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
Page of GC20-1801-10 As Updated March 3, 1980 by TNL GN25-0776
Planning Considerations for Other Virtual Machines RESTRICTIONS: DEVICE SHARING BETWEEN REAL PROCESSORS I. When a device is shared between processors and at least one of the I processors is running VM/370, the shared volume cannot contain more I than one minidisk. The single minidisk may encompass the entire I volume or a small portion of the volume and the remainder of the I volume must not be referenced by CP for use as paging, spooling, I etc., or by any virtual machine. I. Devices shared between processors must not be generated in VM/370's I DMKRIO as having alternate paths. If there are multiple paths from I the VM/370 processor to the shared devices, as well as a path from I the same devices to another processor, the paths from the VM/370 I processor cannot be generated in DMKRIO as alternate paths via the I ALTCH or ALTCU macro operands. lhi§ 1hg deiini1i2n 2i I Eg!hs gng 2t RESTBICTIONS: DEVICE/MINIDISK SHARING ON A SINGLE PROCESSOR I. If more than a single path to a volume exists, DMKRIO may be I generated so that each path is defined as a separate path, not as an I alternate path. When this is done, each path can be attached or I dedicated to a different user, and reserve/release CCws issued by I such users preserve the data integrity. In this case, the integrity I is preserved by the hardware, not by the software reserve/release I support. !,gg!n, definitiQll 2t ,Eath§ in !H1!RIQ gng I of I. A volume may be defined through the Directory to contain one or more I minidisks. Such minidisks must be identified through the MDISK I statement as requesting virtual reserve/release support. These I minidisks may then be shared between virtual machines that support I Shared DASD and the data integrity is preserved by the use of I reserve/release CCWs in the virtual machine channel program. I Alternate paths may be defined to the device when using virtual I reserve/release. The reserve CCW will still be changed to a sense CCW I but the integrity will be preserved by the virtual reserve/release --.:1- ,",vue. Virtual Machine Communication Facility The Virtual Machine Communication Facility (VMCF) allows one virtual
machine to communicate and exchange data with any other virtual machine
operating under the same VM/370 system. The VKCF external interrupt
masking is controlled by PSW bit 7 and CRO bit 31. It is to a user's
advantage to always have CRO bit 31 set to 1 (while VMCF is in use) and
control the interrupts with PSW bit 7 only. This reduces the number of
LCTL instructions.
Part 1. Planning for System Generation 48.3
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