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doc: add section to FAQ on improving accuracy with NTP

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Miroslav Lichvar 2014-09-10 14:43:44 +02:00
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chrony.texi.in
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@ -4628,6 +4628,43 @@ On Linux, if @code{chronyd} is compiled with support for Linux capabilities
the `-u' option or @code{user} directive in the @file{chrony.conf} file to drop the `-u' option or @code{user} directive in the @file{chrony.conf} file to drop
root privileges after start. The configure option @code{--with-user} can be root privileges after start. The configure option @code{--with-user} can be
used to drop the privileges by default. used to drop the privileges by default.
@subsection How can I improve the accuracy of the system clock with NTP sources?
Select NTP servers that are well synchronised, stable and close to your network.
It's better to use more than one server, three or four is usually recommended as
the minimum, so @code{chronyd} can detect falsetickers and combine measurements
from multiple sources.
There are also useful options which can be set in the @code{server} directive,
they are @code{minpoll}, @code{maxpoll}, @code{polltarget}, @code{maxdelay},
@code{maxdelayratio} and @code{maxdelaydevratio}.
The first three options set the minimum and maximum allowed polling interval,
and how should be the actual interval adjusted in the specified range. Their
default values are suitable for public NTP servers, which normally don't allow
too frequent polling, but if you run your own NTP servers or have permission to
poll the servers frequently, setting the options for shorter polling intervals
may significantly improve the accuracy of the system clock.
The optimal polling interval depends on many factors, this includes the ratio
between the wander of the clock and the network jitter (sometimes expressed in
NTP documents as the Allan intercept), the temperature sensitivity of the
crystal oscillator and the maximum rate of change of the temperature. An
example of the directive for a server located in the same LAN could be
@example
server ntp.local minpoll 2 maxpoll 4 polltarget 30
@end example
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
synchronisation. The @code{maxdelaydevratio} option could be added to the
previous example
@example
server ntp.local minpoll 2 maxpoll 4 polltarget 30 maxdelaydevratio 2
@end example
@c }}} @c }}}
@c {{{ S:Computer is not synchronising @c {{{ S:Computer is not synchronising
@node Computer is not synchronising @node Computer is not synchronising