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Re: Set your system clock



On Monday 14 February 2005 15:17, Kirk Strauser wrote:
>On Sunday 13 February 2005 17:36, Glenn English wrote:
>> OTOH, I don't see any reason not to just turn off ntpd and let
>> cron run ntpdate every hour or so (and at boot). That'd keep your
>> clock accurate with less net bandwidth and resource usage.
>
>Big reason #1: ntpdate is deprecated and may be removed at any point
> in the future - see http://www.ntp.org/ntpfaq/NTP-s-config.htm for
> details.

I rather doubt that just yet, since the current (as of FC2 anyway) 
versions of ntpd do in fact call ntpdate to do that one time crash 
setting operation at boot, or restart of ntpd time.  To deprecate 
ntpdate, that functionality would have to be moved into ntpd itself.

>Big reason #2: ntpdate doesn't guarantee a monotonically increasing
> system time (translation: it may set the clock backwards).  Unix
> systems don't like that.

Typically a crash recovery reboot will do exactly that when you keep 
the hardware clock on UT.  In that event, the normal shutdown does 
not have a chance to reset the clock to local time, and hence is off 
by many hours on the reboot.  With ntpdate doing the crash 
re-setting, my clocks get set backwards by 5 hours, but other than 
whats been logged previously in this boot proceedure, everything is 
still future tense to the processes that will run once booted.

However, other than trying to make sense of the system logs, and an 
occasional squawk from smartd which is started before ntpd for some 
reason here, this has been a zero problem problem.  Smartd only 
fuss's once, resetting itself to the new time on the next iteration 
anyway.

>Big reason #3: ntpd only queries remote servers when it believes the
> local time might have drifted by more than an acceptable amount. 
> If you have a very stable clock, then it will ask for the time
> fairly rarely.  A cron job knows nothing about this: it asks once
> per configured time unit, whether the local clock is likely to be
> dead accurate or has already slipped far out of sync.
>
>Big reason #4: if everyone put "0 * * * * ntpdate ntp.example.com"
> in their crontab, then once an hour ntp.example.com gets hammered. 
> This also means that those answers are likely to be far more
> delayed (and therefore inaccurate) then they are during the rest of
> the hour.  Besides, the ntp.example.com admins would hate you.

No one who actually thinks about such nuances schedules such important 
stuff at the top of the hour, every hour.  And that tends to leave a 
slack spot at the top of the hour for those that don't think, but 
just do it.  I expect its probably an mrtg trackable peak though.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
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by Gene Heskett are:
Copyright 2005 by Maurice Eugene Heskett, all rights reserved.



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