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

Tom Smith smith at cag.lkg.hp.com
Fri Feb 4 00:30:40 UTC 2005

Alain wrote:
> 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?

I assume you asked this of me, rather than of Dave. Your method
seems to me to excellent.

If you do that, there should be no need to clean out the
characteristic drift that was so carefully computed over a long
period of time. Cleaning out the drift file is a drastic measure
required only if you do NOT pre-set the clock before starting ntpd
and you end up, as a result, with a bogus re-calculated drift rate.

What you have to calculate into the boot time is the time you have
to block while stepping the clock to the "right" time in the first place.
Putting ntpdate into the boot sequence means that time will be however
long it takes for ntpdate to complete, exactly that long, and no longer
and that when ntpdate completes the rest of your boot sequence, including
ntpd is "safe". That's why people are unhappy about ntpdate being
removed without an equivalent way of doing the same thing with

1) step the clock to a "good enough" time
       do this within a few seconds
       block while doing it
       don't touch/change the pre-computed drift while doing this
2) start ntpd
3) start time-dependent services

It would certainly be possible for ntpd to do this all by itself,
but unless I misunderstand, the only way to do this with just
ntpd at the moment is:

1) ntpd -gq
2) sleep [guess how long ntpd -gq will take, worst case, to set the clock]
    spin on ps looking for ntpd to start and then end
3) start ntpd [normal operation options]
4) start time-dependent services

Which is not the same thing.

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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