Page of GC20-1801-10 As Updated March 3, 1980 by TNL GN25-0776 Planning Considerations for Other Virtual Machines VM/370 CONTROL PROGRAM HANDLING OF A RESERVE CCW VM/370 reserve/release support and the VM/370 alternate path support are
mutually exclusive. The VM/370 CCW translation routine changes a reserve CCW to a sense CCW when alternate paths have been defined to the device
from the VM/370 processor. Data integrity is not preserved when sharing
a device between processors or virtual machines and alternate paths are
defined. When using virtual reserve/release to share a minidisk between
virtual machines on the VM/370 processor, VM/370 still changes a reserve CCW to a sense CCW when alternate paths are defined to the real device.
Part 1. Planning for System Generation 11 n " "to. I

Page of GC20-1801-10 As Updated Karch 3, 1980 by TNL GN25-0776
Planning Considerations for other Virtual Machines
Ho wever, since
reserve/release
al ternate pa ths
si tuations when
sense CCi. the hardware reserve/release is simulated when virtual
is being used, the data integrity is preserved when
are defined. The chart below identifies those
the VK/370 control program changes a reserve CCW to a r-- I I Type IReserve/Release,Virtual Reserve ,CCi Comnd, Alternate IExecutes in the,Release Requestedlsent by , I of I Device
Pa th ,Hardware (2-4 '(V Added to ,VM/370 to, Support ,Channel Switch) I Mode in MDISK) ,Device I Note I IDedicatedlNot defined, ,DASD or , ITape IDefined I IMinidisk INot defined, I I Not applicable, Not applicable
Not applicable, Not applicable Yes No
Reserve 1 Sense 2
Reserve 1 liN ot lil.ad i i es Ye!:; Re serve I , ,------------------------ , I I Not def ined I No No Reserve 3, I 1----------------- I I I Not def inedl No YES Sense , 4, I I -------1 1 ,Defined Not applicable, Not applicable Sense I 5 I 1----------------------- I 11NormalOperation The command is passed unchanged to the hardware. I , 2When the VK/370 system has been generated with alternate path support'
for those devices, it prevents the devices from being reserved. Thisl action causes VK/370 to avoid a possible channel lockout. VM/3701 does not return any indication of this action to the operating system, issuing the CCW command that the device was not reserved. , 3Without the two-channel switch special feature, VM/370 sends
reserve/release CCW command unchanged to the hardware. However,
hardware rejects the command and does not reserve the device.
the
the +Before sending the command to the hardware, VM/370 changes the
reserve CCW command to a sense CCW command and places a virtual
reserve on the minidisk. The real device is not reserved. The
virtual reserve prevents other operating systems running under the
same VM/370 system from accessing the minidisk; however, these same
virtual operating systems may virtually reserve other minidisks
located on the same real volume. Because the two-channel switch
feature is not installed on the channels, only one address path goes
to the device from the VM/370 processor. This path allows V8/370 virtual reserve/release processing to send a sense CCW to the device,
although the reserve CCW command would be rejected by the hardware. 5When alternate paths to a device have been defined (by the ALTCU operand on the RDEVICE macro instruction and the ALTCH operand on the RCTLUNIT macro instruction), VK/370 changes reserve/release CCW commands to sense CCW commands to prevent a possible channel lockout. , Figure 7. Summary of VM/370 Reserve/Release Support
48.2 IBM VM/370 Planning and System Generation Guide