[ntp:questions] NTP Servers in virtual machines

Rob Heemskerk rob.heemskerk at gmail.com
Wed Jun 25 09:43:51 UTC 2014


Op maandag 23 juni 2014 15:54:13 UTC+2 schreef David Woolley:
> On 23/06/14 13:12, Miroslav Lichvar wrote:
> 
> > I think it all depends on the VM implementation and what clocksource
> > is used in the guest. If the guest is using tsc (i.e. its frequency is
> > independent of the host clock), it will need to run its own NTP
> 
> 
> It will still be subject to potentially large scheduling delays between 
> NTP packet arrival and processing.  Also, unless you restrict VM to a 
> single host, the TSC could jump and change frequency when the VM is 
> moved.  If it is impossible to virtualise TSC, it is impossible to hide 
> those jumps.
> 
Not sure what it's worth and how it is implemented on other virtualisation platforms but according to vmware docs they have a virtual tsc. It is set when the vm is powered on but when the vm is moved to a different host (or resumed more generally) it keeps its original power-on tsc rate. 

In practice, on vmware, I see that the system clock jumps a few hundred of ms after a move to another host. Maybe because of delays in the process of setting the system clock to the clock of the new vmware host on resume.



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