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doc: update FAQ

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Miroslav Lichvar 2018-08-29 16:23:42 +02:00
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doc/faq.adoc
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@ -171,6 +171,11 @@ network latency and stability of the system clock (which mainly depends on the
temperature sensitivity of the crystal oscillator and the maximum rate of the
temperature change).
Generally, if the `sourcestats` command usually reports a small number of
samples retained for a source (e.g. fewer than 16), a shorter polling interval
should be considered. If the number of samples is usually at the maximum of 64,
a longer polling interval may work better.
An example of the directive for an NTP server on the Internet that you are
allowed to poll frequently could be
@ -178,15 +183,15 @@ allowed to poll frequently could be
server foo.example.net minpoll 4 maxpoll 6 polltarget 16
----
An example using very short polling intervals for a server located in the same
An example using shorter polling intervals with a server located in the same
LAN could be
----
server ntp.local minpoll 2 maxpoll 4 polltarget 30
----
The maxdelay options are useful to ignore measurements with larger delay (e.g.
due to congestion in the network) and improve the stability of the
The maxdelay options are useful to ignore measurements with an unusally large
delay (e.g. due to congestion in the network) and improve the stability of the
synchronisation. The `maxdelaydevratio` option could be added to the example
with local NTP server
@ -194,17 +199,34 @@ with local NTP server
server ntp.local minpoll 2 maxpoll 4 polltarget 30 maxdelaydevratio 2
----
If your server supports the interleaved mode, the `xleave` option should be
added to the `server` directive in order to allow the server to send the
client more accurate hardware or kernel transmit timestamps. When combined with
local hardware timestamping, sub-microsecond accuracy may be possible. An
example could be
If your server supports the interleaved mode (e.g. it is running `chronyd`),
the `xleave` option should be added to the `server` directive in order to allow
the server to send the client more accurate transmit timestamps (kernel or
preferably hardware). For example:
----
server ntp.local minpoll 2 maxpoll 2 xleave
server ntp.local minpoll 2 maxpoll 4 xleave
----
When combined with local hardware timestamping, good network switches, and even
shorter polling intervals, a sub-microsecond accuracy and stability of a few
tens of nanoseconds may be possible. For example:
----
server ntp.local minpoll 0 maxpoll 0 xleave
hwtimestamp eth0
----
If it is acceptable for NTP clients in the network to send requests at an
excessive rate, a sub-second polling interval may be specified. A median filter
can be enabled in order to update the clock at a reduced rate with more stable
measurements. For example:
----
server ntp.local minpoll -6 maxpoll -6 filter 15 xleave
hwtimestamp eth0 minpoll -6
----
=== Does `chronyd` have an ntpdate mode?
Yes. With the `-q` option `chronyd` will set the system clock once and exit.