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

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On Sat, 3 Sep 2011, Ed W wrote:


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
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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