Monitoring the logger environment

CICS® collects statistics on the data written to each journal and log stream; this data can be used to analyze the activity of a single region. However, because general log streams can be shared across multiple MVS images, it can be more useful to examine the statistics generated by MVS.

About this task

The MVS system logger writes SMF Type 88 records containing statistics for each connected log stream. MVS supplies in SYS1.SAMPLIB a sample reporting program, IXGRPT1, that you can use as supplied, or modify to meet your requirements. Alternatively, you can use some other SMF reporting program. For information about the SMF Type 88 records and the sample reporting program, see z/OS MVS System Management Facilities (SMF).

The main events to monitor routinely are as follows:
  • For coupling facility log streams, the number of structure full events
  • For DASD-only log streams, the number of staging data set full events.

If these events occur frequently, this indicates that the logger cannot write data to auxiliary storage quickly enough to keep up with incoming data, which causes CICS to wait before it can write more data.

Procedure

  1. Consider the following solutions to resolve problems that occur as a result of event-full conditions:
    1. Increase the size of primary storage (that is, the size of the coupling facility structure or, for a DASD-only log stream, the size of the staging data set), in order to smooth out spikes in logger load.
    2. Reduce the data written to the log stream by not merging so many journals or forward recovery logs on to the same stream.
    3. Reduce the HIGHOFFLOAD threshold percentage, the point at which the system logger begins offloading data from primary storage to offload data sets.
    4. Review the size of the offload data sets. Offload data sets must be large enough to avoid too many “DASD shifts”—that is, new data set allocations. Aim for no more than one DASD shift per hour. You can monitor the number of DASD shifts using the SMF88EDS record.
    5. Examine device I/O statistics for possible contention on the I/O subsystem used for offload data sets.
    6. Use faster DASD devices.
    The best CICS system logs performance is achieved when CICS can delete log tail data that is no longer needed before it is written to auxiliary storage by the MVS system logger. To monitor that this is being achieved, your reporting program can examine the values in the SMF88SIB and SMF88SAB SMF Type 88 records, which provide helpful information relating to log data.
    SMF88SIB
    Data deleted from primary storage without first being written to DASD offload data sets. For a system log stream, this value is normally high in relation to the value of SMF88SAB. For a general log stream, this value is normally zero.
    SMF88SAB
    Data deleted from primary storage after being written to DASD offload data sets. For a system log stream, this value is normally low in relation to the value of SMF88SIB. For a general log stream, this value is normally high.
    Note: In any SMF interval, the total number of bytes deleted from primary storage (SMF88SIB plus SMF88SAB) might not match the total number of bytes written to auxiliary storage. Data is only written to offload data sets and then deleted from primary storage when the HIGHOFFLOAD threshold limit is reached.
  2. If the SMF88SAB record frequently contains high values for a CICS system log:
    1. Check that RETPD=dddd is not specified on the MVS definition of the log stream. For information about the MVS RETPD parameter, see Managing auxiliary storage.
    2. Check that no long-running transactions are making recoverable updates without syncpointing.
    3. Consider increasing the size of primary storage.
    4. Consider increasing the HIGHOFFLOAD threshold value.
    5. Consider reducing the value of the AKPFREQ system initialization parameter.