Page of GC20-1801-10 As Updated March 3, 1980 by TNL GN25-0776
Planning Considerations for Other Virtual Machines I. A virtual machine runs under VM/370 and shares a device with another I processor; the operating system in the virtual machine uses I reserve/release CCWs to preserve data integrity. The operating system I running on the other processor can be VM/370, in which case the I virtual machine operating system uses reserve/release CCWs, or a I non-VM/370 operating system with reserve/release capability.
Tc support this environment, the device VM/370 virtual machine by means of the
control statement in the VM/370 directory.
should be dedicated to the
ATTACH command or DEDICATE
In the above shared DASD environments, the use of reserve/release by
virtual machine operating systems and the VM/370 alternate path support
are mutually exclusive. The VM/370 control program changes a reserve CCW to a sense CCW when an alternate path has been defined for the device.
The protecticn offered by the hardware reserve is lost. It is
recommended that a single path be defined in VM/370 for devices which
will be dedicated to virtual machines and then shared between other
virtual machines or processors. I. A virtual machine runs under VM/370 and shares a device with another I processor; the operating system in the virtual machine uses I reserve/release CCWs to preserve data integrity. The operating system I running on the other processor can be VM/370, in which case the I virtual machine operating system should use reserve/release CCws to I maintain data integrity, or a non-VM/370 operating system with I reserve/release capability.
The device can be defined as a minidisk, on the VM/370 processor,
which begins at real cylinder O. Again the use of reserve/release and
alternate path support are mutually exclusive. It should be noted that
virtual reserve/release support should not be used in this environment.
The volume being shared should not contain more than one minidisk or be
used for CP paging, spooling, etc., since reservation by the other
processor could lock out virtual machine users or VM/370 system I/O requests to the same device. VIRTUAL RESERVE/RELEASE The reserve/release software simulaticn in VM/370 provides reserve/
release protection at the minidisk level, including full volume
minidisks. Virtual reserve/release is intended for by the virtual
machines that support Shared DASD (not CMS) runn1ng on the VM/370 processor. Virtual reserve/release simulation is reguested by appending
a character "V" to the mode operand on the MDISK statement in the
directory. All subsequent links to this minidisk are subject to virtual
reserve/release processing. A software locking structure is created to
manage the reservation status by minidisk. The VM/370 control program
then examines virtual machine channel programs and manages the reserve/
release CCWs presented by the sharing virtual machines. The VM/370 control program simulates the hardware reserve by reflecting a "device
busy" condition in response to a virtual machine SIO when the minidisk
is already reserved by another virtual machine. When the minidisk is
released, a "dev ice end" interrupt is reflected to all virtual machines
users who received a "device busy" indication. Diagnose users can also
issue reserve/release CCws. However, no "device busy" or "device end" status is reflected to the virtual machine. If a minidisk is already
reserved, a subsequent Diagnose request for another virtual machine is
queued until the minidisk is released, at which time the Diagnose
request will be redriven.
48 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 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