2. PERFORMANCE REPORTING ANALYSIS › 2.4 I/O Configuration Analysis › 2.4.7 Tape Drive Simulation Analysis Inquiry › 2.4.7.2 Technique Tutorial › 2.4.7.2.3 Analyzing JES2 and JES3 Drive Allocation

2.4.7.2.3 Analyzing JES2 and JES3 Drive Allocation

The inquiry can accommodate both JES2 installations and JES3
installations (using the JES3 Main Device Scheduler (MDS)
High Watermark facility).  JES2 systems allocate and free
tape drives on a job step by job step basis.

JES3 systems can use this same process or, at the beginning
of the job, they can reserve the maximum number of drives
that will be needed by any job step, and then reduce this
number of reserved drives after the step requiring this
maximum number finishes.  Under this process, a new somewhat
smaller number of drives, representing the maximum number of
drives required in any following job step, are reserved.
This process continues until the last step of the job
finishes.  This high water mark device scheduling ordinarily
results in somewhat greater tape drive requirements than does
the step by step process.

To aid detail analyses and assist in rescheduling work, the
DETAIL level BATJOB File data elements JOBCLASS, JOBMCLS,
JOBINQTM, and JOBFETTM are available for display.