| Re: [chrony-dev] can chrony adjust H/W clock too ? |
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Well actually it is already in the docs. In the beginning of the docs it
explicitly states that one of the advantages of chrony over ntp is that it can
keep track of the rtc errors, and compensate for them. There are also various
rtc commands and files described.
Certainly rtc is NOT the primary purpose of chrony -- it is the disciplining
of the system clock. But the docs do point out that rtc is pretty important to
chrony.
On Tue, 20 Apr 2010, Ed W wrote:
On 20/04/2010 19:08, Bill Unruh wrote:
A followup note-- If you use hwclock while using chrony, chrony will get
very
confused in its calculation of the rate of the rtc.(real time clock) since
there are these steps being imposed from outside. Now, you do not need to
use
the chrony rtc control (get rid of the rtcfile directive in chrony.conf).
and that is probably best if you use hwclock as you do.
Do you think the docs could highlight this please? I guess it's obvious once
you know, but my initial config was using both rtcfile and hwclock (one is
distro supplied, the other seemed like a good idea...). In practice the
error isn't gross enough in my situation that the error was very obvious...
Actually most distros run hwclock on shutdown and set the rtc from the system
clock on shutdown. This of course destroys the attempt by chrony to keep track
of the rtc. (there is also a kernel mode where the rtc is set from the system
clock every 11 min, which chrony has to go to some effort to switch off.)
However, my personal opinion is that the advantage of using chrony for rtc and
of the new hwclock is marginal. The latest hwclock also try to estimate the
drift of the rtc and compensate for it. They use a long term average ( rather
than musch more short term that chrony uses) and one could argue that if you
shut down for long times ( hours, days) a long term average is better than a
short term one. Ie, the advantage chrony has WRT rtc is not great. ( Of course
it is not clear that the distros actually use the new hwclock, rather than the
old version with exactly the same name due to disputes over licenses etc.)
Some docs stating the obvious would be cool?
Cheers
Ed W
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William G. Unruh | Canadian Institute for| Tel: +1(604)822-3273
Physics&Astronomy | Advanced Research | Fax: +1(604)822-5324
UBC, Vancouver,BC | Program in Cosmology | unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 | and Gravity | www.theory.physics.ubc.ca/
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