VM/370 dynamically determines when these hardware features have been
installed on a control unit. During VM/370 system generation, VM/370 ignores the operand FEITURE=(2CHINSW) or FEITURE=(4CHINSW) in the RDEVICE macro instruction. However, for compatibility an installation may continue to designate it. For details about the RDEVICE macro
instruction, refer to the 2I§lg! Note: The alternate path support and the real device reserve/release support must be mutually exclusive of each other. However, alternate Fath sUPFort may be used with virtual device reservejrelease support.
For details, refer to the topic "Operating Systems Using DASD Reserve/Release" in this section. When an I/O request is received for a device, VM/370 selects an
available path from any of the paths to a device. ihen the primary
path to a device is busy and an alternate path is available, the I/O operation is initiated immediately without being queued. When all paths
are busy, the I/O request is queued for all of the paths to that device.
The I/O request is then initiated on the first path to become available.
For specific information on channel switching, refer to the OPERATING SYSTEMS USING DASD RESERVE/RELEASE Reserve/release CCW commands prevent several users of the same data
files from simultaneously accessing the same data. It is most useful
when that data is being updated. While VM/370 handles the
reserve/release CCW commands presented by other operating systems running in virtual machines, VM/370 itself does not use reserve/release CCW co.mands. Operating systems use these commands under two conditions: • When running in virtual machines under Vft/370 and sharing data files • When running on other processors and sharing data files with oFerating systems that run under VM/370 VM/370 has two types of reserve/release support: • Real applies to virtual machines issuing reserve/release CCW commands to a dedicated or attached volume. • Virtual --applies to virtual machines issuing reserve/release CCW commands to specially designated minidisks.
Real reserve/release support allows several operating systems, such as MVS, SVS, and VS1, whether running as virtual machines under VM/370 or
on other processors to have data protection on a full volume. The
device is reserved by the hardware when a reserve CCW command is executedG VM/370 supports reserve/release CCW commands for DASD as thcugh each virtual machine has a separate channel path to a shared
device. 20 1M/3iO Operating Systems in a Machine

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