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Making Virtual Server Time Stand Still

Here's how to get control of a VM's virtual clock.

Chris: I found your last column on disabling VMware time synchronization to be very useful and was wondering if you had a similar solution for Virtual PC or Virtual Server?
--Parag

I supposed that the lazy answer to this question would be to convert your Virtual PC or Virtual Server VMs to a VMware platform using VMware Converter and then follow the procedure in my last column, "Virtual Time Freeze." However, I'm sure that this is not what you're looking for.

Virtual PC and Virtual Server both sync a virtual machine's CMOS system clock with the physical host during any initial boot or reboot. Since the clock synchronization occurs at the virtual hardware level, no manipulation of the Virtual PC or Virtual Server additions software will stop the VM from syncing with the physical host. If there is a way to stop time synchronization between the virtual CMOS clock and physical host at the time a VM boots, no one I could find at Microsoft is giving up the answer. Short of resorting to torturing some members of the Microsoft Virtual Server team to learn the answer, I do have a workaround.

Virtual PC and Virtual Server VMs will periodically sync their system clocks with the physical host while they are running. The installed VM Additions in each VM controls the time synchronization. If you want to block the periodic time syncs, you can uninstall the VM additions from each VM or you can make a slight modification to each VM's configuration (.vmc) file. To disable time synchronization, open the VM's associated .vmc file in Notepad and locate the "<Microsoft>\<Components>" section. This section should appear similar to:

<microsoft>
<components>
<host_time_sync>
<enabled type="boolean">true</enabled>
</host_time_sync>
</components>

To disable time synchronization, set the enabled time value to "false." If a <host_time_sync> section is not present in the .vmc file, then you can add it to the <components> section as shown above. So when set to disabled, the .vmc entries required to stop virtual to physical time synchronization are:

<host_time_sync>
<enabled type="boolean">false</enabled>
</host_time_sync>

Keep in mind that this workaround does have limitations. If you reboot a VM, it will re-sync back with its physical host. So, to keep a VM at a consistent date and time regardless of the physical host system's time, you need to save the state of a VM when you are finished working with it. When the VM resumes, its clock will reflect the date and time of the last time the VM's state was saved.

I realize that this solution is far from perfect, since any reboots will destroy any previously maintained time differences between a VM and its host. But hopefully, this workaround will give you the versatility that you need. Short of changing the date and time settings on the physical host prior to starting a VM, there are no other known alternatives for manually controlling the date and time that a Virtual PC or Virtual Server VM assumes when it starts up or is rebooted.

I hope that helps. If anyone else has found a better answer to controlling time synchronization between a Virtual PC or Virtual Server VM and its physical host, please share your solution as a comment to this article.

About the Author

Chris Wolf is VMware's CTO, Global Field and Industry.

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