Sizing coupling facility log streams

You must work out the sizes of the CICS® primary and secondary system log streams, forward recovery logs, and user journals and autojournals.

• Primary system log stream.
  Use the following guidelines when you size CICS primary system log stream, DFHLOG:

  • Minimize the amount of data that is offloaded to auxiliary storage.
    The MVS™ system logger begins the offload process when the high offload threshold (HIGHOFFLOAD) of the log stream is reached. The offload process has two steps:

    1. The MVS logger physically deletes the data in the log stream that the CICS log tail deletion process has marked for deletion.
    2. The MVS logger calculates how much data must be offloaded to auxiliary storage, based on the difference between HIGHOFFLOAD and LOWOFFLOAD, less the amount of data that was deleted since the last offload event.
    To minimize the amount of data offloaded from the CICS primary system log:

    • Define a suitably sized coupling facility structure.
    • Ensure that the log tail deletion process is working effectively. For detailed information about the log tail deletion process, see Log tail deletion.
  • Avoid structure full events.

    A structure full event occurs when the structure space of a log stream becomes full before the offloading of data has completed. For information about monitoring and avoiding structure full events, see CICS logging and journaling.

• Secondary log stream.

  You must size the secondary system log stream, DFHSHUNT, to avoid structure full events. However, typically some data is offloaded from DFHSHUNT to auxiliary storage.

• General logs.

  You must size forward recovery logs, user journals, and autojournals to avoid structure full events. However, because CICS does not delete data from these log streams, data is typically offloaded to auxiliary storage.