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doc: improve FAQ

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Miroslav Lichvar 2020-10-05 13:09:29 +02:00
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@ -428,8 +428,7 @@ are marked with the _*_ or _+_ symbol in the report printed by the `sources`
command.
When the best source (marked with the _*_ symbol) becomes unreachable (e.g. NTP
server stops responding), or the measurements are no longer accepted (e.g. a
change in network routing adds a delay), `chronyd` will not immediately switch
server stops responding), `chronyd` will not immediately switch
to the second best source in an attempt to minimise the error of the clock. It
will let the clock run free for as long as its estimated error (in terms of
root distance) based on previous measurements is smaller than the estimated
@ -437,11 +436,35 @@ error of the second source, and there is still an interval which contains some
measurements from both sources.
If the first source was significantly better than the second source, it can
take many hours (up to 64 times the maximum polling interval) before the second
source is selected. If you do not like this behaviour, you can force a faster
reselection by increasing the clock error rate (`maxclockerror` directive),
shortening the polling interval (`maxpoll` option), or reducing the number of
samples (`maxsamples` option).
take many hours before the second source is selected, depending on its polling
interval. You can force a faster reselection by increasing the clock error rate
(`maxclockerror` directive), shortening the polling interval (`maxpoll`
option), or reducing the number of samples (`maxsamples` option).
=== Does selected source drop new measurements?
`chronyd` can drop a large number of successive NTP measurements if they are
not passing some of the NTP tests. The `sources` command can report for a
selected source the fully-reachable value of 377 in the Reach column and at the
same time a LastRx value that is much larger than the current polling interval.
If the source is online, this indicates that a number of measurements was
dropped. You can use the `ntpdata` command to check the NTP tests for the last
measurement. Usually, it is the test C which fails.
This can be an issue when there is a long-lasting increase in the measured
delay, e.g. due to a routing change in the network. Unfortunately, `chronyd`
does not know for how long it should wait for the delay to come back to the
original values, or whether it is a permanent increase and it should start from
scratch.
The test C is an adaptive filter. It can take many hours before it accepts
a measurement with the larger delay, and even much longer before it drops all
measurements with smaller delay, which determine an expected delay used by the
test. You can use the `reset sources` command to drop all measurements
immediately (available in chrony 4.0 and later). If this issue happens
frequently, you can effectively disable the test by setting the
`maxdelaydevratio` option to a very large value (e.g. 1000000), or speed up the
recovery by increasing the clock error rate with the `maxclockerror` directive.
=== Using a PPS reference clock?