Errors can occur while processing jobs and data. Some of these errors can result in the halting of all system activity. Other errors can result in the loss of jobs or the invalidation of jobs and data. If such errors occur, it is preferable to stop JES2 in such a way that allows processing to be restarted with minimal loss of jobs and data. The checkpoint data set, checkpointing, and the checkpoint reconfiguration dialog all help to make this possible.

Checkpoint data set is the general term used to describe the checkpoint data set that JES2 maintains on either DASD or a coupling facility. The checkpoint data set (regardless of whether it resides on a coupling facility structure or a DASD volume) contains a backup copy of the job and output queues, which contain information about what work is yet to be processed and how far along that work has progressed. Similar to the spool data sets, the checkpoint data set is commonly accessible by all members of the multiple-processor (multi-access) spool complex, but only one member will have control (access) of the checkpoint data set at any one time. Furthermore, the checkpoint data set provides communication among all members of the configuration about jobs and the output from those jobs. JES2 periodically updates the checkpoint data set by copying the changed information from the in-storage copy to the checkpoint data set copy that resides on either a coupling facility structure or on DASD. Information in the checkpoint data set is critical to JES2 for normal processing and in the event that a JES2 or system failure occurs.

Checkpointing is the concept of keeping a copy of the checkpoint data set that contains essential workload information on either a coupling facility structure or a DASD volume. This copy is updated from the in-storage copy of the checkpoint as each JES2 member of a multi-access spool updates the in-storage checkpoint data set. Checkpointing allows JES2 to be stopped and then restarted with little or no loss of job or data integrity.

The checkpoint reconfiguration dialog is a dynamic means by which the current checkpoint configuration can be changed (for example, adding a checkpoint device or moving the checkpoint data set to a different device). It is possible to enter the checkpoint reconfiguration dialog to continue processing without necessitating a JES2 restart. This process increases system availability.