Note: When a reserve is issued to a device that has alternate path support (defined in the RDEVICE and RCTLUNIT Ve/370 system generation
macro instructions), V!/370 changes a reserve CCW command to a sense CCW commando Virtual reserve/release support allows several operating systems, such
as MVS, SVS, and VS1, to all run as virtual machines under the same V!/370 operating system and to have data protection when using the same
data files en the same minidisk.
Te use virtual reserve/release, specify "V" in the mode operand of
the !DISK directory statement. Also subject to virtual reserve/release
processing are the virtual machine users who use the same minidisk by
way of LINK statements.
By using the VM/370 virtual reserve/release support, one operating
system running in a virtual machine can prevent other operating systems
running under the same VM/370 system from accessing the reserved
minidisk. Hcwever,. a minidisk protected by virtual reserve/release sUFport may net be protected from access by an operating system running
on other processors. When using V!/370 considerations:
reserve/release support, note the following
For devices shared between processors, volumes mounted on these
devices can contain only one minidisk against which the virtual
machine issues reserve CCW commands. (A minidisk may encompass all or part of that one volume.)
Do not Flace spooling and paging minidisks on volumes that will be
reserved.
To reserve devices between processors, define them with separately
addressable paths. (Omit the ALTCU operand on the RDEVICE macro
instruction and the ALTCH operand on the RCiLUNIT macro instruction
during system genera tion.) When using real reserve/release support, define the devices to be
reserved with separate paths, not alternate path support. When using virtual reserve/release support, each minidisk is
protected. Thus, any number of minidisks located on the same volume
can be protected. In addition, alternate path support can be defined
for the real device on which the minidisk is located, provided other
processors do not share the same volume. Section 1. General Considerations 21

within a single processor, reservejrelease CCW commands permit several operating systems running as virtual machines under one VM/370 system to use both virtual and real reserve/release to protect
data from cne another. Real reserve/release support is for dedicated
devices.
In this example, 230 and 330 are alternate device addresses for a particular DASD to be shared by USERA and USERB (two virtual machines
running on the same real computing system). share this device:
1. Generate the virtual machine operating system for USERA to support
both the device at 230 (two-channel switch) and the reserve/release
software.
2. Generate the virtual machine operating system for USERB to support
both the device at 330 (two-channel switch) and the reserve/release
software.
3. Generate VM/370 as though 230 and 330 were different devices (with
different control units and channels).
4. Issue the CP ATTACH command to attach device 230 to USERA and
device 330 to USERB. If the system generated for USERB is to run in a real machine,
rather than a virtual machine: • Generate the VM/370 system with device 230 but not 330. • Issue the CP ATTACH command to attach device 230 to USER!. In both cases: • The device addresses generated for systems to run in a virtual machine need not be the same as on the real machine. • The devices used by virtual machines must be dedicated (attached or
defined with a DEDICATE statement in the VM/370 directory). While theoretically possible, do not share the CP SYSRES and any
other CP-owned disk between two processors. VM/370 checks all CCW commands passed by operating systems running in
virtual machines. It bases reserve/release CCW command processing on:
the type of device, the presence or lack of alternate path support, and
whether the MDISK statement in the VM/370 directory contains a "V" on
the mode operand. For the hardware to execute the reserve/release CCi commands, the two-channel switch special feature must have been
installed. Depending upon the various combinations of these items, IM/370 either permits the reserve CCW command to execute on the hardware
or changes the reserve CCW command to a sense CCi command. To determine
the conditicns when a "reserve" is changed to a "sense" CCW command,
refer to Figure 2.
22 IBM VM/370 Operating Systems in a Virtual Machine