3.5 Performance

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After your initial log streams allocation, performance data is produced in a number of forms that can be useful to determine if any adjustments are necessary to your installation:

SMF 88 data produced by the MVS System Logger
SMF 70–78 data produced by various MVS components

There are several tools which aide in the analysis of log stream performance problems.

IXGRPT1 formats and reports the SMF 88 data produced by the MVS System Logger
ERBRMFPP reports on the data captured in the SMF 70 -78 records

Transactional VSAM log streams are both funnel and active log streams. For the active log streams, the Undo log set, Transactional VSAM will take care of maintaining currency of data within the log streams. For this reason, after an accurate tuning, we should see that offload are eliminated or minimized for these log streams.

On the other log streams, forward-recovery and log-of-logs, Transactional VSAM does not provide any mechanism for maintaining the currency of the data. For these reasons, on these log streams, a large amount of data is expected and so offloads are expected as part of a normal process.

There are some fields in the SMF88 that you might want to check against the active log streams to determine if they are tuned properly: DASD SHIFT, STRUCTURE FULL, ENTRY FULL and OFFLOAD.

As regard to the funnel type log streams, you should check the fields DASD SHIFT, STRUCTURE FULL, ENTRY FULL and SC1/2/3 in the SMF88 for this particular log stream.

Refer to the 8.6.3, "SMF Type 88 records and IXGRPT1 program" on page 281 for a description of these fields and what to do in case one of these conditions happens in your installation.

If you are running Transactional VSAM with CICS, there is no effect on CICS logging for DFHLOG and DFHSHUNT. There should be minimal impact for the log-of-logs and for the forward recovery logging since they are funnel-type log streams. Mainly, you should ensure that there is enough space on the DASD pool for the offload data sets, since the volume of data is probably going to increase.

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