Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829 The MSC handles volume mounts, demounts and relinquishes. If the MSC order was for a volume demount, then normal ending status from the indicates that the volume is demounted. The CP RDEVBLOK corresponding
to the device is reset to free status, and can now be used for
allocation to another task.
If the request is for a volume mount, DMKHSS examines the MSSRETRY bit. This bit is turned on only when CP determines which VUA is used to
mount a volume. For example, CP makes the determination for normal MDISKs, LINKs, and DEDICATEs of MSS volumes with no real address
specified. If the MSSRETRY bit is off, the mount order is sent to the MSC and the resulting MSC return code is passed back to CP. However, if CP determines which VUA will mount the volume, DMKMSS scans the MSC mounted volume table (MVT) to see if the volume is already
staged in any SDG. If the volume is not staged, then DMKMSS sends the
mount order to the MSC and the resulting KSC return code is passed back
to CP. If the volume is staged in any SDG, DMKMSS compares that SDG number with the SDG number of the VUA on which CP wants to mount the
volume. If a match is found, the mount order is sent to the MSC and the
resulting MSC return code is passed back to CP. In the case of a SDG number match, there may be no cartridge picks required to fulfill the
mount request. This situation reduces the amount of time spent waiting
for a volume mount to complete. It also reduces the amount of
mechanical work the MSS has to do.
If the SDG numbers do not match, DMKMSS examines the mount status of
the volume. The volume mayor may not be mounted. This is because the MVT shows that there are cylinders still allocated to that volume even
though it may not be mounted.
If the volume is mounted by another processor, the correct SDG number
is placed in the MSSCOM, the VOLMNTD bit in MSSCOM is turned on, and
reason code X'10' is returned to CP. If the volume is not mounted (but
still has cylinders in the staging space allocated to it), the correct SDG number is placed in MSSCOM, the VOLMNTD bit is left off, and reason
code X'10' is returned to CP. For a mount that can be retried and has a
reason code of X'10', CP attempts to select a VUA from the correct SDG. If CP finds one, MSSCOM is updated to reflect the new VUA and CP reissues the request= If no VUAs are available in the correct SDG and
the VOLMN1D bit in MSSCOM is off, the MSSRETRY bit in MSSCOM is turned
off and the original mount request is reissued. If no VUAs in the
correct SDG are available, and the VOLMNTD bit is on, the request is
terminated with reason code X'10'. If the MSC request was for a volume mount, the MSC ending status
indicated that the MSC was processed. If the MSC accepts the mount order, the MSC will order the staging adapter to generate a pack change
interrupt (an unsolicited device end) on the device when that device has
been CP receives the pack change interrupt, the RDEVBLOK is
set to indicate that the volume is mounted, and any VM/370 task waiting
for the volume is marked dispatchable. If the mount order was rejected,
no further processinq of the mount request occurs. The previously
allocated RDEVBLOK is marked free and processing continues with the next Part 2. Control Program (CP) 172.1

Paqe of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829
The VM/370 control program destages any changed cylinders on a volume
at detach time when the user count for the entire volume goes to zero.
Destaging is accomplished when CP issues a relinquish order to the MSS via the central server application program (DMKMSS). A relinquish order
is issued at detach time for volids mounted on SYSVIRT virtual unit
addresses and VIRTUAL virtual unit addresses. This is the case only
when the volume was mounted by the control program on behalf of the
guest operating system. Refer to the Generation Guide that identifies VS1/VS2 APARs to be applied to the central-server--virtual machine operating system to support the VM/370 relinquish function (APAR 11344).
172.2 IBM VM/370 System Proarammer's Guide