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@ramboll.se ________________________________________ Ramböll Sverige AB Krukmakargatan 21 P.O. Box 17009 SE-104 62 Stockholm, Sweden www.rambollrst.se -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org For additional commands, e-mail: opensuse+help@opensuse.org