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

Tom Smith smith at cag.lkg.hp.com
Thu Feb 3 21:13:47 UTC 2005

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.

Tom Smith                       smith at alum.mit.edu,smith at cag.lkg.hp.com
Hewlett-Packard Company                          Tel: +1 (603) 884-6329
110 Spit Brook Road ZKO1-3/H42                   FAX: +1 (603) 884-6484
Nashua, New Hampshire 03062-2698, USA           Mobile: +1 978 397 3411

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