On 26/04/13 07:57, staticsafe wrote:
> On 4/25/2013 19:50, Alan McKinnon wrote:
>> On 26/04/2013 01:42, William Kenworthy wrote:
>>> Does anyone know a good guide to using time sync in VM's, for both
>>> windows and linux (gentoo) guests using libvirt?  Especially for guests
>>> that are resumed, or the whole virtualisation system is hibernated? (ntp
>>> refuses to resync after guest pause/save/restore/resume (known problem),
>>> even with "tinker panic 0"
>>
>>
>> That's not a bug, it's by design.
>>
>> If ntpd detects the clock is out by more than X seconds [1], it will not
>> try to correct the difference, concluding that something is wrong and a
>> human must decide. It can't easily tell the difference between a resumed
>> guest (or even that it was resumed at all) and a severe problem.
>>
>> We fixed this by taking the easy route of least resistance;
>>
>> 1. run ntpdate on startup/restart once before ntpd starts
>> 2. start ntpd as normal
>> 3. a colleague wrote a $MAGIC_HOOK to detect resumed guests that runs
>> ntpdate once
>>
>> True, it's a brutal solution and uses a baseball bat where some finesse
>> might be less ugly, but it suits our needs just fine.
>>
>> [1] I forget what X is and am too lazy to look it up. Is it 30 seconds
>> or thereabouts?
>>
>>
> 
> "When first started, the daemon normally polls the servers listed in the
> configuration file at 64-s intervals. In order to allow a sufficient
> number of samples for the NTP algorithms to reliably discriminate
> between correctly operating servers and possible intruders, at least
> four valid messages from the majority of servers and peers listed in the
> configuration file is required before the daemon can set the local
> clock. However, if the difference between the client time and server
> time is greater than the panic threshold, which defaults to 1000 s, the
> daemon will send a message to the system log and shut down without
> setting the clock." [0]
> 
> [0] - http://doc.ntp.org/4.1.1/debug.htm
> 


Keep reading :)

Check out "tinker panic o" I mentioned, or the -g argument to ntpd

The docs say its a "once only" adjustment in one place, but I am not
sure thats actually the case.

BillK



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