[ntp:questions] NTP Servers in virtual machines

Rob nomail at example.com
Mon Jun 23 14:27:44 UTC 2014


David Woolley <david at ex.djwhome.demon.invalid> wrote:
> On 23/06/14 13:12, Miroslav Lichvar wrote:
>> I think it all depends on the VM implementation and what clocksource
>> is used in the guest. If the guest is using tsc (i.e. its frequency is
>> independent of the host clock), it will need to run its own NTP
>
> It will still be subject to potentially large scheduling delays between 
> NTP packet arrival and processing.  Also, unless you restrict VM to a 
> single host, the TSC could jump and change frequency when the VM is 
> moved.  If it is impossible to virtualise TSC, it is impossible to hide 
> those jumps.

In practice on a modern VMware environment the time will be within
about 2-3 ms for a Linux system.  On a VMotion it will jump a few
hundred ms and then regulate back.
When your NTP server is a Windows server and you want to be within 1
second you have different worries.

> Generally you are going to put ntpd on a VM because the IT manager 
> doesn't consider it very important.  If it is not important, he is 
> probably not going to worry about the effect of moving it around hosts.

That is right.



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