Bug#534978: clock drift in Xen domU with clocksource=xen

[Moritz Muehlenhoff <jmm@inutil.org>]

On Tue, Feb 23, 2010 at 02:06:41PM +0100, Markus Hochholdinger wrote:
> Here is my solution to this problem, lenny xen kernel:
> * dom0 with clocksource=jiffies and /proc/sys/xen/independent_wallclock=0
> * domU with clocksource=jiffies and /proc/sys/xen/independent_wallclock=0
> * ntpdate/ntp only in dom0, NOT in the domUs
> 
> I tested it the following way:
> While changing the time in the dom0 with date and/or hwclock doesn't change 
> the time in the domU. But changing the time in the dom0 with ntpdate/ntpd 
> changes the time in the domU.
> While ntpd is running in the dom0, I can change the time in the domU with date 
> ("hwclock --show" in the domU prints nothing), but within 5 minutes, the time 
> in the domU will be automatically corrected to the dom0 time. It seems ntp 
> does this, because if i don't have a ntpd running in the dom0, the changed 
> time in the domU wouldn't correct itself.
> 
> Hope this helps somebody. If this setup is stable for me for a few weeks, 
> perhaps I'll write it to 
> http://wiki.debian.org/Xen#A.27clocksource.2BAC8-0.3ATimewentbackwards.27 .
> 
> I still don't understand why ntp on the dom0 can "change" the time in the 
> domU, while date and hwclock doesn't!? But it works as I expect it.

It would be nice if you could add this information to 
http://wiki.debian.org/Xen 

Cheers,
        Moritz