For a coupling facility log stream, the high offload threshold defined by an installation for the coupling facility space allocated to a log stream applies also to the staging data sets in use by connections to that log stream. For example, if you define a high threshold of 80% for a log stream, system logger will begin off-load processing when either the coupling facility structure allocated for a log stream or the staging data set for any connection to the log stream reaches or exceeds 80% usage.

This means that when a staging data set reaches the high threshold, system logger immediately off-loads data from the coupling facility to log data sets, even if the coupling facility usage for the log stream is below the high threshold.

Thus, if your staging data sets are small in comparison to the coupling facility structure size for a log stream, the staging data sets will keep filling up and system logger will offload coupling facility data to DASD frequently. This means that your installation experiences frequent offloading overhead that could affect performance.

If your staging data sets are too small, you also run the risk of filling them up completely. If this occurs, system logger immediately begins offloading the coupling facility log data in DASD log data sets to harden it. System logger applications will be unable to log data until system logger can free up staging data set space. See the recovery topic in the system logger chapter of z/OS MVS Programming: Assembler Services Guide.

In general, the sizing guideline for staging data sets is to make them large enough to hold all the log data in the coupling facility. See Plan space for staging data sets for complete sizing staging data sets.