Re: [ntp:questions] NTP Servers in virtual machines

[Rob Heemskerk]

Op maandag 23 juni 2014 15:54:13 UTC+2 schreef David Woolley:
 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.
 
Not sure what it's worth and how it is implemented on other virtualisation 
platforms but according to vmware docs they have a virtual tsc. It is set when 
the vm is powered on but when the vm is moved to a different host (or resumed 
more generally) it keeps its original power-on tsc rate. 

In practice, on vmware, I see that the system clock jumps a few hundred of ms 
after a move to another host. Maybe because of delays in the process of setting 
the system clock to the clock of the new vmware host on resume.

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[ntp:questions] NTP Servers in virtual machines

[Rob Heemskerk]

Hi,

Why are NTP Servers running on virtualized hardware (vmware) unsuitable to 
serve time to clients?

I've read this statement several times but can't find a good motivation. I've 
searched the official documentation, FAQ, the NTP support wiki, this news 
group, google search.

I found this in the support wiki but it seems outdated: 
NTP server was not designed to run inside of a virtual machine. It requires a 
high resolution system clock, with response times to clock interrupts that are 
serviced with a high level of accuracy.  

As kernels are tickles nowadays response times to clock interrupts and 
(interrupt backlogs) do not seem to be relevant anymore.  

Could we say it is safe to run ntp servers on a virtualized platform or do we 
still need a few (4?) dedicated pieces of hardware to run our internal NTP 
servers?  


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