Re: [chrony-users] Control when the driftfile is written

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

On Thu, 20 Feb 2014, aobrien@xxxxxxxxxxxxxxx wrote:

> Quoting Bill Unruh <unruh@xxxxxxxxxxxxxx>:
>
> > On Thu, 20 Feb 2014, Andrew O'Brien wrote:
> >
> > > Hello list,
> > > 
> > > I'm interested in running chrony on an embedded device where the 
> > > filesystem is normally mounted read only.  My plan was to have a cron 
> > > job remount rw and ask chrony to write the driftfile but I can't seem to 
> > > figure out that last part.
> >
> > Well, you could have a small filesystem mounted in ram as a tmpfs, and then
> > have a cron job remount the disk/flash based filesystem rw and copy the 
> > drift
> > file over from the tmpfs filesystem.
> >
> > > 
> > --
> > William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
> > Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
> > UBC, Vancouver,BC  |   Program in Cosmology |     unruh@xxxxxxxxxxxxxx
> > Canada V6T 1Z1     |      and Gravity       |  www.theory.physics.ubc.ca/
>
> That's a good idea but what I was worried about was that chrony would only 
> write the file when it was shutting down.  As the device is normally just 
> powered off (hence the ro filesystem) it would never get written.
>
> Does anyone know if it writes the file more often than that?

Once an hour

From reference.c

 if (drift_file) {
    /* Update drift file at most once per hour */
    drift_file_age += update_interval;
    if (drift_file_age < 0.0 || drift_file_age > 3600.0) {
      update_drift_file(abs_freq_ppm, our_skew);
      drift_file_age = 0.0;
    }


So if you had a cron job to write it to permanant from temporary say once and
hour, or even once a day, it would not be too old and should be a reasonable
estimate of the drift.

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--
William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
UBC, Vancouver,BC  |   Program in Cosmology |     unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1     |      and Gravity       |  www.theory.physics.ubc.ca/