Tuning for DASD-only logging

The primary storage used by a DASD-only log stream consists of:

No data is written to coupling facility structures. In its use of staging data sets, a DASD-only log stream is similar to a coupling facility log stream defined with DUPLEX(YES) COND(NO).

When the staging data set reaches its HIGHOFFLOAD limit, data is either deleted or offloaded until the LOWOFFLOAD limit is reached.

The following principles apply to DASD-only log streams as much as to coupling facility log streams:

Start of changeThe basic principles of sizing the staging data set for a DASD-only log stream are the same as those for sizing a staging data set for a coupling facility log stream, as described in Tuning the size of staging data sets. You should take the values that you obtain as a starting point, and monitor your logger environment to adjust the size of the staging data set.End of change

If you are migrating from CICS/ESA 4.1 or CICS/ESA 3.3, you are strongly recommended to use the DFHLSCU program to size your staging data sets. For information about DFHLSCU, see the CICS® Operations and Utilities Guide.

If for any reason you cannot use DFHLSCU, you can use the following formula to calculate a starting point for the size of the staging data set for the system log. The formula calculates the value to be specified on the STG_SIZE parameter of the logstream definition; that is, the size is expressed as a number of 4K blocks.

Staging
DS size [No. of 4K blocks] = (AKP duration) * No. of log writes per second
                                              for system log
where:
AKP duration = (CICS TS 390 AKPFREQ) / (No. of buffer puts per second)
  

Start of changeThe values for the number of log writes per second and buffer puts per second can be taken from your CICS statistics. In CICS Transaction Server releases, the Logstream statistics fields collect these statistics as "write requests" (LGSWRITES) and "buffer appends" (LGSBUFAPP), and you can divide the totals by the number of seconds in your statistics interval.End of change

Start of changeIf you want to make a more accurate estimate for the size of the staging data set, consult the following documents:

You are also recommended to consult the IBM Redpaper which is cited in Logging and journaling: performance considerations.End of change

Related tasks
Logging and journaling: performance considerations
Monitoring the logger environment
Performance implications of average blocksize
Performance implications of the number of log streams in the coupling facility structure
Setting LOWOFFLOAD and HIGHOFFLOAD parameters on log stream definition
Tuning the size of staging data sets
Setting the activity keypoint frequency (AKPFREQ)
Specifying the log defer interval (LGDFINT)
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