Re: [opensuse] chrony and hwclock



On Fri, 2011-07-15 at 12:44 +0200, Per Jessen wrote:
Roger Oberholtzer wrote:

I am exploring changing the way our remote openSUSE systems (not
connected to the internet) maintain proper time. We have GPS devices
in these system, but have thus far only used the time for our own
devious purposes. We have never tried to maintain the system time. The
popular wisdom on the gpsd list (all things related to GPS access on
Linux) is that one should use chrony (http://chrony.tuxfamily.org)
rather then NTP for this purpose. The reason given is that NTP does
not deal well with a time source that is perhaps seldom available. See
http://chrony.tuxfamily.org/FAQ.html#question_2.1

I guess it will depend quite a bit on on how you define "seldom" - as
long as access to a time source is regular, e.g. daily, I think the
local system will be able to keep the clock stable in between accesses.

That seems to be the point of chrony. Access to the time source need not
be regular. Obviously the longer it is not connected the more time will
suffer. I guess it assumes that the error in the real time clock is
basically linear. To the extent this is true, it could perhaps do a good
job. I worry that the error in the rtc may not be linear, especially if
(1) the system is disconnected from the power mains and runs on the PC
battery, or (2) temperature changes drastically (hot in summer and cold
in winter).

I define 'seldom' as being when the GPS is on and has a fix. For
example, if our vehicle is turned on in a garage, the GPS may not
provide PPS signals and a GPS-based time. It too is providing an
internal time maintained between satellite access. So, chrony may not
get usable info at these times. But, chrony claims it will still set the
clock rather accurately as it records the drift of the clock as part of
what it keeps when the system goes down. When it comes up, it can set
the time using only this information. NTP does not do this. I can
confirm this. On my openSUSE machine, I use NTP and a wireless network
enabled by Network Manager (not the traditional if up method). So, when
NTP starts at boot, there is no network, and so it never gets the time
correct - it stays with whatever the BIOS has. I have to restart NTP by
hand after I log in and Network Manager gets me a connection. Seems
chrony deals with this better.

Or so I plan on finding out. I am now trying to see how complete the
'new' PPS driver support is in openSUSE 11.2 (kernel 2.6.31). I see it
is present. Which is a good first sign. Next is to see that it works.


Yours sincerely,

Roger Oberholtzer

OPQ Systems / Ramböll RST

Office: Int +46 10-615 60 20
Mobile: Int +46 70-815 1696
roger.oberholtzer@xxxxxxxxxx
________________________________________

Ramböll Sverige AB
Krukmakargatan 21
P.O. Box 17009
SE-104 62 Stockholm, Sweden
www.rambollrst.se


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