Marco, just some basics:
The reference implementation of NTP (ntpd) polls the reference time sources in cyclic intervals, filters the replies, and tries to disciplined the sysrtem clock such that the determined time offset *and* clock drift are minimized. However, this only works well if the *undisciplined* system time is increasing in a stable way.
In a VM the virtualized timer ticks may be stable (i.e. occur in constant intervals) , or not, depending on the type and version of the virtualization software.
For example, the VMware folks have spent quite some effort to get timekeeping inside their VMs working very good, so you can use ntpd inside the VM to get the system time synchronized pretty accurately.
In *earlier* versions of VMware this did *not* work very well, so the suggested solution at that time was to use VMware's built-in timekeeping mechanisms to keep the time inside the VMs synchronized.
If a built-in mechanism is used to synchronize the time inside a VM then the resulting accuracy depends on how this has been implemented by the maintainers of the virtualization software. In worst case the VM's system time is simply stepped in periodic intervals.
I've recently run a test under VMware with ntpd running under Linux inside a VM where the reference time source was a GPS PCI card installed in the physical machine was passed through to the VM. The loopstats showed that the time offset determined by ntpd was below +/- 5 microseconds, which I found pretty good.
On the other hand, we had discussions here with a different virtualization software (not kvm) where you obviously had no chance to use ntpd successfully since the system time itself had much too much jitter.
Unfortunately I haven't tried kvm, yet, so I can't tell how good accurate timekeeping is supported by kvm.
Martin -- Martin Burnicki Meinberg Funkuhren Bad Pyrmont Germany _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
