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Limiting Access to Devices Through Job Reserve

In addition to reserving certain devices for particular jobs, you can use the job reserve facility to limit access to a group of devices. You can limit access to remote devices or non-tape devices defined on the system as tape devices, thus eliminating the need to cancel local jobs that incorrectly allocate remote or non-tape devices.

You typically limit access to these devices by defining esoteric device groups to z/OS. The esoteric device groups you create are subsets of the generic device groups created by z/OS. You can then specify the esoteric group name on the UNIT parameter in your JCL to allocate one of these devices. However, there are two potential problems with this method:

• Because remote and non-tape devices are also defined in a generic device group, any JCL request that specifies the generic group on the UNIT parameter can access these devices.
• If no UNIT parameter is specified in the JCL, then z/OS uses the generic device specification contained in the catalog entry.

You can overcome these z/OS limitations with the job reserve facility. To do this, you reserve all of the devices in the group to a non-existent (or dummy) job name. This approach takes advantage of the fact that job reserve processing eliminates no devices in the EDL if the reserve specifications eliminate all of the devices in the EDL.

For instance, a job that specified a generic UNIT parameter would have the reserved devices eliminated from its EDL, allowing it to only allocate local devices. If a job specified a UNIT parameter that only included the reserved devices, then device elimination would be bypassed since all of the devices in the EDL would be eliminated by the reserve specification. This allows the job to allocate from this reserved set of devices.

Important! TPCF ignores the reserve only if all of the devices in the EDL are eliminated.

Example:

This example illustrates the procedure you can use to limit access to remote tape devices through the job reserve facility. Assume that device groups 0580-058F and 0680-068F belong to the same generic device group. This example assumes that any request for an input tape that does not specify a UNIT, or that specifies a generic unit, is allocating a local device. If greater control over this process is required, then it is necessary to use the TPCEDLXT exit routine.

1. This step illustrates how you would create two esoteric device groups for devices 580-58F and 680-68F, defined to the system as remote devices and local devices. Specify the following z/OS SYSGEN statements:

   UNITNAME NAME=REMOTE,UNIT=(0580,F) /* REMOTE TAPE DRIVES 
   UNITNAME NAME=LOCAL,UNIT=(0680,F)  /* LOCAL TAPE DRIVES
   

2. This step illustrates how to limit allocation of devices located at the remote data center. Issue the CA MIA VARY command to specify a non-existent job name for the device group 0580-058F:

   VARY 0580-058F,JOB=NONEXIST
   

3. This step illustrates an output DD statement you specify in a job to request a local device allocation:

   //OUTPUT DD DSN=LOCAL.DATASET,UNIT=LOCAL,DISP=(NEW,CATLG)
   

   Because UNIT=LOCAL is specified and z/OS does not need to search its catalog for a volume number on an output request, the only candidates for this job are devices from the 0680-068F group. However, at the end of this process, the data set is cataloged, and the generic device group for the data set is stored in the z/OS catalog.

4. This step illustrates how the job reserve facility handles the override process that occurs on an input tape request for which no volume serial number is provided, and no UNIT is coded in the JCL. You can run a job named LOCLREAD with the following DD statement to request a tape device for input:

   //INPUT DD DSN=LOCAL.DATASET,DISP=(OLD,KEEP)
   

   When z/OS searches the catalog for the volume where this data set is stored, it uses the generic device group stored in the catalog entry of the data set.

   Devices in both groups 0580-058F and 0680-068F are eligible candidates for this request because they are all defined to the system in the same generic device group. However, because the devices in groups 0580-058F are reserved for job name NONEXIST, TPCF eliminates devices in this group and allows a device from the 0680-068F groups to be allocated.

5. This step illustrates how the job reserve facility handles the situation that occurs when allocation of a remote tape device is really desired. Run an output job named RMOTWRIT to create a tape data set:

   //OUTPUT DD DSN=REMOTE.DATASET,UNIT=REMOTE,DISP=(NEW,CATLG)
   

   Because UNIT=REMOTE is specified on this statement and z/OS does not need to search its catalog for a volume number on an output request, the only candidates for this request are devices from the 0580-058F group.

TPCF typically eliminates devices in this group because they are reserved by job name NONEXIST. However, the job reserve feature protects you from the problem of eliminating all eligible devices. In these cases, TPCF ignores the job reserve and allows the devices in groups 0580-058F to be eligible candidates for this allocation request.

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