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Prior to the availability of shared tape support, each real device address could be attached to only one virtual machine at a time, requiring the definition of an independent real address for each virtual machine that would share the tape drive. You do this by defining paths to multiple channel interfaces on a single control unit as if there were multiple control units, each with a single channel interface.
The Shared Tape Support feature of z/VM alleviates this restriction. With Shared Tape Support, a real tape device can be attached to an unlimited number of guest systems using the same address. This is accomplished using the new MULTIUSER option on the CP ATTACH command. The MULTIUSER option implies the NOASSIGN option. Guests to which a device is attached as MULTIUSER must provide their own serialization for the device.
For example, if a device is defined to a z/VM host system (VMH) with address 3A0, it can be attached to one z/VM guest system (ZVM1) and two z/OS guest systems (ZOS1 and ZOS2) using the following CP ATTACH commands:
CP ATTACH 3A0 TO ZVM1 AS 880 MULTIUSER CP ATTACH 3A0 TO ZOS1 AS 880 MULTIUSER CP ATTACH 3A0 TO ZOS2 AS 880 MULTIUSER
Each guest system knows the device as address 880. CA MIA running on each guest system serializes access to the device. CA MIA runs on guest systems ZOS1 and ZOS2. CA MIA for z/VM runs on guest system ZVM1. The device must be defined to CA MIA on each system using the same global name. Global name G880 is used in this example. For CA MIA for z/VM running on VM1, the device must be defined with global name G880 in the UNITS MIM file. For CA MIA running on ZOS1 and ZOS2, the device must be defined with global name G880 in the MIMUNITS member.
Note: The Shared Tape Support enhancement is not designed to share a device between CMS users or between CMS users and guest systems. The use of the MULTIuser ATTACH option assumes that the user to whom the device is attached serializes access to a device. CMS users do not have the means to serialize access to a device with other CMS users or with guest systems. There is no serialization of the device if it is attached to multiple CMS users or to a CMS user and a guest as MULTIuser. Data corruption can occur as the CMS users and guests access the device at the same time.
CA MIA for z/VM, in conjunction with CA MIA, provides serialization of devices between CMS users and z/OS guests while using the Shared Tape Support enhancement to minimize the number of paths required per device. Sharing devices between CMS users and an unlimited number of guests using CA MIA requires only two paths per device:
For example, to share device 3A0 between CMS users on VMH and guests ZVM1, ZOS1, and ZOS2 in the complex defined above, the following additional setup is required:
In this configuration, CA MIA for z/VM on VMH does not manage device address 3A0 and is not aware of the CP ATTACHes of 3A0 to the guests. However, CA MIA on the guest systems manages device address 880, which is the address the guests use for access to the device through z/VM address 3A0. Any time a guest allocates device 880, CA MIA on the guest informs CA MIA for z/VM on VMH that device with global name G880 is in use by a guest.
If a CMS user on VMH wants to attach the device, then a CA MIA for z/VM ATTACH command (MI ATTACH) is used to attach device address 4A0/G880 to the user. CA MIA for z/VM on VMH recognizes that the ATTACH is for the device with global name G880. If G880 is already in use on a guest (through address 3A0), then CA MIA on the guest informs CA MIA for z/VM on VMH that the device is busy. CA MIA for z/VM on VMH does not allow the attach to proceed. If the device is not busy on a guest, then CA MIA for z/VM on VMH attaches the device and informs CA MIA on the guests that the device is not available for use by the guests.
Note: The preceding configuration provides for sharing a device between CMS users on a host z/VM and an unlimited number of guests using only two paths to the device. If configuration constraints prohibit defining more than one path per device, then the Autopath solution uses the CA MIA for z/VM Autopath for z/VM feature in conjunction with the CA MIA Autopath feature to allow for sharing between CMS users and guest systems using only one path per device. With the Autopath feature, a device is automatically moved between guest and host systems using attach and detach commands based on need for the device. While the Autopath feature has the advantage of needing only one path, the disadvantage is that the device is moved between host and guest system by detaching and attaching the device. With the Shared Tape Support configuration, the device remains attached to all guests all the time. There is no need to actually attach and detach the device. The disadvantage of shared tape support is that two paths are required. The Autopath feature is discussed in greater detail later in this section.
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