Types of JES3 setup

JES3 allocates devices in three different ways: job setup, high watermark setup, and explicit setup. The type of setup to be used is specified during JES3 initialization, but can be changed for a job by parameters on the //*MAIN statement.

Job setup: For job setup, JES3 allocates all the JES3-managed and jointly-managed devices required in the job before the job is initiated. JES3 mounts the initial volumes necessary to run all steps before the job executes. To request job setup, code:

   //*MAIN  SETUP=JOB

When volumes are no longer needed, they are demounted, if removable, and the devices unallocated, that is, made available for use by another job. If you specify the FREE=CLOSE DD parameter, JES3 unallocates the device when the data set is closed.

If you are using the dequeue at demount facility (early volume release) for multivolume data sets, JES3 unallocates volumes when they are demounted. For information on the dequeue at demount facility, see the TYPE=J OPEN macro option inz/OS DFSMSdfp Advanced Services.

Table 1. JES3 Job Setup (SETUP=JOB)
Devices and volumes to be allocated	Tape						Direct access
Volumes on Devices Set Up Before Execution	1	2	3	4	5	6	8	9	10	11	12
Job Steps STEP 1 tape volume=1,2 direct access volume=8,9	U	U	A	A	A	A	U	U	A	A	A
STEP 2 tape volume=2,3,4 direct access volume=8	A	U	U	U	A	A	U	A	A	A	A
STEP 3 tape volume=4 direct access volume=9,10,11	A	N	N	U	A	A	A	U	U	U	A
STEP 4 tape volume=1,5,6 direct access volume=8,11,12	U	N	N	N	U	U	U	N	N	U	U
Total Devices Used by the Job for Setup	6 Tape						5 Direct Access

Legend
U: The device is allocated and in use
A: The device is allocated but not in use
N: The device is no longer needed and can be unallocated.

High watermark setup: For high watermark setup, JES3 reserves for a job the maximum number of devices of each type needed for any one job step. JES3 premounts only some volumes before the job executes. When you must use fewer devices for a job, high watermark setup is better than job setup. To request high watermark setup, code:

High watermark setup for tapes, direct access, graphics, printers, and punches:
```
   //*MAIN SETUP=HWS  
```
High watermark setup for tapes only, with job setup for direct access:
```
   //*MAIN SETUP=THWS  
```
High watermark setup for direct access, with job setup for tapes:
```
   //*MAIN SETUP=DHWS  
```

When the last step that uses a device no longer needs it, JES3 unallocates it.

In Table 2, volumes mounted after STEP1 are indicated by placing the volume number in the box for the step in which it is allocated. For example, Volume 3 is mounted at STEP2.

Table 2. JES3 High Watermark Setup (SETUP=HWS)
Devices and volumes to be allocated	Tape			Direct access
Volumes on Devices Set Up Before Execution	1	2	4	8	9	11
Job Steps STEP 1 tape volume=1,2 direct access volume=8,9 Volume 1 is mounted at STEP1 and then demounted until needed in STEP4. Volume 8 is mounted for STEP1 and STEP2 and then demounted until needed in STEP4.	U	U	A	U	U	A
STEP 2 tape volume=2,3,4 direct access volume=8 Volume 3 is mounted at STEP 2.	U 3	U	U	U	A	A
STEP 3 tape volume=4 direct access volume=9,10,11 Volume 10 is mounted at STEP 3.	A	A	U	U 10	U	U
STEP 4 tape volume=1,5,6 direct access volume=8,11,12 Volumes 1, 5, 6, 12, and 8 are mounted at STEP 4. Volumes 1 and 8 are mounted on any available device.	U 1	U 5	U 6	U 12	U 8	U
Total Devices Used by the Job for Setup	3 Tape			3 Direct Access

Legend
U: The device is allocated and in use
A: The device is allocated but not in use
N: The device is no longer needed and can be unallocated.

Explicit setup

Explicit setup is directed by the user. Explicit setup requires the same number of devices as job setup. JES3 premounts volumes according to the instructions coded in:

   //*MAIN  SETUP=(stepname.ddname,...)
   //*MAIN  SETUP=(stepname.procstepname.ddname,...)

To request that JES3 not explicitly set up certain volumes, code:

   //*MAIN  SETUP=/(stepname.ddname,...)
   //*MAIN  SETUP=/(stepname.procstepname.ddname,...)

The advantage of explicit setup over high watermark setup is that you can force volumes to stay mounted on devices until they are no longer needed. The disadvantage is that JES3 does not unallocate devices early: JES3 allocates a certain number of devices before job execution and does not unallocate any until the job completes execution. In contrast, with job setup and high watermark setup, JES3 can unallocate devices at the end of any step, if the devices are no longer needed.

In the explicit setup shown in Table 3, four devices are allocated for both tape and disk instead of the three allocated using high watermark setup. The volumes to be explicitly mounted, for example, volumes 1 and 8, are not unallocated and then remounted for the last step.

Table 3. JES3 Explicit Setup (SETUP=ddname)
Devices and volumes to be allocated	Tape				Direct access
Volumes on Devices Set Up Before Execution	1	2	3	4	8	9	10	11
Job Steps STEP 1 tape volume=1,2 direct access volume=8,9	U	U	A	A	U	U	A	A
STEP 2 tape volume=2,3,4 direct access volume=8	A	U	U	U	U	A	A	A
STEP 3 tape volume=4 direct access volume=9,10,11	A	A	A	U	A	U	U	U
STEP 4 tape volume=1,5,6 direct access volume=8,11,12 Volumes 5, 6, and 12 are mounted in STEP 4.	U	A	U 5	U 6	U	A	U 12	U
Total Devices Used by the Job for Setup	4 Tape				4 Direct Access

Legend
U: The device is allocated and in use
A: The device is allocated but not in use
N: The device is no longer needed and can be unallocated.

Altering JES3 Device Allocation: To keep JES3 from allocating devices before the first step and holding them until a later step needs them, break a multiple-step job into several smaller jobs in a dependent job net.