Element/entry ratio and the number of log streams per structure

AVGBUFSIZE is set at the structure level and dictates the ratio for the whole structure. If many applications write significantly differing amounts of data to their log streams at significantly differing intervals, some applications might experience unexpected DASD offloading, incurring increased processor usage.

The DASD offloading is unexpected because the log stream might not yet have reached the HIGHOFFLOAD threshold. Generally, the greater the number of log streams per structure, the greater the chance that the element/entry ratio is inappropriate for certain applications that use the log streams.

Each log record places an entry in the list entry area of the structure, and the data is loaded as one or more elements in the list element area. If the list entry area exceeds 90% of its capacity, all log streams are offloaded to DASD. DASD offloading commences regardless of the current utilization of the log stream, and continues until an amount of data equal to the difference between the HIGHOFFLOAD threshold and the LOWOFFLOAD threshold has been offloaded.

For example, the list entry area might exceed 90% of its capacity while log stream A is only 50% used. The HIGHOFFLOAD threshold is 80% and the LOWOFFLOAD threshold is 60%. Even though log stream A has not reached its HIGHOFFLOAD threshold, or even the LOWOFFLOAD threshold, data is offloaded until 20% of the log stream has been offloaded. This is the difference between 80% and 60%. After the offloading operation has completed, log stream A is at 30% utilization (50% minus 20%).

Thus, the log stream used by an application that issues few journal write requests might be offloaded to DASD because of frequent journal write requests by other applications that are using other log streams in the same structure.

However, if multiple log streams share the same structure, a situation where list entry storage reaches 90% utilization occurs only where all the log streams have a similar amount of logging activity.