DEQ at Demount Facility for Tape Volumes

This facility is intended to be used by long-running programs that create an indefinitely long tape data set (such as a log tape). Use of this facility by such a program permits the processed volumes to be allocated to another job for processing (such as data reduction). This processing is otherwise prohibited unless the indefinitely long data set is closed and dynamically deallocated.

You can invoke this facility only through the RDJFCB/OPEN TYPE=J interface by setting bit JFCDQDSP (bit 0) in field JFCBFLG3 at offset 163 (X'A3') to 1. The volume serial of the tape is dequeued when the volume is demounted by OPEN or EOV with message IEC502E when all the following conditions are present:

The tape volume is verified (where a tape is considered verified after file protect, label type, and density conflicts have been resolved) for use by OPEN or EOV (see page DEQ at Demount Facility for Tape Volumes for more information concerning verified).
JFCDQDSP is set to 1.
The program is APF authorized (protect key and supervisor/problem state are not relevant).
The tape volume is to be immediately processed for output. That is, either OPEN verifies the volume and the OPEN option is OUTPUT, OUTIN, or OUTINX; or EOV verifies the volume and the DCB is opened for OUTPUT, OUTIN, INOUT, or EXTEND, and the last operation against the data set was an output operation (DCBOFLWR is set to 1).

For EOV to find JFCDQDSP set to 1, the program must not inhibit the rewrite of the JFCB by setting bit 4 of JFCBTSDM to 1.

The tape volume is considered verified after file protect, label type, and density conflicts have been resolved. The volume is dequeued when demounted after this verification, even if further into OPEN or EOV processing the volume is rejected because of expiration date, security protection, checkpoint data set protection, or an I/O error.

When the volume serial is dequeued, the volume becomes available for allocation to another job. However, because the volume DEQ is performed without deallocating the volume, care must be exercised both by the authorized program and the installation to prevent misuse of the DEQ at demount facility. A discussion of such misuse follows:

The authorized program must not close and reopen the data set using the tape volume DEQ at demount facility, if it does, one of the following can occur:
- The dequeued volume can be mounted and in use by another job. When the volume is requested for mounting, for the authorized program, the operator is unable to satisfy the mount. Therefore, the operator must either cancel the requesting job, cancel the job using the volume, wait for the requesting job to time out, or wait for the job using the volume to terminate.
- The dequeued volume can be allocated to another job, but not yet in use. The operator mounts the volume to satisfy the mount request of the authorized job. When the volume is requested for mounting by the other job, the operator is unable to satisfy the mount request, and is faced with the same choices as in the previous item.
- The dequeued volume can not yet be allocated to another job and the volume is mounted to satisfy the mount request of the authorized job. Another job can allocate the volume and, when the volume is requested for mounting, the situation is the same as in the previous item.
It is the responsibility of the installation that permits a program to run with APF authorization to ensure that it does not close and reopen a data set using the DEQ at demount facility.
Care should be exercised when an authorized program uses the DEQ at demount facility (data set 1), but processes another tape data set (data set 2). Assume the same volume serial numbers have been coded in the DD statements for data set 1 and data set 2. As the volumes of data set 1 are demounted, they are dequeued even though those volumes still might be requested for data set 2. All the problems explained in the preceding list can occur as data set 2 and another job contend for a dequeued volume.
This problem should not occur, given the intended use of the DEQ at demount facility; that is, a long-running application creating an indefinitely long tape data set. This type of application is not normally invoked through batch execution with user-written DD statements.
After a volume has been demounted and dequeued because of the DEQ at demount facility, the volume is not automatically rejected by the control program when mounted in response to a specific or nonspecific mount request. Without the use of the facility, the control program can recognize (by the ENQ) that the volume is in use, and reject the volume. Therefore, operations procedures, in effect to prevent incorrect volumes from being mounted, should be reviewed in the light of reduced control program protection from such errors when the DEQ at demount facility is used. Specifically, if a volume is remounted for an authorized program and the volume had been used previously by that authorized program, duplicate volume serial numbers will exist in the JFCB, and the control program will be unable to release the volume during EOV processing.
Checkpoint/restart considerations are discussed in z/OS DFSMSdfp Checkpoint/Restart .