[CentOS-virt] ntp on kvm

Tue Jul 21 08:42:12 UTC 2009
Nigel Metheringham <nigel.metheringham at dev.intechnology.co.uk>

On 21 Jul 2009, at 00:41, Johnny Tan wrote:
> * Setting independent_wallclock=1 (or is that a xen-only thing)?
[Can't comment - I'm not a Xen user]

> * Passing kernel parameters? If so, which? The vmware KB
> mentions "notsc divider=10" -- is that vmware-specific?

The divider=10 will help - reduces the number of timer interrupts  
through the system which tends to mean it gets itself less confused  
when counting them :-) [problem tends to be that the interrupts get  
batched due to the guest not being running all the time].

> * Just don't run ntpd on virtual guests? And let it get its
> time from the host.

VMware BP is now to run local ntpd - up to last year or so it was use  
the correction from vmtools (which I never found to work well).   
However VMware does have this back channel through vmtools to set the  
time, so that will only work well under Xen if it has an equivalent.

My suggestions, from my position of complete ignorance on Xen (but I  
do quite a lot under VMware) is to tweak your kernel params  
(definitely divider, maybe some other tweaks depending on  how the  
virtual hardware appears, but on our systems we use clocksource=acpi_pm)

> The last seems like the easiest/cleanest, if it really does
> always get its time from the host.

Linux time generally picks up from the RTC on boot, and then relies on  
counting interrupts (gross simplification alert!).  In a virtualised  
environment where your clocking is being externally tampered with this  
does not work well.

Hope that this helps but you have opened a very messy can of worms here!

[ Nigel Metheringham             Nigel.Metheringham at InTechnology.com ]
[ - Comments in this message are my own and not ITO opinion/policy - ]