Re: [chrony-users] Fallback clock on event "Can't synchronise: no reachable sources"

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On Fri, Feb 07, 2014 at 02:10:32PM +0100, Ulrich Schwesinger wrote:
> I have a system with a write protected system partition and an NTP
> server running within a local network. Sporadically, chrony reports
> the above mentioned error.
> The error persists maybe within a time range of 20-30 seconds.
> Within that time interval, the system time seems to diverge/drift by
> a couple of 10 milliseconds, which seems unreasonable large for me.

How did you measure the error in that period?

> (a) What is chrony exactly doing upon the event "Can't synchronise:
> no reachable sources". It does not seem to keep the current system
> time. Is it falling back on hardware clock maybe?

No, nothing with RTC. It should keep the system clock as it was set on
the last update unless chronyd is configured with the fallbackdrift

> (b) Or might something be wrong with the drift estimate, causing
> this large divergence during the short time interval

Did it switch to another source before the "no reachable sources"
message? If there are multiple sources and the worst source becomes
unreachable as last, the clock will be set by the measurement from the
worst source, possibly introducing a large time or frequency error.

> (c) Are there some files I could look at the trace back what's
> happening within these time intervals?

The tracking log would be very helpful here. You can enable logging
in chrony.conf like this:

logdir /var/log/chrony
log measurements statistics tracking

Miroslav Lichvar

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