Each installation planning for coupling facility log streams needs to plan how to assign log streams to coupling facility structures. This can be done in a variety of ways: multiple log streams sharing one structure, one structure for each log stream, related log streams using a structure, and so forth. The important point is that the approach be systematic, easy for your installation to track, and easy to recover data for, in the event of a system, sysplex, or coupling facility failure.

For coupling facility limitations, see the PR/SM™ Planning Guide. Structure limitations are documented with the IXCL1DSU utility for the CFRM and LOGR policies. See Format utility for couple data sets for the IXCL1DSU utility.

When you have more than one log stream using a single coupling facility structure, system logger divides the structure storage evenly among the log streams that have at least one connected system logger application. For example, if an installation assigns three log streams to a single structure, but only one log stream has a connected system logger application, then that one log stream can use the entire coupling facility structure. When an application connects to the second log stream, system logger dynamically divides the structure evenly between the two log streams. When an application connects to the third log stream, system logger allocates each log stream a third of the coupling facility space. For this reason, it is important that the log streams that share a coupling facility structure be equal in size and usage rates. Knowing this can help you understand and plan how much coupling facility space you have for each log stream and predict when log stream data will be written to DASD as the coupling facility space becomes filled.

Start planning your configuration by determining the number of coupling facility structures available for logging, then determine the number of log streams needed for all system logger applications. Group log streams with the same characteristics together in coupling facility structures. IBM® recommends that you limit the number of log streams per structure to 10 or fewer. This is because system logger connect and log stream recovery processing (which affect the restart time of system logger applications) have been optimized to provide parallelism at the coupling facility structure level. Therefore, the more structures you use, the greater parallelism you'll get during log stream connect and rebuild processing.

For example, if you have 10 log streams, each one defined to its own unique coupling facility structure, system logger can process connect requests to each log stream in parallel. On the other hand, if you define all 10 log streams to the same coupling facility structure, system logger must sequentially process log stream connect requests for the log streams.