[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Tue Mar 13 23:08:15 UTC 2012

On 3/13/2012 6:42 PM, Chuck Swiger wrote:
> On Mar 13, 2012, at 3:19 PM, Ron Frazier (NTP) wrote:
>> At that time, I had the GPS as the only selectable clock, for testing.  I'm monitoring the internet servers for comparison.  I've since selected 1 NIST server as the preferred and only selectable clock, and am monitoring the GPS and other internet clocks for comparison.
> ntpd is probably better able to judge which timeserver to use then forcing the choice via prefer, as it continues to evaluate the servers available to it and will use another one if the chosen peer happens to become unreachable.
>> My thinking was simply that if all the available clocks vanished (internet down), or the GPS went insane, I just want the system to coast on its internal clock until a viable clock source returns.  I figure it may gain or lose up to 10 ms / minute this way, but at least it shouldn't do anything radical like jumping 50 seconds.
> If you let ntpd run long enough to estimate the intrinsic drift, it will continue to adjust the clock even without external timesources. However, if your system clock really is off by a rate of 1:600 or worse, it's broken and ntpd probably wouldn't be able to fix it, at least without tinkering the max allowable slew rate-- running ntpdate via cron might do.
>> I looked at the docs page for the "orphan" command for a few minutes and my eyes just glazed over.  I decided to try the local clock option instead for now.
> Why would you want to use the local clock option?  It's rarely the right solution, barring unusual circumstances...
> Regards,

I was speculating that perhaps my only selectable clock, the GPS, 
failed, and that something went nuts, and that's why I found the clock 
50 seconds off this morning.  However, I don't have any evidence of a 
GPS failure.  In any case, I just figured the local option might prevent 
any major clock changes if all other sources are not available.  I only 
wanted the local option to kick in if there were no other sources.  I 
did not have that option in the ntp.conf file when I started this 
thread.  When I started this thread, my GPS was the only selectable 
clock, and there was no local option.

For now, I'm mainly wanting to compare the GPS to one primary other 
source because I've been experiencing a slow drift in NMEA time with a 
variation of about 120 ms and an oscillation period of several days.  
I'm trying to isolate the source, either the GPS, or the subsystem 
that's getting time from the internet servers.  I'm assuming all the 
internet servers are not drifting, but my instrumentation, ie the 
Meinberg time server monitor, or whatever drives it, could be off.  
That's why only have one source selectable, either the GPS, or one NIST 




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Ron Frazier
timekeepingdude AT c3energy.com

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