Using CQS system checkpoint

CQS system checkpoint, the checkpoint data sets that are used for recovery, applies to a CQS if it manages at least one queue structure. If a CQS manages only a resource structure, system checkpoint does not apply.

At a system checkpoint for recovering CQS information, CQS writes log records that contain restart and recovery information to the CQS log. CQS does not stop activity while the checkpoint is in progress.

CQS performs a system checkpoint in each of the following situations:

• When a client issues a CQSCHKPT FUNC=CHKPTSYS request
• When the number of log records that CQS writes reaches the value specified on the SYSCHKPT= parameter in the CQSSLxxx PROCLIB member data set
• When the client is IMS and you enter the /CQCHKPT SYSTEM command
• When a client RESYNC ends
• When structure checkpoint ends successfully
• At the end of a restart

In addition, CQS takes system checkpoints during significant events, such as a shutdown.

CQS checkpoint data set

For each structure pair, CQS maintains a checkpoint data set. CQS writes to its checkpoint data set and uses it during restart. The checkpoint data set is dynamically allocated during CQS initialization. You define the checkpoint data set DSNAME for a structure using the CHKPTDSN= parameter in PROCLIB member data set CQSSLxxx.

CQS also takes Structure Checkpoints; see Using CQS structure checkpoint for details.

How CQS restarts after system checkpoint

During CQS restart, CQS reads the log records from the last system checkpoint and restores the environment for committed data objects and backs out uncommitted data objects on queue structures. The frequency of system checkpoint affects this restart. CQS must read more log records when checkpoints are infrequent than when the checkpoints occur more often. Because the CQS log is shared by multiple CQSs, CQS restart time is affected by the number of log records written by the multiple CQSs, not just the CQS that is being restarted.

CQS takes an initial system checkpoint at the end of a restart.