[ntp:questions] Isolated Network Drift Problem

Hal Murray hal-usenet at ip-64-139-1-69.sjc.megapath.net
Fri Nov 21 00:38:34 UTC 2008

If you have an isolated network, it will drift unless you
have a local refclock.  You can get a low cost GPS unit for
under $100.

On the other hand, you should be able to tune the drift
file by hand so that it is within a few seconds per day.

I don't know of a good description of how to do that.  It's
simple, but a step by step description would be handy and
save some trial and error.

I'd do roughly this:
  (re)Start ntpd
  Record the time and drift
  Wait N hours/days
  Compare the system time with the correct time
  Compute the drift in PPM (parts per million)
  1 second per day is 11.6 ppm (1000000 / 86400)
  stop ntpd
  edit the drift file to adjust the number by that amount
    (I don't know which direction.  It might take two tries.)
  loop back to top

The main problem is temperature.  (The crystal in your
system makes a pretty good thermometer.)  The temperature
is determined by both your air conditioning setup and the
load on your system.  So you probably get to start over
if you move the machine to a different environment or
change the load pattern.  Obviously, it helps if the
load is constant, for example a system that does nothing
else other than run ntpd.  (Old slow boxes work fine.)

These are my opinions, not necessarily my employer's.  I hate spam.

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