[ntp:questions] Re: ntpd, boot time, and hot plugging
brad at stop.mail-abuse.org
Sun Feb 6 19:08:16 UTC 2005
At 8:46 AM -0500 2005-02-06, Richard B. Gilbert wrote:
> I'm suggesting that ntpd query the usual suspects using iburst and
> then unconditionally set (not slew) the clock.
I will allow that perhaps there should be a startup mode where
this behaviour is used, but I do not believe that this should be the
If nothing else, we have the 34 year problem whereby you could
easily have your clock mis-set to a completely inappropriate value,
if it's not set closely enough on startup.
> Assuming that you have
> a more or less accurate drift file, and use it, why would that not give
> a fast startup and a time accurate to within, say, twenty milliseconds?
> The time budget would be something like eight seconds to send four
> queries, get responses, and make initial time and frequency settings.
> Compared with 214 seconds to remove 107 milliseconds of a 127
> millisecond offset, that looks pretty good when you are in a hurry to
> get you system back on line.
I don't think that setting a hard eight second limit would be
wise. If nothing else, there are potentially serious delays that can
be caused while waiting for DNS queries to be answered.
If your primary nameserver is located on the same machine as your
ntpd server, there could be very nasty name resolution deadlock
issues which might result, if your startup sequence chooses to start
named after ntpd, instead of the reverse. Throw DNSSEC into the mix,
and you could be in for a very serious world of hurt.
If all goes well, even without a good local Stratum 1 timeserver,
and without a good drift file, you can get pretty much full startup
of ntpd in about eight seconds.
But then all hell could break loose if things don't go well, and
tying yourself to a hard eight second time limit would be about the
worst possible thing you could do under those circumstances.
Brad Knowles, <brad at stop.mail-abuse.org>
"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."
-- Benjamin Franklin (1706-1790), reply of the Pennsylvania
Assembly to the Governor, November 11, 1755
SAGE member since 1995. See <http://www.sage.org/> for more info.
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