[ntp:questions] Re: ntpd, boot time, and hot plugging

Alain alainm at pobox.com
Thu Feb 3 23:03:22 UTC 2005

Tom Smith escreveu:
> David L. Mills wrote:
>> I get nervous about nonquantitative statements, since they might start 
>> urban legends. A "decent" frequency file is one created when first 
>> starting ntpd without the file and letting it determine the intrinsic 
>> frequency error. This takes about fifteen minutes. However, the 
>> frequency file itself is written only after the first hour and at 
>> hourly intervals after that. The discipline should be stable even if 
>> the frequency file is present and intentionally set as much as +-500 
>> PPM in error and that even with a large initial time offset. This has 
>> been confirmed by simulation; however, the simulations assume the 
>> adjtime() system call operates as in original Unix model; the Solaris 
>> adjtime() is a killer when large offsets are involved.
> A physicist I worked with early in my career taught me a very
> useful law. "Different things vary."
> I couldn't tell you how many ntp.drift files I've encountered
> with a vlaue of +-500.000. It's a lot. There are many ways this
> can occur, but all of them involve ntpd starting up against a large
> offset with its reference clocks and/or shutting down while it is
> working one off, the latter usually because of an NTP misconfiguration,
> but also sometimes because of thunderstorms in July. Others have
> also observed how this happens on mobile systems that get booted
> and shut down a lot.
> Once a system is in this state, it depends on the specifics of the OS
> how long or even if that system will "settle". I can assure you that
> for some systems, if not most, this is most assuredly not 15 minutes,
> might be days, or might, for all practical purposes, be never. These
> are the systems on which the ordinary non-NTP-expert system
> manager or field support team will go through several rounds of
> battery or crystal or motherboard or system replacement before
> anyone tells them to just delete the drift file and start over.

So, if I manage to set the initial time within a good aproximation od 
"real" time using ntpdate using 5 servers as explained earlyer, would 
you recomend to delete the drift file and let it start all over again?

Does this afect the time for the server to start serving?


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