|Re: [chrony-users] Chrony using RTC to resynch system time: Not a useful feature ?|
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On Sat, 3 Sep 2011, thomas.schmid@xxxxxxxxx wrote:
In my project we were planning to use the "rtcfile" feature of chrony to resynchronize the system
time from the RTC using its RTC-measurements it has taken previously. Now there is some doubt
if that is an useful feature at all:
1. Current kernels seem to update the RTC when they find that the system time is NTP-controlled.
That has been going on for about 10 years by now. That is why chrony keeps the
kernel clock in its unsynchronized state, since the 11 min rule only applies
if the kernel clock is labeled as synchronized.
IMO doesn't that render chrony's RTC measurements useless because of this external influence?
It would if chrony did not have ways to avoid it. Note the same applies to
running hwclock to set the rtc on shutdown which many distributions use in
their shutdown scripts. You have to disable that.
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) ?
Not at all sure where this info is from. The main problem is that the rtc on a
shutdown computer is cold, and on a running computer is warm. This means that
the rtc drift rate is different on shutdown than estimated during running.
However this is probably only a few PPM (ie a few seconds per month of shutdown)
Ie chrony will get rid of the gross rtc drift rate. but you should not trust
it to better than a few PPM.
If you have evidence that the battery vs power performance of the rtc differs
(above and beyong temperature effects) that would certainly be interesting to
Note that hwclock (in at least one of its versions, as I believe there are two
different streams out there) can estimate the drift rate of the rtc as well.
3. In the light of point 1, what is the real use for chrony's "rtcsync" on modern kernels ? According
See my answer to point 1
to our dev, the feature of the "kernel synching the RTC duringNTP-synched system time" is not
something you can turn off or on. Or maybe I just do not understand the feature :-) ?
Yes, you can. You keep the kernel clock sychronized flag off.
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