[ntp:questions] once 'ntpd' stops, does the drift file value continue to get used for clock adjustments?

Steve Kostecke kostecke at ntp.org
Fri Mar 18 01:43:46 UTC 2011

On 2011-03-17, Gautam Thaker <gautam.h.thaker at lmco.com> wrote:

> I have a stable drift value on my fedora 12 node and everything is
> running well. (offsets are well below 1msec). If I now stop 'ntpd'
> will my time start to drift relative to the 'correct time' by the PPM
> value in the drift file?

The value in the "drift file" is a snapshot of the frequency correction
being applied to the system clock. The contents of the drift file are
updated approximately hourly.

The purpose of the drift file is to provide ntpd with an approximate
frequency correction (somewhere between -500 and +500) for use at
startup. This "ballpark" correction is close enough to allow ntpd to
quickly adjust the clock.

> Or is it that ntpd has already set things up so that corrections as
> per last value of drift file will continue indefinitely?

When ntpd stops the clock continues slewing at the last rate and
direction requested.

> If it is the latter and if indeed my drift value is quite good (as
> ntpd had run for many days already), it seems I can go for many more
> days w/o ntpd and not get very much out of sync at all.

ntpd continuously steers (i.e. slews) the clock to bring it as close as
possible to the apparent correct time. These adjustments compensate for
variations in clock behavior which are induced by factors such as system
load and environmental conditions.

Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/

More information about the questions mailing list