Log buffer pools

Insufficient log buffers can cause applications to wait until a log buffer is available, which can affect WebSphere MQ performance. RMF reports might show heavy I/O to volumes that hold log data sets.

There are three parameters we can use to tune log buffers. The most important is OUTBUFF. If the log manager statistic QJSTWTB is greater than 0, increase the size of the log buffer. This controls the number of buffers to be filled before they are written to the active log data sets (in the range 1 to 256). Commits and out-of-syncpoint processing of persistent messages cause log buffers to be written out to the log. As a result this parameter might have little effect except when processing large messages, and the number of commits or out of syncpoint messages is low. These parameters are specified in the CSQ6LOGP macro (see the WebSphere MQ for z/OS System Setup Guide for details), and the significant ones are:

OUTBUFF

This controls the size of the output buffer (in the range 40 KB through 4000 KB).

WRTHRSH

This controls the number of buffers to be filled before they are written to the active log data sets (in the range 1 through 256).

You should also be aware of the LOGLOAD parameter of the CSQ6SYSP macro. This specifies the number of log records that are written between checkpoint records. The range is 200 through 16 000 000 but a typical value for a large system is 50 000. If a value is too small you will receive frequent checkpoints, which consume CPU and can cause additional disk I/O.