Devices assigned per step

The number of devices assigned for a job step is not necessarily the sum of the device requirements for each DD statement.

The following tend to reduce the total devices assigned for a step:

A volume can be allocated to only one device. Therefore, when more than one DD statement asks for the same volume, the system allocates the same volume on the same device.
Requests for direct access space on public and/or storage volumes can be allocated to the same volume. Therefore, when more than one DD statement requests such space, the system can allocate the same volume on the same device.
Requests for the same public tape volume are allocated to that volume. Therefore, if a DD statement requests a public tape and specifies VOLUME=REF, the system can allocate the same volume on the same device.

The following tend to increase the total devices assigned for a step:

A permanently resident or reserved volume cannot be demounted. Therefore, the system assigns a permanently resident or reserved volume to its own device, on which it is mounted. The volume is assigned to its own device even if the DD statements specify that the device was to be shared with other volumes.
A direct access volume is requested by more than one DD statement in a step; the volume is shared by the data sets. The system assigns that volume to a device and does not assign any other volumes to that device, even if the DD statements specify that the device was to be used for other volumes.
The system allocates additional devices for a VSAM data set, if the data set resides on more than one type of device.
The system allocates a direct access device for a private catalog, if it is associated with and/or used to retrieve volume information about a requested data set.
For a generation data group (GDG), the system may have to assign additional devices to satisfy the device type needs for each generation data set in the GDG.
When DD statements request conflicting device assignments for a tape volume, the system assigns the volume involved in the conflict its own device. For example:
```
   //DD1  DD  UNIT=3592,VOLUME=SER=(V1,V2)
   //DD2  DD  UNIT=3592,VOLUME=SER=(V2,V3)  
```
Volume serial V2 has conflicting device assignments. Therefore, the system assigns the three volumes to three devices. If the DD2 had requested unit affinity, UNIT=AFF=DD1, the system would have assigned only one device to all three volumes.