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

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makestep is an command to be run when chrond is started. When started it is
probably that the clock, whether on a server, a laptop or whatever, is out.
So, instead of trying to fix a huge offset at the startup of chronyd by
slewing the clock, it says to step the clock if it is out by over 1 sec. on
the first three updates.

The skew is the undertainty in the rate of the clock compared with the ntp
source. If larger than 100PPM uncertainty (not rate but uncertainty in the
rate) do not use that source to determine the rate.

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On Mon, 2 Jul 2018, Pedro Côrte-Real wrote:


I'm a user of Ubuntu both on servers and desktops/laptops. With Ubuntu
18.04 switching to chrony by default I decided to use that on all
machines. When setting up the roll-out to all machines using puppet I
checked the default config to see if I neded to do any changes. Two
lines stood out:

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"

No idea where you got "below 100"  If your clock is out by say 100 years, it
is also stepped.

It seems at least the second line is a recommended config:

This seems very strange to me as a default for an NTP tool. I have two
main use cases that I assume are common:

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.

The 3 means "in the first three clock measurements". If the server is always
on, then it will not be "in the first 3 measurements" situation so this is
irrelevant. If you just set up your server and just switched it on, that
server's clock could well be out by a huge amount, and you do not want to wait
while chrony tries to slow away 100 years of offset by changing the clock rate
by the max of 10%. It would take 1000 years to do so.

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?

Both are reasonable defaults. Most would have something like
makestep .1 3 which would step the clock if out by .1 sec in the first three measurements.



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