DASD-only logging
The primary storage used by a DASD-only log stream consists of a data space owned by the MVS™ logger and staging data sets. You can tune for DASD-only logging to improve performance.
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.
- Size system logs so that system log data produced during the current activity keypoint interval, plus data not deleted at the previous activity keypoint, is retained in primary storage
- For the system log, avoid “staging data set full” conditions and offloading to auxiliary storage.
The basic principles of sizing the staging data set for a DASD-only log stream are the same as for sizing a staging data set for a coupling facility log stream, as described in Tuning the size of staging data sets. Take the values that you obtain as a starting point, and monitor your logger environment to adjust the size of the staging data set.
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 log stream definition; that is, the size is expressed as a number of 4 KB 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)
The 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 log stream 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.
- The IBM® Redpaper Performance Considerations and Measurements for CICS and System Logger, REDP-3768. This document supplies guidance on the interactions between CICS and z/OS® System Logger, provides examples of different CICS and System Logger configurations, and demonstrates the tuning process.
- The IBM Redbooks® publication Systems Programmer's Guide to: z/OS System Logger, SG24-6898. This document explains how to obtain and use an IXGRPT1 report to estimate the size of a staging data set for a DASD-only log stream. (IXGRPT1 is a sample program provided with z/OS.)