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doc: add new questions to FAQ

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Miroslav Lichvar 2018-05-21 14:49:53 +02:00
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----
=== Using a Windows NTP server?
A common issue with Windows NTP servers is that they report a very large root
dispersion (e.g. three seconds or more), which causes `chronyd` to ignore the
server for being too inaccurate. The `sources` command may show a valid
measurement, but the server is not selected for synchronisation. You can check
the root dispersion of the server with the ``chronyc``'s `ntpdata` command.
The `maxdistance` value needs to be increased in _chrony.conf_ to enable
synchronisation to such a server. For example:
----
maxdistance 16.0
----
=== Using a PPS reference clock?
A pulse-per-second (PPS) reference clock requires a non-PPS time source to
determine which second of UTC corresponds to each pulse. If it is another
reference clock specified with the `lock` option in the `refclock` directive,
the offset between the two reference clocks must be smaller than 0.2 seconds in
order for the PPS reference clock to work. With NMEA reference clocks it is
common to have a larger offset. It needs to be corrected with the `offset`
option.
One approach to find out a good value of the `offset` option is to configure
the reference clocks with the `noselect` option and compare them to an NTP
server. For example, if the `sourcestats` command showed
----
Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev
==============================================================================
PPS0 0 0 0 +0.000 2000.000 +0ns 4000ms
NMEA 58 30 231 -96.494 38.406 +504ms 6080us
foo.example.net 7 3 200 -2.991 16.141 -107us 492us
----
the offset of the NMEA source would need to be increased by about 0.504
seconds. It does not have to be very accurate. As long as the offset of the
NMEA reference clock stays below 0.2 seconds, the PPS reference clock should be
able to determine the seconds corresponding to the pulses and allow the samples
to be used for synchronisation.
== Issues with `chronyc`
=== I keep getting the error `506 Cannot talk to daemon`