For better system performance, you can store the entire system log within the coupling facility to prevent the log from spilling to disk.

Recommendation: Take frequent sync points to minimize the amount of data being held in the coupling facility.

Generally, the volume of data that DFSMStvs keeps in the primary system log at any one time covers between two and three activity keypoints. The activity keypoint frequency, which is measured in blocks of log data, and defined by the AKP parameter in the IGDSMSxx member of SYS1.PARMLIB, determines this volume.

The way that DFSMStvs manages the system log data makes the frequency of activity keypointing an important factor when planning the size of the primary log stream. Use the AKP parameter to specify keypointing frequency. Review the activity keypoint (AKP) frequency defined for each DFSMStvs instance. The larger the value, the more coupling facility space you need for the system logs. But do not set AKP so low that transactions last longer than an activity keypoint interval. DFSMStvs manages the size of the system log by deleting old, completed units of work (log tail deletion). If you need long-term data retention, then you might want to copy the data from the log stream into alternative archive storage.

A log tail is the oldest end of the log. At each activity keypoint, DFSMStvs deletes the tail of the system log by establishing a point on the system log before which all older data blocks can be deleted. If the oldest "live" unit of work is in data block x, all data blocks older than x (x-1 and older) can be deleted. DFSMStvs keeps the two most recent, complete activity keypoints on the primary system log and deletes data from complete units of work older than this.

The system log is designed to ensure the availability of logged data following a system or DFSMStvs failure, thereby maintaining data integrity. Do not use the system log for any other purpose. Forward recovery logs cannot be written to system log streams.

Related reading: For more information about the AKP parameter, see z/OS MVS Initialization and Tuning Reference.