the direct access storage devices (DASD). This is a preliminary form of sharing and has limited capabilities.
The channel-to-channel (CTC) function simulates an input/output (I/O) device that can be used by one system control program (SCP) to communicate with another SCP. It provides the data path and synchronization for data transfer. When the CTC option is used to connect two channels that are associated with different systems, a loosely coupled multiprocessing system is established.
To a large extent, batch jobs and interactive users can run on any system in this configuration because all disk data sets can be accessed from any z/OS image. Jobs (and interactive users) can be assigned to whichever system is most lightly loaded at the time.
A Systems Complex, commonly called a sysplex, is one or more (up to 32) systems joined into a cooperative single unit using specialized hardware and software. A sysplex generally provides for resource sharing between communicating systems (tape, consoles, catalogues, and so forth).
Sysplexes are often isolated to a single system, but Parallel Sysplex technology allows multiple mainframes to act as one. It is a clustering technology that can provide near-continuous availability.
A Parallel Sysplex is a symmetric sysplex using multisystem data-sharing technology. This is the mainframe’s clustering technology. It allows direct, concurrent read/write access to shared data from all processing servers in the configuration without impacting performance or data integrity.
A Parallel Sysplex allows the linking up to 32 servers with near linear scalability to create a powerful commercial processing clustered system. Every server in a Parallel Sysplex cluster can be configured to share access to data resources, and a “cloned” instance of an application might run on every server.
Another unique advantage of using Parallel Sysplex technology is the ability to perform hardware and software maintenance and installation in a non-disruptive manner.