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/