[ntp:questions] Re: NTP Leap Second on Windows XP
mayer at ntp.isc.org
Tue Jan 3 03:24:30 UTC 2006
Maarten Wiltink wrote:
> "Jerry Baker" <jerry at novalid.invalid> wrote in message
> news:jh%tf.3767$aB1.462 at trnddc02...
>>Others have reported large drift values. My driftfile has been steady
>>around -70 for over a year, but as soon as the leap second approached,
>>my values pegged at 500. They have been bouncing around since and today
>>have climbed to -59 and still climbing slowly. See
> As soon as it approached? I could understand wild hopping and bouncing
> just after. That is in fact what I saw, and many others too.
> I watched the leap on two Linux hosts, a kernel 2.0.40 and a kernel
> 2.2.26. They were running "ntpq -p -c rv |tee -a timelog" in a tight
> loop, both about three updates per second. Both repeated 23:59:59.
> Then the fun started. As observed by Johan Swenker, also an xs4all
> customer, their four NTP servers started disagreeing. I watched the
> confusion for a bit, went to sleep, got up, watched and listened to
> the Wiener Philharmoniker's New Year's concert, and only around half
> past noon UTC started fixing things. I watched some more wild hopping,
> stepping, bouncing, creaking and groaning, and then disconnected my
> main NTP server from its, still useless, batch of reference servers
> to run free for a while.
> This is a Red Hat box that uses a file /etc/ntp/step-tickers to step
> the time before starting the NTP daemon through the normal SysV init
> scripts. Since stepping was going to do more harm than good, I renamed
> the file. Then I added "disable ntp" to /etc/ntp.conf to let it run
> free. Judging from the "ntpq -p" report, it had by now stepped to some
> 150 ms ahead of real time, so I wrote a new value to the drift file
> and restarted the daemon (/etc/rc.d/init.d/ntpd restart). No step;
> polling four time servers but not adjusting the clock; and the
> frequency correction supplied by the drift file.
> As it turned out, I had guessed wrong. The old drift value was very
> near zero and I had replaced it with 10. So the clock drifted ahead
> even further, at a rate of 10 µs per second. When I came back to look,
> it had (still judging from the "ntpq -p" report) amassed a 900 ms
> difference with my best guess of actual time. Some rough calculation
> of when I intended to look again and how much offset to correct
> until then convinced me to write -27 to the drift file and restart
> the daemon again.
> This host serves five internal nodes and not all of them followed
> very well. One in particular almost reached -128 ms offset. Almost.
> I suppose a 37 PPM instantaneous differential is a lot to ask.
> By the time the offset had been reduced to about -30 ms, the ISP's
> four NTP servers were still not agreeing but I was willing to take
> that chance. I removed the "disable ntp" line from the configuration
> file, restarted the server, and renamed step-tickers back. (In that
> order. I still didn't want the step.) Looking back, I might have also
> reset the drift. After that, things settled down nicely.
> Next time, I'll probably not wait for the confusion to start, and
> turn off clock adjustment and coast preemptively. It's not a problem
> if the network is off by a second, and I now know how to make manual
> Maarten Wiltink
Maarten, I'm not sure why you would do this. First of all step-tickers
is a Red-hat thing for using ntpdate before starting ntpd. You are far
better off just starting ntpd with the -g option and iburst on the
server lines. All this messing with the drift file doesn't really help
you. You should let ntpd do its job and set the drift value.
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