[ntp:questions] ntp or other time stuff on virtual server?

David Woolley david at ex.djwhome.demon.co.uk.invalid
Mon Nov 17 21:58:09 UTC 2008

Unruh wrote:
> A question has come up on another newgroup whethr or not the time can be
> disciplined via ntp (or chrony) on a virtual server. I would suspect not
> (imagine the mess if both ntp on one virtual server and chrony on another
> were both trying to discipline the same hardware clock, and I cannot see
> how the virtual OS could keep track of its own separate clock since it only
> gets some subset of the time slices on the real system) but perhaps I am
> wrong. Can a virtual server discipline its own clock? If not, how is the
> adjtimex system call trapped? Surely each virtual machine cannot influence
> the real system, which is what would happen if the adjtimex call on each
> virtual machine were passed to the adjtimex system call on the real
> machine. 
You should discipline the host and run vmware tools or equivalent on the 
guests.  You should not run anything with tight timing requirements on a 
guest, as the guest's time will have large short term variations because 
the host will dish out time roughly in proportion to the CPU it gives.

A link to the VMWare paper on this was posted last week.

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