[ntp:questions] running NTP as server only

folkert folkert at vanheusden.com
Wed Aug 18 21:01:24 UTC 2010


> > Is it possible to run the NTP daemon only as a server and not as a
> > local-clock maintainer?
> > Reason: I have a virtual machine which gets its time via the vmware
> > tooling from the hardware server it is running on. Now this virtual
> > machine needs to distribute the time to clients.
> Virtual machines make terrible timesources-- 10's to 100's of
> milliseconds of jitter are not unusual.

I don't think that is in all situations the case. Depends on the
scheduling by the hypervisor. iirc ibm pseries lpars don't have this
problem. That is why they have (x)ntp running in each of them normally.

> If you need to run ntpd on that specific hardware, run it in the
> "host ESX" or Xen's Dom0 instead, and not in one of the hosted
> virtual machines.

Yes, that is what I propose:
- let the hypervisor sync to a reliable accure timesource
- sync the vms to the hypervisor with some mechanism. e.g. on vmware you
  have the vmware tooling which runs in the vm and syncs time to the
  hypervisor (and also things like memory ballooning etc)
- let the vm then distribute the notion of time it got from the
  hypervisor to clients

Somewhere this week I'll test how this works: I put together what I was
asking, a program which picks the time from the local clock and then
"sends" this via ntp. Then I'll have two systems (which run directly on
hardware, not a vm) that will have a couple of low-stratum servers to
sync against as well as my vm. If they then declare my solution as a
falseticker and/or with a high jitter, I then know it won't works.

Folkert van Heusden

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