Re: [chrony-users] Defaulting to stepping the clock

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On Mon, Jul 02, 2018 at 01:29:32PM +0100, Pedro Côrte-Real wrote:
> maxupdateskew 100.0
> makestep 1 3
> If I'm reading the documentation correctly this means "step the clock
> in the first three corrections if the step is above one second but
> below 100"

That 100.0 is actually a limit for the estimated error in frequency
and is not related to the offset.

> 1) For a server never step the clock and if the drift is large
> complain loudly because something has gone very wrong. Servers are
> always on and should be always syncing so if their clock drifts a lot
> something has gone wrong.
> 2) In a desktop/laptop stepping the clock is probably always ok if
> going forward but may be bad if going back. So just accept frequency
> adjustments both going forward and backwards. Machines are turned
> on/off, suspend/resume and so it's less important to complain loudly.
> Instead maintain monotonic clocks that are synchronized quickly even
> if their frequency needs to shift a log.
> Given all this why were these defaults chosen? Are there recommended
> settings to approximate 1) and 2)? Is the recommendation to do
> something else?

The recommended configuration is supposed to minimize the number of
people complaining that NTP does not work. I agree that for most
people it would be better to completely disable stepping of the clock.
But those few that have a computer which for some reason can start
with system time far from the true time, it would be a big problem.
They would need to figure out what's going on and use the chronyc
makestep command to fix it. That's not a good user experience.

I don't see a big difference between servers and desktops/laptops.
What matters is the RTC. On real HW it needs to be present, have a
battery, must not drift too much and the local/UTC setting must be
correct. Similar requirements apply to virtual machines. It needs to
work with suspend/resume and migrations.

Even if we could be assume that everything always works as expected,
someone or something would need to set the RTC for the first time.
When you bought a new server/desktop/laptop, did it always come with
the clock correctly set?

Miroslav Lichvar

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