Offloading log data from interim storage by freeing and/or moving it to DASD

As applications write data to a log stream, the interim storage defined for log data begins to fill, eventually reaching or exceeding its high threshold. You specify (in the LOGR policy) high and low thresholds for each log stream to define when system logger begins and ends the process of moving, or offloading, log data to DASD log data sets. It is important for you to understand the way system logger offloading works so that:

You can set the high and low thresholds that control offloading.
You understand when, why, and how often offloading will occur for your installation.
You understand when log data is physically deleted from the log stream.
You can plan coupling facility, log data set, and staging data set sizes to control how often offloading occurs.

The high threshold and offloading work differently for coupling facility versus DASD-only log streams:

For a coupling facility log stream, system logger off-loads data from the coupling facility to DASD log data sets when coupling facility usage for the log stream reaches the high threshold. The high threshold is the point, in percent, of coupling facility structure usage for a log stream, where system logger will begin offload processing.
For a DASD-only log stream, system logger off-loads data from local storage buffers to DASD log data sets. The high threshold, however, is the point, in percent, of staging data set usage. Thus, for a DASD-only log stream, offload processing moves log data from local storage buffers to DASD log data sets, but the offloading is actually triggered by staging data set usage.

For either type of log stream, when a log stream reaches or exceeds its high threshold, system logger begins processing to offload enough of the oldest log stream data to get to the low offload threshold point specified in the log stream definition. Note that a log stream might exceed the high threshold before offloading starts, because applications might keep writing data before system logger can begin offload processing.

The low threshold is the target where offloading is to stop, leaving roughly the specified percentage of log data for that log stream in the coupling facility or staging data set.

For a coupling facility log stream, the high threshold you define for the coupling facility structure also applies to the staging data sets for the log stream; offloading is performed when either a staging data set or the coupling facility structure for a log stream hits its high threshold.

While offload processing is occurring, system logger applications can still write data to the log stream; system logger accepts and completes write requests while offloading is in progress. Since system logger calculates the amount of data to offload at the time offload processing starts, by the time offloading has finished, the amount of data in the coupling facility or staging data set exceeds the target low threshold.

When the offload completes, system logger issues an ENF 48 signal.