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query on NTP drift file

 
zhaogui
Super Advisor

query on NTP drift file

Can anyone help to answer the following question?
when the driffile says 161.961, how do you interpret that ? Is is in seconds or milliseconds ? Also is it cumulative since ntp daemon startup ?

Thanks in advance,
2 REPLIES 2
Robert_Jewell
Honored Contributor

Re: query on NTP drift file

For clarification the driftfile entry in the /etc/ntp.conf should point to the /etc/ntp.drift file.

The xntpd daemon computes the internal clock's average drift and uses this to determine how often to poll the NTP servers.

The ntp.drift file itself is used by xntpd to record the average drift time of the systems clock.

I cannot find any indication of what is kept in the drift file and what its value means though.

-Bob
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Michael Steele_2
Honored Contributor

Re: query on NTP drift file

Drift is an attempt to factor in aging of a clock. For as a clock gets farther and farther away from the master stratum 1 clock its drift increases.

Drift is also called skew.

Your NTP configuration has the option of syncing with more than one NTP server, varying in stratums beginning with 1 and going down to, I believe, stratum six. The further a computer is from stratum 1, the less accurate its timing with the master source. So your 'ntpd' will write the difference to the drift file between stratums.

Several NTP clients record their drift and then switch dynamically to the server with the best figures, or, the stratum server with the least drift.

Here's a definition from NIST:

Unless indicated otherwise, the offset of two clocks is the time difference between them, while the skew is the frequency difference (first derivative of offset
with time) between them. Real clocks exhibit some variation in skew (second derivative of offset with time), which is called drift;

Here's the NIST RFC link.

http://www.faqs.org/rfcs/rfc1305.html
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