Tape device selection

When allocating non-SMS-managed tape devices, Device Allocation considers all of the eligible devices for a particular request and attempts to choose the best possible device from the list of eligible devices.

There are many factors that influence the selection of tape devices:

The state of a particular device can influence its selection. Factors such as whether a device is online, allocated, autoswitchable, Assigned to a Foreign Host (AFH), or has an automatic cartridge loader (ACL) installed or active can influence whether a device is likely to be chosen by a specific request.
The type of device (for example, 3480 or 3490) can influence its selection.
The Tape Device Selection SSI (SSI Function Code 78) can influence device selection. For more information, see z/OS MVS Using the Subsystem Interface.
Factors such as unit or volume affinity influence device selection.
When the tape device selection algorithm has a number of equally preferable devices to choose from, it attempts to choose a device at random from those equally preferable devices. It does this so that the same device is not chosen repeatedly. The randomization algorithm will allow any eligible device to be chosen, but it may tend to favor certain devices over others and does not guarantee that all devices are chosen equally over a large number of allocation requests.

JES3-managed devices are not allocated by Device Allocation; instead they are selected by JES3 which uses its own selection algorithm to choose devices.