Re: [chrony-users] Chrony using RTC to resynch system time: Not a useful feature ?

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

On Sat, 3 Sep 2011, Ed W wrote:

> Hi
>
>
> > 1. Current kernels seem to update the RTC when they find that the system time is NTP-controlled.
>
> I believe chrony disables this unless you re-enable it again with the
> appropriate chrony.conf config (something like rtcsync?)
>
> >    IMO doesn't that render chrony's RTC measurements useless because of this external influence?
>
> Miroslav's very sensible response when I asked this was "measure it".
> Chrony can kick out measurements of the rtc deviation and you can
> observe how long this takes to stabilise on your system.  For me it was
> more than 9 mins

Or look at www.theory.physics.ubc.ca/chrony/chrony.html
where you can get at about 5 years of rtc rate graphs generated by chrony.

> > 2. RTCs seem to have completely different stability performance depending on if they are run from
> >    their own battery or if the PC is powered. So even if the kernel would not update the RTC while
> >    the system is running, the measurements are still invalid for a time correction based on a battery-
> >    run RTC (ie PC had been off for a while) ?
>
> This is sadly quite correct, but it may not be a totally useless
> correction to try and make
>
> Chrony tries to observe the RTC drift rate while the system is on.  It
> then has an option to set the initial system time to "RTC + estimated
> offset" when chrony is started.
>
> So during boot, modern kernels will set time to RTC, chrony later on
> during boot will set the time to RTC+offset.  The resulting time will be
> incorrect, but if the rtc drift while the system is off is closer to the
> drift while the system is on than zero, then you have a better estimate
> of current time than not using the offset...
>
> Likely this is really most interesting for:
> - Systems that are rebooted regularly
> - Don't set RTC to system time at shutdown
> - Have some limited network connectivity which prevents fixing up the
> clock at startup using ntp
>
>
> Probably rtcsync is a sane default for most modern machines that are on
> most of the time and regularly have network access at boot

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