18

When the ntpd daemon is running, the file: /var/lib/ntp/ntp.drift gets updated periodically. Example:

17:20 hostname 118 ~> ls -l /var/lib/ntp/ntp.drift
-rw-r--r-- 1 ntp ntp 7 May 20 16:46 /var/lib/ntp/ntp.drift
# So it looks like it was last updated ~34 minutes ago

The file has one number in it, for example, looking at a 4 virtual hosts, I find these values, respectively:

-22.086
-10.214
-13.669
6.045

I assume these are seconds per day(?), but not sure. man ntpd mentions a different drift file /etc/ntp.drift which doesn't seem to exist. The man page doesn't explain what units are being used for the drift.

Questions:

  • Is /etc/ntp.drift actually /var/lib/ntp/ntp.drift on Ubuntu?
  • What units is the drift expressed in?

Thanks!

23

The drift file is /var/lib/ntp/ntp.drift. This is fairly standard. For RH/Fedora, it's /var/lib/ntp/drift.

The units for the drift file are "PPM", or "parts per million". Your clock will drift due to fluctuations in the frequency oscillating the quartz crystal on your motherboard. A fluctuation of just 0.001% (0.00001, or 10 PPM) means losing or gaining about 1 second per day. NTP has finer grained control than that, so we look at errors of margin using 0.0001% (0.000001, or 1 PPM). Thus:

  • 1 PPM = 1 part per million = 1 microsecond per second = 3.6ms per hour = 86.4ms per day

Thus, my drift file shows the value of "2.643" which means my clock is off by 2.643 parts per million, which means it's currently off at 228.3552ms per day.

2

According to the web site https://groups.google.com/forum/m/?fromgroups#!topic/comp.protocols.time.ntp/coDks98gw0U the value is a calculationn made per poll and needs to be divided by 4096 to get the actual drift value in milliseconds

  • Thanks so much for the link. I read the thread and I'm still not clear on this. One post mentions the drift number divided by 4096 is in "parts-per-million" meaning microseconds of drift per second of time. Anyway, appreciate the help. – arielf May 23 '13 at 8:10

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