Page of GC20-1801-10 As Updated March 3, 1980 by TNL GN25-0776
Planning Considerations for Other Virtual Machines
Two assembler language macros, RESERVE and DEQ, are available to effect
the reserving and releasing of a device. The data integrity of shared
devices can be maintained by application program use of the RESERVE macro or by operating system components which automatically issue the
reserve macro if the target of their update operation is to a shared
device. CMS does not make use of these macros in its CMS file system.
In addition, CMS does not support these macros in the as simulation or CMS/DOS simulation packages. The SHAREOPTIONS operand which appears on
the access method services control statement has no function in CMS. No
attempt is made by CMS VSAM to reserve or release system resources. The
use of shared DASD by virtual machines should be limited to those quest
operating systems which will maintain the integrity of shared data, such
as catalogs, VTOCS, program libraries, etc., and will support the use of
the RESERVE and DEQ macros used by application programs running under
thosE operating systems. The only other alternative is the use of the
hardware reserve or release CCWs by an application program running under CMS. In this case the application program issues the hardware reserve
and release CCWs in a SIO or DIAGNOSE operation to the shared device. VM/370 reserve/release support can be addressed in two forms: Shared DASD and Virtual Reserve/Release. Shared DASD refers to the use of reserve/release CCW strings by
virtual machine or processor operating systems for the purpose of
preserving data integrity. The integrity of the data is preserved by the
hardware on a device basis during the interval of time between the
reserve and release CCWs by not allowing access to the reserved device
via any other path. Virtual reserve/release is the software simulation of reserve/release CCWs for minidisks. Since virtual devices associated with a minidisk all
map to the same real channel interface to the device, the hardware
protection is lost and a software locking structure is required to
maintain the data integrity during reserve/release seguences.
The VM/370 control program and the CMS operating system do not issue
reserve CCws. The use of reserve/release remains the full responsibility
of the operating system running in the virtual machine. The VM/370 initialization routine issues a release CCW to tape and DASD devices to
dynamically determine whether the two-or-four channel switch feature is
installed. I SHARED DASD Operating systems that support Shared DASD use reserve/release CCws to
preserve data integrity in the following environments: I Two virtual machines running under VM/370 with each operating system I having a separate channel path to the device to be shared; each I virtual operating system uses reserve/release CCws to preserve data I integrity.
Reserve/Release CCWs are recognized by the VM/370 control program CCW translation routine and are executed by the hardware to preserve data
integrity. In this environment devices should be generated, at system
generation time in DMKRIO, as separate devices. Each device should be
dedicated to a virtual machine by means of the ATTACH command or
DEDICATE control statement in the directory. Part 1. Planning for System Generation 47
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
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