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Should I deselect the "Synchronize with an Internet Time Server" checkbox under the VM's "Date and Time -> Internet Time Settings" tab if the "Time Synchronization Service" for a Hyper-V-based Virtual Machine is enabled?

One of the Integration Services that Hyper-V provides is the Time Synchronization Service, which can be enabled/disabled by going to a VM's Settings->Integration Services setting in the Management section. I believe this is checked by default.

When you install a Windows Server 2008 OS in a VM on the Hyper-V server, it comes with the "Synchronize with an Internet Time Server" option set, pointing to "time.windows.com".

I'd think that if the parent Hyper-V server is set to one time server, and the child VM is pointing to a different time server, there would be a momentary blip if the two are not spot on with their times when the synchronization services run.

So the question is, which time sync service should I use? I'm assuming not both. And what is the advantage of one over the other?

Note: This question assumes that the machines are not joined to a domain. If they were, the machines would also try to update their time against the domain controller with the primary domain controller role too, right?

Thanks!

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4 Answers 4

So the question is, which time sync service should I use?

Hyper-V. The clock on a vm can get out of sync VERY fast. Most modern OS (and this includes windows) rely on a software clock, which - sadly - is unstable on a VM. It will fall behind in the VM can not allocate it enough CPU slices, which it assumes to have. Hyper-V is resyncing VERY often.

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I generally disable the time sync service for non windows VMs and sync them against the firewall. I've found that Windows in Hyper-V with the time sync service on hyper-v works fine but all the 'nix systems seem to drift one way or the other really fast. I've generally got a cron job running on those to sync everywhere from 2 to 5 minutes.

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I've had good luck using NTP with Fedora and Ubuntu. CentOS was a major pain with NTP. For that to work correctly, I had to add notsc divider=10 to grub.conf, and ntp.conf required some editing, and I scaled the polling period down to 4. –  Greg Askew Feb 3 '11 at 19:35
    
For some reason the worst offender for me is suse. I'm just getting into CentOS so I'll keep that setting in mind. Thanks. –  ErnieTheGeek Feb 3 '11 at 19:46

I think this depends on the role. For Domain Controllers, I treat them as a regular DC, and use the normal NTDS rules for Windows time synchronization, and do not use Hyper-V.

For AD to function, it's more important that all DC's have the same time than for the time to be accurate.

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up vote 0 down vote accepted

I decided since the Hyper-V integration services already provide time synchronization with the host Hyper-V server, and since it is already synchronizing it's time using the Internet Time functions, that it is best to just disable the internet time synchronization on the VM instance.

Done by right clicking the time/date, going to "Adjust Time/Date" - Click on the "Internet Time" tab -> Click on the "Change Settings" button and de-selecting the "Synchronize with an Internet Time Server" checkbox.

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