Re: [chrony-users] chronyc tracking question

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Wed, Apr 19, 2017 at 02:34:21PM -0400, Chris Perl wrote:
> I'm trying to figure out what metric(s) from `chronyc tracking' I
> should be logging and graphing to answer the question of "how far off
> is my system from its reference".

> The System time field from `chronyc tracking' represents the
> difference between what chrony believes the "true" time to be and the
> system time.  But, chrony is constantly updating its notion of what
> the "true" time is.  In the absence of updates, it simply does its job
> and drives that difference to 0.

Yes. That's a good description. If you would like to improve the
manual pages, I'll gladly accept patches :).

> If that is true, does that mean that I should be graphing the "System
> time", but that I need to be sure I've got sufficiently fresh samples
> to make sure the notion of the "true" time is correct?  Or, should I
> just be graphing the "Last offset", or perhaps something else
> entirely?

The root distance (root delay / 2 + root dispersion) is the
uncertainty of the "true" time, which is increasing when no updates of
the clock are made. You can graph "system time" +/- "root distance" to
show the maximum assumed error of the clock at given time.

Graphing "last offset" can be useful to show the stability of the
synchronization and estimate the minimum error of the clock.

-- 
Miroslav Lichvar

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