| Re: [chrony-dev] [RFC] Transparent fallback from NTP reference clock to RTC |
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- To: chrony-dev@xxxxxxxxxxxxxxxxxxxx
- Subject: Re: [chrony-dev] [RFC] Transparent fallback from NTP reference clock to RTC
- From: Miroslav Lichvar <mlichvar@xxxxxxxxxx>
- Date: Mon, 7 Jul 2025 09:12:23 +0200
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On Thu, Jul 03, 2025 at 06:53:19PM +0000, Christoph Schittel wrote:
> > > Can you please explain the use case? >
>
> At the time the limit of 16s was implemented in the kernel, this was an
> reasonable choice. Today for many applications even a (possible) deviation
> of 1s or less may be regarded as too big and the system should be signaling
> it by setting STA_UNSYNC. Reading David R. Mills documentation reads like
> the syncing service should set STA_UNSYNC. The kernel setting it, when
> maxerror reaches 16s was just meant as last resort.
Applications that need to know the maximum error should examine the
timex maxerror directly. Different applications running on the same
system may have different requirements, so one threshold in chrony to
set the flag globally could not work for all applications.
In this thread it was about setting the flag in order to disable the
kernel 11-minute system clock->RTC copy process to avoid a synchronization
loop when the system clock is synchronized to RTC by chronyd.
> (Remote) monitoring services could benefit, because hosts with sync problems
> could be very easily detected. E.g. NAGIOS instead monitors the clock
> difference between itself and remote hosts. This implies the monitoring host
> has the "right" time. This could be improved by reading remote hosts
> STA_UNSYNC, if it will be set when needed (which should be the admin's
> choice, not the kernel's).
NTP has root delay + dispersion (combined as root distance) for
estimating the maximum error. That's what should normally be
monitored, not a flag.
--
Miroslav Lichvar
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