[gentoo-user] Re: Latest unstable ntp not generating ntp.drift file.

[Francesco Talamona]

On Thursday 06 January 2011, Dale wrote:
> Francesco Talamona wrote:
> > On Wednesday 05 January 2011, Dale wrote:
> >> Now on my old system, it would adjust the drift file and the
> >> adjustments  would get smaller and smaller.  On the new rig, as
> >> you can see it stays about the same.  I would like it to get to a
> >> point where it doesn't have to sync so often.  I read on the
> >> website where they are needing more servers to help with the load
> >> and I don't want to be one of the ones putting a load on it.
> >
> > Maybe you copied over /etc/adjtime from the previous machine. I
> > would try to regenerate it...
> >
> > Ciao
> >
> > 	Francesco
>
> I'm sure I didn't copy that.  I copied ntp.conf but that is all.  I
> didn't even notice that one being there.  I got to see what purpose
> that has.
>
> I may delete it tho and see what happens.  It would generate a new
> one if I restart the service correct?
>
> Dale
>
> :-)  :-)

It makes the hardware clock take care of the systematic drift. If a 
wrong value is stored in it, it can interfere with ntp in the way you 
described: every time ntp runs it always corrects for the same amount.

From man 8 hwclock:

"The  Hardware  Clock  is usually not very accurate.  However, much of 
its inaccuracy is completely predictable - it gains or loses the same 
amount of time every day.
       This is called systematic drift.  hwclock's "adjust" function 
lets you make systematic corrections to correct the systematic drift."

and:
"It is good to do a hwclock --adjust just before the hwclock --hctosys 
at system startup time, and maybe periodically while the system is 
running via cron."

This is my "recipe":
let ntpdate sync your clock, then /sbin/hwclock --systohc
and you are done. From that moment on ntp takes care of the non 
systematic error, while the drift is zeroed "by hwclock".

Somewhere it is suggested to run /sbin/hwclock --adjust once a year.

I experienced what you describe when I built my new machine, I had 
copied /etc/adjtime (without knowing what it was) from the previous, it 
took me a good deal of googling...

BTW I don't have ntp.drift, I only use ntpdate and the clock is always 
correct.

HTH
	Francesco

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