[ntp:questions] R: Re: debugging strange ntp in virtual environment

David Woolley david at ex.djwhome.demon.invalid
Thu Sep 19 07:20:36 UTC 2013


On 19/09/13 06:41, Riccardo Castellani wrote:

> one the problem ? Virtualization ? I'd like to know if ntpd adjusts only OS
> time or it adjust hardware clock too.

ntpd only adjusts the software clock, however, the guest OS may well try 
to update the hardware clock.  Normal VM hosts will virtualise the 
hardware clock, so this should not affect the host.

The basic problems with running ntpd on VMs are that simulated real time 
can run at variable rates, compared with true real time, and there can 
be large scheduling delays, which compromise the accuracy of offset 
measurement.

Both can result in very large jitters and make it difficult for ntpd to 
work out the underlying clock frequency error.




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