JES2 checkpoint
The record of every job and output that JES2 is holding, shared by all the JES2 systems that use the same spool.
Also written checkpoint dataset, MAS, multi-access spool, spool volume, $HASP050
JES2 keeps two kinds of data. The spool, on one or more spool volumes, holds the jobs' JCL and output themselves. The checkpoint holds JES2's records of every job and piece of output: which queue it is on, its status and where its data is on the spool.
In a sysplex, several JES2 systems usually share one spool, which is called a multi-access spool, or MAS. They all read and update the same checkpoint, taking turns, so each system knows about every job. The checkpoint is kept in a dataset or in a coupling facility structure, usually with a second copy for safety.
Both are critical. If the spool fills up, JES2 cannot accept new output, and warns with messages such as $HASP050 about a shortage. Jobs may stop until old output is purged or more spool space is added. If the checkpoint is lost or damaged, JES2 may need a cold start, which loses every job and all output on the spool.
That is why sites purge old output regularly, and why system programmers watch spool usage closely. Your own part is simple: delete job output you no longer need.
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- JES2The job entry subsystem: the part of z/OS that queues jobs, decides when they run, and collects their output.
- SpoolThe system-managed holding area where job output waits to be read, printed or purged.
- SysplexA group of mainframe systems joined together so they behave as one, sharing work and data so that no single failure stops the service.
- Coupling facilityA special LPAR that holds shared information for all the systems in a sysplex, so they can work on the same data at the same time.
- System programmerThe person who installs, configures, tunes and upgrades the operating system and its subsystems.