[ntp:questions] ntpd -q and driftfile

Richard B. Gilbert rgilbert88 at comcast.net
Tue Mar 22 20:50:36 UTC 2011


On 3/22/2011 1:22 PM, unruh wrote:
> On 2011-03-22, David J Taylor<david-taylor at blueyonder.co.uk.invalid>  wrote:
>>> He can do that if he wants to. That was why the -q option was designed
>>> into ntpd-- one time setting of the clock time. However it cannot create
>>> a drift file. If he wants a drift file then as you say, he must run it
>>> continuously.
>>
>> You could look at it that way, Bill, if all he needs is a one-off setting.
>> However, Prashant says he wants to run it periodically, which doesn't
>> really make sense.  Rather than a periodic run, he should leave ntpd
>> running continuously, which is what ntpd is designed for.
>
> ntpd -q is a replacemtn for ntpdate, which was typically run from cron,
> and he is doing, and it is an "acceptable" procedure if for example you
> do not want a daemon running which could have (unknown to you) security
> issues. It is also a bit unclear how to switch off the server role of
> ntpd and he may not want others querying his machine. On the other had
> it comes at a cost of far worse clock discipline and the probability of
> the computer jumping backwards in time.
>
>>
>> Prashant, the fact that the server capability is already built-in to ntpd
>> in no way detracts from using ntpd just as a client for your PC to keep
>> the time spot-on.  Just leave it running all the time as it uses very
>> little resources.  It will then compute the drift information after (IIRC)
>> one hours of running, and it will keep your PC clock correct by adjusting
>> the rate, rather than bang-bang adjustments of the time.
>
> One hour? More like 10 hr. ntpd is really bad at recovering from
> changes, and switchon is a big change.
> After 1 hr the drift is liable to be way off, as ntpd alters the drift
> to bring the clock back into line.

That depends on how accurate you want your time to be.  NTPD should have 
the right day, hour, and minute within a very few minutes or even 
seconds.  If you want microsecond accuracy you run in a temperature 
controlled environment and endeavor to keep it running 24 hours a day, 
seven days a week, 365 days a year!

There are people who want fast startup.  NTPD does not do fast startups.
If you want to turn everything on at 8:30AM and shut it all off again at 
5:00PM, NTPD is not good choice!!






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