Dynamic allocation processing can bring devices online and have volumes mounted.
This function is optional for TSO/E users, because it is time-consuming and requires operator intervention; it is not always desirable in an interactive environment. If selected, the option is assigned through the UADS entries.
By default, other users of DYNALLOC can always have volumes mounted and devices brought online. The ALLOCxx member of SYS1.PARMLIB, described in z/OS MVS Initialization and Tuning Reference, defines the defaults for mounting volumes and bringing devices online. If you do not want volumes mounted or devices brought online during dynamic allocation, you can indicate it in the SVC 99 parameter list.
In addition, the operator may inform dynamic allocation that a volume is not to be mounted or that a device is not to be brought online. If the operator prevents the mounting or bringing online of a volume or device, the allocation request fails.
If you allow volume mounting, dynamic allocation waits for tape volumes to be mounted. Batch allocation processing, by contrast, does not wait for tape volumes to be mounted. When a volume is mounted for a dynamic allocation request, the system verifies that it is the correct volume when it opens the data set.
If the option to have volumes mounted and devices brought online is not in effect, tape and direct access devices that have an outstanding mount request, or that are not ready, are not eligible for use by dynamic allocation.