Controlling automatic tape switching

In a sysplex, there are MVS™ operational considerations for two types of tape devices:

A dedicated tape device is varied online to one system at a time. For a second system to use that same device, an operator issues VARY commands (first VARY OFFLINE, then VARY ONLINE) to make the device available to the second system.
An automatically switchable tape device can be online to more than one system at a time. For one system to use an automatically switchable tape device, then another system to use the same device, an operator does not have to issue any VARY commands. In many ways, automatically switchable tape devices are similar to JES3-managed devices. They require that the systems in the sysplex communicate with each other.

Through system commands, the operations staff plays a key role in setting up and maintaining automatic tape switching (that is, using automatically switchable tape devices). For example, a device is automatically switchable after the following operational actions are taken:

The device is defined as automatically switchable.
The VARY AUTOSWITCH command, as described in Defining automatically switchable devices, turns the AUTOSWITCH attribute on and off.
The device is varied online through the VARY ONLINE command.
Before z/OS® Release 2, the connection between participating systems and the coupling facility is active and an IEFAUTOS structure is defined in the active coupling facility resource management (CFRM) policy.
Systems in a sysplex store the status of online automatically switchable tape devices in IEFAUTOS.

With z/OS Release 2 and higher, the ATS STAR design improves the availability and system management characteristics of the previous automatic tape switching function. ATS STAR does not use the IEFAUTOS structure but instead uses global resource serialization and XCF services to maintain serialization when it allocates shared tape devices. To maximize the performance of the ATS STAR function, use the global resource serialization STAR configuration rather than the ring configuration.