Advanced Topics › Tape Device Allocation and Allocation Recovery Considerations › z/OS Device Allocation with CA MIA

z/OS Device Allocation with CA MIA

The two CA MIA facilities, GTAF and TPCF, interact with the z/OS allocation mechanism in different ways.

The function of GTAF in the allocation process is to prevent concurrent allocations of the same tape drive on multiple systems from occurring. GTAF extends the device serialization from one system to all systems in the CA MIA complex. To accomplish this goal, GTAF performs the following actions:

• GTAF controls cross-system access to the device group locks that z/OS uses to serialize device allocation.
• GTAF communicates allocation information among systems after a device has been selected for a job.

One of the functions of TPCF in the allocation process is to influence device selection. To do this, TPCF intercepts the EDL created by the z/OS allocation mechanism and eliminates or preferences devices according to the criteria you establish.

Note that CA MIA facilities interact with the z/OS allocation mechanism only when a managed device is on the EDL for a job. Therefore, GTAF only serializes allocation for managed devices, and TPCF intercepts the EDL only when a managed device is on the list. In addition, TPCF acts only on managed devices from these lists. TPCF does not act on devices that you have not placed under CA MIA management.

Note: To determine if a device is managed, see Verifying Whether Devices are Defined to CA MIA in this chapter.

The following steps provide an overview of the role of CA MIA in the z/OS allocation process:

1. z/OS allocation creates the EDL for a DD for the job, based on the device groups that z/OS creates during SYSGEN.
2. TPCF intercepts the EDL and marks devices ineligible if:
   • The elimination logic of the TPCEDLXT exit routine specifies that the device should be eliminated.
   • The device is reserved for another job.
   • The device is not reserved for this job and other devices are reserved FORCE=YES for this job.
   • The device is OVERGENNED and SETOPTION TPCF OVGINELIG is set to YES or JOBRESV.

   If all eligible devices are eliminated from the EDL by TPCF, then TPCF returns the original EDL to z/OS allocation.

   z/OS or other vendor products may also eliminate devices from allocation at this time.

3. z/OS requests the device group locks for all device groups in the EDL. GTAF makes the job wait until it determines that the locks are not in use by jobs on other systems. When the locks are available, GTAF releases the job to continue through z/OS allocation. GTAF tells the other systems that the locks are in use on this system.
4. z/OS examines the EDL and eliminates ineligible, inaccessible, allocated, and offline devices from possible allocation. Because TPCF may have varied additional devices offline (due to the preferencing status that you have assigned to these devices), z/OS may eliminate additional devices from possible allocation. These devices include OVERGENNED, NOTAVAILABLE, and EXTERNALLY DEDICATED devices.

   If there are no devices that can be allocated, the job enters allocation recovery. If there is one device, then the job allocates it and then goes to Step 7. If there are multiple devices, the job goes to Step 5.

5. TPCF preferences the remaining devices based on the preferencing criteria specified. TPCF preferences devices that:
   • Are reserved for this job
   • Are dedicated to this system
   • Are selected by the TPCSRMXT exit
   • Have a preference value set

   z/OS or other vendor products may also preference devices at this time.

6. z/OS chooses a device for allocation from the remaining devices based on the preferencing for the devices.
7. z/OS marks the device as allocated and the job releases the device group locks. GTAF tells the other systems which device was allocated and that the group locks are no longer in use by this system.