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.