The maximum number of tape devices or direct access devices required
to satisfy any DD statement is the unit count in the UNIT parameter
except when volume affinity is present. If volume affinity is present,
the number of devices might be more than the unit count in the UNIT
parameter. For more information, see Devices assigned per step.
However, if the UNIT parameter also specifies P, for parallel
mount, the system uses the
greatest of the
following numbers to determine how many devices and volumes to allocate:
- Unit-count in the UNIT parameter
- Volume-count specified in the VOLUME parameter
- Number of serial numbers implicitly or explicitly specified
- With SMS, volume-count in the data class
The number of devices is affected by the DD statement parameters
as follows:
- DD statement specifies
- System action
- UNIT=AFF
- The system obtains the device requirements from the referenced
DD statement. All of the devices used for the referenced DD statement
are shared with the referring statement's data set.
- Generation data group (GDG)
- The system determines the number of devices needed by totaling
the devices needed for each generation data set. Each generation
data set is handled as a single request.
- VSAM data set
- The system determines the number of devices needed based on
the device/volume configuration of the data set. If the data
set is on more than one type of device, the system determines the
total number of devices required and allocates them. The system
may override the unit count or parallel mounting, if specified.
- Unit name that includes different device types
- The system allocates devices of the same type.