Running Multiple IMS Systems

You can run multiple IMS systems on a single z/OS image or on multiple z/OS images. One instance of an IMS system (a control region and all its associated dependent regions) is referred to as one IMS system. In many cases, these IMS systems would be production and testing systems. A batch IMS system (for example, DB batch) is also considered one IMS system.

Running Multiple IMS Systems on a Single z/OS Image

The number of IMS subsystems you can run on a single image of z/OS depends on many factors, including the size of each IMS system (the amount of z/OS common service area [CSA] required by each IMS is often one of the most limiting factors in the equation). In most installations, you can run up to four IMS subsystems, although some installations run as many as eight small subsystems concurrently.

Each IMS subsystem should have unique VTAM access method control block (ACB) and IMSID (IMS subsystem identifier to the operating system) names. The dependent regions use the IMSID to connect to the corresponding IMS control region. If the dependent region starts and there is no control region running using that IMSID, the dependent region issues a message to the z/OS system console and then waits for a reply. Each IMS subsystem can have up to 999 dependent regions. However, there are other limiting factors, such as storage limitations due to pool usage.

Running Multiple IMS Systems on Multiple z/OS Images

There are basically three ways to run multiple IMS subsystems on multiple z/OS images:

• Multiple Systems Coupling (MSC): MSC supports only IMS-to-IMS connections. For more information about MSC, see "Multiple Systems Coupling (MSC)" on page 183.

• Inter System Communications (ISC): ISC is another way to connect multiple IMS subsystems. ISC is more flexible than MSC, in that ISC supports connections to IMS and other z/OS products, such as CICS. For more information about ISC, see "Intersystem Communication (ISC)" on page 184.

• Parallel Sysplex: Running multiple IMS subsystems in a Parallel Sysplex environment is a good way to balance workload, build scalability into your systems, and provide maximum availability. For more information on this topic, see "Parallel Sysplex" on page 48, and Chapter 28, "IMSplexes" on page 495.