[Dovecot] "Time just moved backwards" in Dovecot in a Xen DomU

[Patrick Domack]

Hmm, I have been running dovecot inside xen for almost 3 years now  
without any time issues. I checked my logs and I have no ntp time  
reset messages for the last month.

I think it's more possible ntp is stepping the time instead of slewing  
it (http://www.ntp.org/ntpfaq/NTP-s-algo.htm section 5.1.1.4), OR, the  
ntp servers your using or network connection you have are giving you  
lots of jitter. I personally try to pick 3 good low jitter, low  
latency servers and 1 higher latency. Only running ntpd on the dom0  
and nothing on the domU's.

Hmm, checking the lots on my home machine, even it doesn't have a time  
reset log message, and it's network can be overloaded for hours at a  
time.

Quoting Thierry DOSTES <tdostes at ibsm.cnrs-mrs.fr>:

> Hi,
>
> Here comes an extract from Debian Wiki about Xen to allow a domU to  
> keep its own time :
>
> ----
> (...) your domU is likely using the xen clocksource instead of its  
> own clock ticks. In practice, this seems to be the cause of  
> infrequent lockups under load (and/or problems with suspending). A  
> workaround is to decouple the clock in the domU from the dom0:
>
> In your dom0 and domU /etc/sysctl.conf add the line:  
> xen.independent_wallclock=1. On the dom0, edit the configuration  
> file of the domU (e.g. /etc/xen/foobar.cfg and add (or expand) the  
> extra-line: extra="clocksource=jiffies".
>
> These settings can be activated without rebooting the domU. After  
> editing the configuration files, issue sysctl -p and echo "jiffies">  
> /sys/devices/system/clocksource/clocksource0/current_clocksource on  
> the domU prompt.
>
> Because the clock won't be relying on the dom0 clock anymore, you  
> probably need to use ntp on the domU to synchronize it properly to  
> the world.
> ----
>
> Hope this helps.
>
> Thierry.
>
>
> PGNet Dev a écrit :
>> hi,
>>
>> On Mon, Oct 5, 2009 at 4:02 PM, Timo Sirainen <tss at iki.fi> wrote:
>>
>>> And no ntpd in your DomU?
>>>
>>
>> nope.
>>
>> service ntp status
>>  Checking for network time protocol daemon (NTPD):   unused
>>
>>
>>>> any suggestions as to what/how to fix?
>>>>
>>> If no one here can give you a good answer, I'd try some Xen mailing
>>> list. I'm sure a lot of people are running Dovecot in Xen without time
>>> problems. If you do find out the problem, please let us know also.
>>>
>>
>> i've been poring over the lists ... found nothing yet :-/
>>
>