[ntp:questions] ntpd -q and driftfile

David J Taylor david-taylor at blueyonder.co.uk.invalid
Tue Mar 22 19:57:44 UTC 2011

"unruh" <unruh at wormhole.physics.ubc.ca> wrote in message 
news:slrniohsae.ksv.unruh at wormhole.physics.ubc.ca...
> I have seen the drift out by 400PPM at around the hour mark. ntpd
> massively overswings to correct any intial error in the clock ( say a
> few ms) , and then gradually
> settles back  toward the correct drift rate. Thus the drift printed into
> the file during that transient really has little relation to the true
> drift rate, and is not very helpful.

I have seen the drift become unstable, but not after just starting ntpd, 
and not recently.  Perhaps there was a bug which is now fixed?

> Now, if the clock is close to the correct time, and the system had a
> reasonable drift rate in the file before hand, then the drift rate
> published after an hour may well be pretty close to the right rate. But
> if the clock was say out by 50ms, then it could get pretty hairy.
> This problem is caused by ntp having no memory, except for the drift
> rate and the current time. It thus cannot use the past history of
> offsets to make a better estimate of what the true drift rate is. The
> drift rate it has now is a sum of the true drift rate and the drift
> rate being used to correct the clock offset. While with history, it
> would be trivial to calculate the true drift rate and disentangle the
> two ntpd does not do that. (chrony does).

Perhaps "if" is the operative word here - I have not seen this to be a 
problem on my systems within the last year or two, and they aren't 
rebooted very often.


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