| Re: [chrony-users] Chrony source selection algorithm documentation |
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On Thu, Mar 10, 2016 at 07:58:09PM +0100, Henrik Bieler wrote:
> I still would like to know a way of preferring the ntpd over the gps
> when they are online. And switching back to gps when they go offline.
>
> Maybe someone can explain how chrony selects sources or point to
> documentation.
What stratum do the NTP servers have? My guess is that the NTP sources
are considered as unsynchronized for having stratum larger than the
local clock. This is a requirement of the NTPv3 protocol which
prevents synchronization loops (see "test 7" in RFC 1305). It was
later removed in NTPv4 (implemented in chrony >= 2.0), which made it
more vulnerable to synchronization loops, but allows synchronization
to sources with larger stratum.
In your case, if chronyd (version 1.30) selected the GPS, it is
stratum 1 and will ignore sources with stratum 2 or larger for a
possibility they are directly or indirectly synchronized to it. You
can check the measurements log to see if TEST7 is failing.
In order to outvote the GPS source as a falseticker you would need at
least two stratum 1 servers. Considering that less than 10% of the
pool.ntp.org servers are stratum 1, I think it's quite unlikely to
happen. My suggestion would be to use two or three stratum 1 servers
with open access from the list of servers maintained at the NTP
website instead of the pool.
http://support.ntp.org/bin/view/Servers/StratumOneTimeServers
--
Miroslav Lichvar
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