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Re: [Xen-users] Xen : Time issues

To: smainklh@xxxxxxx, Xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] Xen : Time issues
From: Jeff Lane <dreadpiratejeff@xxxxxxxxx>
Date: Tue, 7 Jul 2009 17:37:39 -0400
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On Tue, Jul 7, 2009 at 11:43, <smainklh@xxxxxxx> wrote:
> Recently we noticed that the DomU's clocks are not synced with the Dom0 and 
> the
> clocks are wrong.
> So we tried to sync DomU's via openntpd but it generates errors in nagios. 
> Logs
> indicate "Time goes back".
> So we want to sync DomU with Dom0 and sync Dom0 with openntpd.
> What do you think about this solution ?
> Is there a way to sync DomU with Dom0 ?

> What is the best way to sync DomU's ?

First of all, here's a Google search that yields a lot of hits
regarding Xen and time...

This is a fairly common problem with Xen (and pretty much any
virtualization package).  I believe that in addition to differences
between hardware clocksources, the problem stems from the fact that
domUs do not run in real time.  Instead, they are constantly paused in
memory as other domUs take up CPU cycles.

I've personally seen several instances where domUs either suffer from
time loss, or time gain (I've even seen some in beta versions of Red
Hat and SuSE that actually gain about an hour of time for every 1.5
minutes of actual system time used...)

So there are some tricks you can do to change the clocksource and such...

Also, it became our practice to enable ntp on all domUs and sync them
to something other than dom0, in addition to the problems with
clocksource.  So we'd have a central time server set up, and all dom0
machines and domU virtual machines would sync against that.

Not necessarily elegant, as the ntp clients were set to hit the ntp
server every 5 - 10 minutes or so, but it worked.

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