IBM SC34-7012-01 manual Keeping system log data to a minimum, About this task

work. With this information, CICS continues reading backwards, but this time reading only the records for units of work that are identified in the activity keypoint. Reading continues until CICS has read all the records for the units of work identified by the activity keypoint.

This process means that completed units of work, including shunted backout-failed and commit-failed units of work, are ignored in this part of the log scan. This is significant for the retrieval of user-written log records. User-written records cannot be presented at the XRCINPT global user exit program (see “Writing user-recovery data” on page 115) if they are part of units of work that CICS skips in this part of the log scan process.

Figure 10 illustrates the way CICS performs log processing during a CICS restart.

CICS S Y S T E M L O G S T R E A M

AKP

n-1

CICS uses AKP data		CICS	reads back
to skip-read in-flight		sequentially to
UOW records only.		last	complete AKP
	AKP
	n

Abnormal termination of CICS

Figure 10. System log scan during restart

Here are some steps you can take to ensure that system log stream sizes, and thus restart times, are kept to a minimum:

vKeep to a minimum the amount of data that has to be read. This means specifying an activity keypoint frequency that is non-zero, and which is:

–Long enough to avoid excessive keypointing

–Short enough to avoid large volumes of data between keypoints.

For information about calculating system log stream structure sizes, see the CICS Transaction Server for z/OS Installation Guide.

vExcept for your own recovery records that you need during emergency restart, do not write your own data to the system log (for example, audit trail data).

vIn particular, do not write information to the system log that needs to be kept. (See “Avoiding retention periods on the system log” on page 115).

vIf you write a long-running transaction that updates recoverable resources, ensure that it takes a syncpoint at regular intervals.

Keeping system log data to a minimum

CICS log manager controls the size of the system log stream by regularly deleting the oldest completed unit of work records (log-tail deletion).

About this task

This operation is associated with activity keypoints. It is important, therefore, that you choose the correct activity keypoint frequency (AKPFREQ) - that is, one that allows CICS to keep the system log down to a reasonable size and to keep it within the coupling facility (if one is used):

Chapter 11. Defining system and general log streams 113