Re: [chrony-users] questions about gps with ntp fallback

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On Tue, Nov 17, 2015 at 07:40:08PM -0500, James Knight wrote:
> server <ntpserver> minpoll 4 maxpoll 6 iburst
> refclock SOCK /var/run/chrony.ttyS1.sock refid PPS prefer iburst

The iburst option is valid only for server/pool lines. With refclocks
the polling interval is constant. I'm wondering why chronyd doesn't

> What I do not understand is what I observe when I unplug or power down
> my GPS receiver. If I simulate a GPS disconnect (ex. power failure,
> cable accidentally removed, etc.), I would expect that synchronization
> of the system will fallback to the NTP source over time. While it does
> appear to happen, sometimes it takes over an hour and a half to do so.

> I assume what is happening is that chrony perceives/calculates that
> the time information from the GPS source is still "more correct" than
> the NTP source even though it is disconnected. Only when the
> calculated time from the GPS source cannot be guaranteed better than
> the NTP source, it switches over. Is this assumption correct? If not,
> could someone explain what I am observing?

Yes, it's supposed to be a feature, not a bug :). It was added in
chrony-2.0 to avoid reselecting to a significantly worse source when a
refclock is temporarily unreachable. A source can remain selected for
as long as its samples are not older than the oldest sample from other
sources and the dispersion estimated at the current time is still
better. There can be up to 64 samples per source, so with an NTP
source using maxpoll 6, it could take up to 64*2^6 seconds before the
refclock source is unselected. 

Miroslav Lichvar

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