[CentOS] Xen clock drift

Luke Dudney listmail at lukedudney.com
Wed Jan 9 15:41:18 UTC 2008


On 08/01/2008 15:15, Brian Mathis wrote:
>> From: Jack Bailey <jack at internetguy.net>
>> Date: Jan 8, 2008 3:17 AM
>> To: centos at centos.org
>>
>> Hello All,
>>
>> Consider a CentOS-5.1 Xen server (2.6.18-53.1.4.el5xen) hosting two
>> domains running CentOS-5.1 (2.6.18-53.1.4.el5).  One domain has a fairly
>> accurate clock, the other domain has a clock that gains ungodly amounts
>> of time, roughly one minute every two or three minutes.  For a fix, one
>> suggestion is to run this command in DomU:
>>
>> echo 1 > /proc/sys/xen/independent_wallclock
>>
>> This didn't change anything.  As an experiment, I wrote a script to call
>> ntpd -q, sleep 60, and repeat indefinitely.  Here are a couple of
>> snippets of output:
>>
>> goodclock# ksh ./xenclockdrift
>> ntpd: time slew +0.001211s
>> ntpd: time slew +0.001200s
>> ntpd: time slew +0.001855s
>> ntpd: time slew +0.001532s
>> ntpd: time slew +0.001603s
>> ntpd: time slew +0.001320s
>> ntpd: time slew +0.001931s
>>
>> badclock# ksh ./xenclockdrift
>> ntpd: time slew -0.000193s
>> ntpd: time set -57.356377s
>> ntpd: time slew +0.002352s
>> ntpd: time slew +0.003018s
>> ntpd: time set -57.417488s
>> ntpd: time slew +0.012089s
>> ntpd: time slew -0.000985s
>>
>> These domains are fully virtualized and set up identically, except
>> "badclock" is allocated two processors versus one processor for
>> "goodclock".  DomU's clock is running normally.
>>
>> Anyone know what's going or know how to fix it?
>> Jack
>>     
>
>
> I'm not sure with Xen, but on VMWare one should not be using NTP at
> all.  Time syncing should be done with the vmware tools and the host,
> with NTP only running on the host (not the guests).  Using NTP on a
> vmware system will result in similar behavior to what you are seeing
> here.

At least, that is the theory. Empirically, using the vmware tools time 
sync feature will push a slow VM's clock forwards, but it won't push a 
fast clock backwards. I'm yet to see a "best practice" for ensuring 
proper time synchronisation within VMware VMs, but for now, NTP seems 
the best option.

cheers
Luke





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