JES help

JES2 uses a list structure for its checkpoint data set structure. The JES2 checkpoint function performs two separate functions:

• Job and output queue backup to ensure ease of JES2 restart

• Multi-access spool (MAS) member-to-member workload communication to ensure efficient independent JES2 operations

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.

An installation can place checkpoint data set(s) (primary or secondary) on a coupling facility structure, DASD or a combination of both. Placing the primary checkpoint on a coupling facility structure provides more equitable access for all members of a MAS than that available through DASD because of the coupling facility lock. JES2 uses the coupling facility lock to serialize access to the checkpoint data set. When the checkpoint resides on a coupling facility, the first-in, first-out (FIFO) method of queuing ensures that all MAS members can have equal access to the data.

Attention: IBM does not recommend placing both checkpoints on coupling facility structures or placing the primary checkpoint on DASD and the secondary checkpoint on a coupling facility structure. If both checkpoints reside on coupling facilities that become volatile (a condition where, if power to the coupling facility device is lost, the data is lost), your data is less secure than when a checkpoint data set resides on DASD. If no other active MAS member exists, you can lose all checkpoint data and require a JES2 cold start. Placing the primary checkpoint on DASD while the secondary checkpoint resides on a coupling facility provides no benefit to an installation.

IBM recommends initializing JES2 with the checkpoint data sets defined on DASD, and then forwarding them to a previously-defined coupling facility through the checkpoint reconfiguration dialog.

Adequately determining the size of your checkpoint data set is critical to JES2's ability to initialize, and should be large enough for future expansion.

4K Records

During JES2 initialization, JES issues the $HASP537 message that displays the number of 4K records in your checkpoint. This number will not allow for any increase to these parameters nor expansion by the $ACTIVATE command which is necessary to enable new functions at the current JES2 level. Use this number plus any increase you might need for future expansion as input to this field. A numeric value in 4K records is expected. If the value from the $HASP message is 100 4K records, enter 100 plus some percent for future expansion.