Using automatic tape switching (ATS STAR)

When you configure your tape devices in a sysplex, you must consider whether to have all your tape drives on one system, or whether you will spread them out. You can share tape devices across many systems, both with and without the need for manual intervention by the operator. IBM® offers the following ways for many systems to use the same tape devices. A tape device can be:

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. When the system selects that device for allocation to a job step, the device is at once both assigned to the specific system and allocated to the job step. Automatically switchable tape devices require that the systems in the sysplex communicate with each other.

Automatic tape switching (that is, using automatically switchable tape devices) can benefit a sysplex by:

Requiring fewer tape devices in a sysplex
By using automatically switchable tape devices, you can reduce the overall number of tape devices needed by a sysplex. With tape devices easily available to multiple systems, a sysplex can use its tape resources more efficiently.
Reducing operations cost
Automatic tape switching can reduce operating costs by managing tape drives at the sysplex level and eliminating costly operator intervention.

For a system to consider a device automatically switchable, these conditions must be true:

The device must be defined as automatically switchable.
You can define the device as automatically switchable through HCD.

You can issue the VARY devnum AS ONLINE command when the device is offline, and then VARY it online.
The device must be varied online through the VARY ONLINE command or the IEEVARYD macro.

A system not participating in automatic tape switching but connected to a device that is defined as automatically switchable can vary that device online. During the time the device is online to the nonparticipating system, the device is not available to the participating systems. After the device is varied offline, it becomes available to participating systems.

For general information about automatic tape switching, see z/OS HCD Planning.