Usage mode of a device

Exclusive use (usage mode EXCLUSIVE)

A device is used exclusively when it is assigned to only one VM and cannot be assigned to any other.

Exclusive use is said to apply in the following cases:

• The device is assigned to the VM explicitly by /ADD-VM-DEVICES with the operand TYPE=*NONE/*VC.

• The device (unless it is a disk device or CRD device) is assigned to the VM implicitly.

• Exclusive use is set with /MODIFY-VM-DEVICE-USAGE.

• Shared use of disks of a pubset is converted to exclusive use when the disks are assigned implicitly and only to this VM and the pubset is imported exclusively to this VM in the guest system (/IMPORT-PUBSET ...,USAGE=*EXCLUSIVE).

• An alias device is used exclusively if its base device is used exclusively (see "Parallel access volume (PAV, SU /390)").

/MODIFY-VM-DEVICE-USAGE USAGE=*SHARED can be used to set shared use for a device that is being used exclusively by a VM, provided it is the kind of device for which shared use is possible (e.g. a disk device).

Inputs/outputs for exclusively assigned devices take place between the guest system and the device directly.

Shared use (usage mode SHARED)

Shared use describes the situation where a device is assigned to at least one VM and can be assigned to other VMs (see also the section "Shared disk devices").

Shared use is said to apply in the following cases:

• The device is assigned to the VM explicitly by /ADD-VM-DEVICES with the operand TYPE=*SD.

• The disk device (neither FastDPAV alias device nor emulated disk) or CRD device is assigned to the VM implicitly.

• Shared use is set with /MODIFY-VM-DEVICE-USAGE.

• When a pubset which is used exclusively is exported (/EXPORT-PUBSET), exclusive use of the disks of the pubset is converted to shared use if the disks are assigned to the VM implicitly (reversal of the procedure for /IMPORT-PUBSET...,USAGE=*EXCLUSIVE).

• An alias device is shared if its base device is also shared (see "Parallel access volume (PAV, SU /390)").

A device that is assigned to a VM for shared use can be assigned to the VM exclusively by means of /MODIFY-VM-DEVICE-USAGE USAGE=*EXCLUSIVE, provided it is not already assigned to any other VMs.

The usage mode (SHARED) cannot be modified for CRD devices.

The usage mode of a device entered for the VM in its VM definition can be modified with /MODIFY-VM-DEFINITION. Detailed information can be found in the section "Working with VM definitions".

Input/output handling for shared devices

The handling of inputs/outputs for shared devices depends on the number of VMs to which the device is assigned:

Direct I/O

If the device is assigned to only one VM, the inputs/outputs take place between the guest system and the device directly.

On SU x86, these inputs/outputs are handled by X2000. There is no hypervisor overhead.

In the information commands of VM2000, shared devices with direct I/O are identified as such by SH(D).

Indirect I/O

If the device is assigned to a number of VMs, the inputs/outputs for the shared device are coordinated.

	On SU /390, these inputs/outputs are coordinated by the VM2000 hypervisor. The VM2000 hypervisor also receives the asynchronous input/output results and delivers these to the VM concerned.
	On SU x86, these inputs/outputs are coordinated by X2000. There is no hypervisor overhead.

In the information commands of VM2000, shared devices with indirect I/O are identified as such by SH(I) for the usage mode.