Page of GC20-1801-10 As Updated March 3, 1980 by TNL GN25-0776 Planning Considerations for Other Virtual Machines
The following message is issued when the channel-set is connected to
the attached processor: DMKCPU6231 - CHANNEL-SET CONNECTED TO PROCESSOR nn
Operating Systems Using Reserve/Release
Shared DASD is the term used to describe the capability of accessing
direct access devices from two or more systems. The systems can be in
virtual machines on the same real processor or on different real
processors. Device access by the sharing systems is sequential.
Sharing of DASD devices can occur when: I. A two- or four-channel switch attaches a device's control unit to two t or tour channels. I. String switching is utilized and the control units to which they are I switched are on channels of two different systems.
With Shared DASD, an I/O operation may be started to a shared device
from any of the systems able to access the device by means of the
switch. Each sharing system vies for the programmable switch to gain
device access. The first requesting system gets the switch set to its
interface so that it may perform I/O operations to a shared device.
When the switch returns to the neutral position, any other system, or
the same one, may select the shared device and have the switch set to
its interface.
It is important to note that none of the sharing systems is aware of
what the other is doing with the data on the shared devices. Data
integrity is the responsibility of the using program. For this reason,
the hardware command, RESERVE, may be issued by a program to retain
exclusive use of a shared device while a critical update to data is
being performed. Device RELEASE is issued to terminate the exclusive
reservation. If a shared device has been reserved for exclusive use,
the system channel through which the reserve was issued will lock out
any other channel, on the same or different system, from accessing the
device.
Reasons for Sharing:
There are several reasons an installation would elect to share devices
between systems: I. Scheduling of jobs is simplified and operator intervention is I minimized. Instead of being moved from one system to another, the I volume remains mounted and available to each system able to access I the data by means of the two-or four-channel switch or string I switch. I. Updating of data is minimized. One update to a shared data set is I needed, instead of the multiple updates that would be required if I each of several systems had its own copy of the data set. I. Backup and switchover in the event of hardware failure is facilitated I in a multi-system environment if the needed data is accessible to I surviving systems without moving it. I. Direct access storage space may be saved, as one copy of the data is I required instead of multiple copies.
46 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
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