Sizing for DASD-only 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.

Sizing DFHLOG

To size the CICS primary system log stream, DFHLOG:
  • Minimize the amount of data that is offloaded to secondary storage
    The MVS™ system logger begins the offload process when the high offload threshold (HIGHOFFLOAD) of the log stream is reached. The offload process consists of two steps:
    1. The MVS logger physically deletes the data in the log stream that has been marked for deletion by the CICS log-tail deletion process.
    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 has been deleted since the last offload event.
    To minimize the amount of data offloaded from the CICS primary system log, you must:
    • Define a suitably sized staging data set. You can alter the size of a staging data set without deleting the log stream. To do so, use the UPDATE LOGSTREAM request of the MVS IXCMIAPU utility to change the value of the STG_SIZE parameter.
    • Ensure that the log-tail deletion process is working effectively. For detailed information about the log-tail deletion process, see Log tail deletion.
  • Avoid staging-data-set-full events

    A staging-data-set-full event occurs when a log stream's staging data set becomes full before the offloading of data has completed.

    For advice on monitoring and avoiding staging-data-set-full events, see Monitoring the logger environment.

Sizing DFHSHUNT

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

Sizing general logs

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

Setting attribute values for log stream and system definitions

Table 1 summarizes how you decide on the values for various attributes on the log stream definition, and system definition.
Table 1. How to decide on the values of attributes
Facility Attribute Method or Value
Primary system log stream (DFHLOG) HIGHOFFLOAD 80
LOWOFFLOAD Use the formula in Sizing your DASD log streams to calculate a suitable value.
MAXBUFSIZE 64000
STG_SIZE Use the formula in Sizing your DASD log streams to calculate a suitable value.
Secondary system log stream (DFHSHUNT) HIGHOFFLOAD 80
LOWOFFLOAD 40 – 60
MAXBUFSIZE 64 000
STG_SIZE 500 (4 KB blocks)
General log stream HIGHOFFLOAD 80
LOWOFFLOAD 0
MAXBUFSIZE 64 000
STG_SIZE Use the formula in Sizing your DASD log streams to calculate a suitable value
CICS system AKPFREQ 4 000
Startup might take longer than you experienced when using earlier releases. This is caused in part by the allocation and formatting of the staging data sets. Increased startup time depends on factors like these:
  • Size of staging data set (STG_SIZE)
  • DASD speed
  • DASD contention
You can reduce startup time by avoiding large staging data sets.