JES2 checkpoint function

The JES2 checkpoint function performs two separate functions:
  1. Job and output queue backup to ensure ease of JES2 restart
  2. Multi-access spool (MAS) member-to-member workload communication to ensure efficient independent JES2 operation.

The function(s) the checkpoint data set will perform in your configuration depends on whether you have a JES2 single-member MAS, or a multi-member MAS with as many as 32 members.

Checkpointing is the periodic copying of a member's in-storage job and output queues to the checkpoint data set, which can reside on either a DASD volume or a coupling facility structure. Checkpointing ensures that information about a member's in-storage job and output queues is not lost if the member loses access to these queues as the result of either hardware or software errors. Because all members in a JES2 MAS configuration operate in a loosely-coupled manner, each capable of selecting work from the same job and output queues, checkpointing allows all members to be aware of current queue status.

Within the single-member environment, the checkpoint function operates solely as a backup to the “in-storage” job and output (SYSOUT) queues maintained by JES2.

In a MAS environment the checkpoint data set not only backs up the job and output queues, it also links all members. It is the commonly accessible repository of member activity that allows each member to communicate and be aware of the current work load. The checkpoint data set contains a record of member values that describe the overall configuration of the MAS environment and specific characteristics and information that describes the current status of each member. The checkpoint allows all members to access and update (write to) the checkpoint data set and also allows all members to refresh their in-storage queues by reading from the checkpoint data set.

The following discussion describes:
  • Placement of the checkpoint data set

    This topic explains how workload performance can be enhanced by placing a checkpoint data set on a coupling facility structure rather than on DASD.

  • Checkpoint data set specifications

    This topic explains how to specify your primary and secondary checkpoint data sets during initialization, using the CKPTDEF initialization statement for checkpoints that reside on both a coupling facility structure and on DASD.

  • Determining the size of your checkpoint data set

    This topic explains how to determine the number of 4K records required for a member's checkpoint data set(s).

  • Placing a checkpoint on a coupling facility for the first time

    This topic describes how to start JES2 with a checkpoint data set residing on a coupling facility for the first time. Also, it contains guidance on how to correct errors in specifying the size of the coupling facility structure.

  • The checkpoint cycle

    This topic describes the stages through which JES2 reads and writes checkpoint records in a MAS environment. This data is especially helpful when using ID TRACE 17.

  • Checkpoint configuration modes

    This topic explains how you decide what mode to specify for your checkpoint data set(s) (DUPLEX with backup, DUPLEX without backup, or DUAL), using the MODE= and DUPLEX= subparameters on the CKPTDEF initialization statement.

  • Accessing the CKPTn data set in a MAS

    This topic describes how you can use the HOLD= and DORMANCY= parameters on the MASDEF initialization statement to control how access to the checkpoint is shared in a MAS environment.

  • Replacement data set (NEWCKPTn) definition

    This topic describes how you can specify replacement checkpoint data set definitions to ease recovery when JES2 loses access to the CKPTn data set(s).

  • Checkpoint reconfiguration: An overview

    This topic describes how you can redefine the JES2 checkpoint data set configuration dynamically to replace, discontinue use, or resume using a checkpoint data set.

  • Recovering from member failures on other JES2 members

    This topic explains how you can use JES2 operator commands or the AUTOEMEM option to make jobs from a failed member eligible for restart on an active MAS member.

  • Providing copies of the JES2 checkpoint to application programs

    This topic explains how authorized application programs can obtain a copy of the checkpoint data set.