ALTERNATE PATH SUPPORT VM/370 alternate path supports the two-channel and two-channel switch
additional features. In the real hardware configuration, the
installation of the two-channel switch permits a real connection from
two real channels to a single control unit. Installation of the
two-channel switch additional feature allows a connection from four
different real channels to a single real control unit. The two-channel
switch plus the two-channel switch additional features are often
referred to as the "four-channel switch." Vo/370 alsc supports the string switch hardware feature that permits
a string of real devices to be physically connected to two real control
units. with this facility and the four-channel switches installed, VM/370 allows one to eight paths to a given device, as shown in Figure
1.
Section 1. General Considerations 19

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