Hi-- On Aug 18, 2010, at 2:01 PM, folkert wrote: >> 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.
It depends on the hypervisor, the hardware, the workload, etc. > iirc ibm pseries lpars don't have this > problem. That is why they have (x)ntp running in each of them normally. The type of latency you might see from a lightly loaded box might well be a lot better than what you see from a virtual webhosting company which jams as many VMs as it possibly can before the clients yell about slow web performance... >> 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 I agree up to the last part. The quality of time you get from querying ntpd running in the hypervisor should be noticably / obviously better than what you would get from ntpd running in a VM. Set up both and compare the jitter your clients see for yourself. > 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. > Agree? Yes, testing is a fine idea. Regards, -- -Chuck _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
