Advanced Topics › z/VM Considerations › Defining Multiple Addresses per Device

Defining Multiple Addresses per Device

Prior to the implementation of shared tape support, each device address could only be attached to one virtual machine at a time, requiring the definition of an independent real address for each virtual machine that shares the tape device. As discussed above, this restriction is eliminated with Shared Tape Support and can be avoided using the Autopath feature. However, you still need to define multiple addresses per device if you are not using the Autopath feature and if any of the following are true:

• You are running at a level of z/VM that does not support Shared Tape Support.
• You are running at a level of z/VM that does support Shared Tape Support but you do not plan to use the Shared Tape Support feature.
• You are using CA MIA and the Shared Tape Support feature to share devices between CMS users on a z/VM host system and guest systems using the two-path configuration described in the previous section.

To define multiple addresses per device, define paths to multiple channel interfaces on a single control unit as if there were multiple control units, each with a single channel interface.

Unsupported device definitions should be used so z/VM does not set its own path group ID on the drives. For example, if supported devices are defined on two paths, then both paths will have the same path group ID. If one of the z/OS systems goes offline, then both paths are disbanded. If the drives are defined as unsupported, then each z/OS guest defines its own path group ID. This way, the path to one z/OS guest is not affected by a reset of the other path.

Note that the drives defined as unsupported are not usable in z/VM environments, either by CMS users or by CP functions such as SPTAPE or MONITOR. You can define one path as a supported tape drive, such as 3480, and another path as unsupported to handle this type of situation.

IBM hardware permits up to eight paths to a single tape drive. In general, when you plan to run multiple z/OS guests under z/VM, only one of the addresses you define to z/VM for a device should be supported. All extra paths should be defined as unsupported and only be used by z/OS or other operating system guests, such as z/VM guest systems. For information on the configuration statements required for defining tape devices, see the documentation provided by your hardware vendor.