Backup and Recovery of MSS Volumes The process of creating backup copies of MSS volumes. and restoring frem those backup copies, can be controlled through the OS/VS access methods
services COPYV command. This command can operate without system operator intervention.
For each active volume in the MSS. there may be one or more cOFi volu.es. At any time, the active volume may be copied to a copy volume with the access method services COPYV command. All volume mounts and
data transfer are controlled by this command. If at any time it is
necessary to restore the level of a volume to that of a copy, the OS/VS access methods services RECOVERV command is used.
All the OS/VS access methods services commands can be run from either
a real processor or a VS virtual machine. If the ess communicator
virtual machine is in operation, these commands can be run from that
virtual machine while it is acting as the communicator.
174 IBM VM/370 System programmer's Guide

Timers In a Virtual Machine
This section describes the results obtained in using timers in a virtual
machine created by CP. Interval Timer Virtual location 80 (X'50'), the interval timer, contains different
values than would be expected when operating in a real machine. On a
real machine, the interval timer is updated 300 times per second when
enabled and when the real machine is not in manual state. The interval
timer on a real machine thus reflects system time and wait state time. In a virtual machine, the interval timer reflects only virtual processor time, and not wait time. It is updated by CP whenever a virtual machine
passes control to CP, and this one updating reflects the entire time the
virtual machine had control. Note that during the time a virtual
machine has control, the virtual interval timer does not change; the
virtual processor time used is added to the virtual interval timer when CP regains control. For some privileged instructions, CP may be able to
simulate the instruction and still return control to the virtual machine
before the end of that virtual machine's tiae slice. In such cases, the
virtual interval timer is updated but only for those privileged
instructions that require normal or fast reflect entry into the
dispatcher. For those privileged instructions that do not require entry
into the dispatcheL, the virtual interval timer is not updated until CP gets control at the end of the time slice.
If the virtual machine assist feature or Extended Control - program
Support is ON, more time is charged to the virtual interval timer than
if the feature is OFF. When the virtual machine assist feature is OFF, the time spent by CP to simulate privileged instructions is Bot charged
to the virtual interval timer; Whereas, with the feature ON, the time
spent i§ charged to the virtual interval timer.
The virtual interval timer assist feature is the updating of the virtual
interval timer and presentation of timer interrupts to the virtual
machine by the hardware. When the software simulates the interval
timer, updating occurs only when CP takes over This usually
results in an update frequency of once per time slice and repeatability
of timed intervals suffers greatly under these conditions. When the
virtual interval timer assist feature is active, the update frequency is
the same for both virtual and real interval timers, 300 times a second.
In order for the virtual interval timer assist feature to be active,
the following conditions must be met: • VM/370 must be running on a Model 135-3, 138, 145-3, or 148. • The virtual machine must have enabled the virtual machine assist and
the virtual interval timer (SET • The virtual machine must have enabled both the virtual machine assist
and the virtual interval timer assist (SET ASSIST ON TMR). Part 2. Control Program (CP) 175