[ntp:questions] problem with synchronizing two comps to each other

Brad Knowles brad at stop.mail-abuse.org
Thu May 19 16:18:08 UTC 2005


At 5:04 PM +0200 2005-05-19, Maarten Wiltink quoted "Mike" 
<miodek.STOP.SPAM at thebestnudes.com>:

>>  so you're telling, that it will be easier to update time from external
>>  server? beacause I thought, that if 1 comp (master) keeps the time,
>>  the other one (client) will adjust to it over LAN network. That it
>>  will give beter results in setting the same time on both comps.
>
>  No, it will avoid one specific failure mode. The one where one server
>  is running fast, and one is running slow, both with respect to real
>  time, and neither speed difference is out of spec (500 PPM) for NTP,
>  but the difference between them is.

	It will avoid that failure mode, as well as failure modes 
involving the entire network running fast or slow (as clocked from 
the bogomaster), so that aggregate measurements over time may be 
skewed.


	Mike -- Please note that my original message mentioned 
microseconds, and I know that you were talking about milliseconds. 
However, there are issues that computers face in synchronizing their 
clocks to an upstream server which will affect you in pretty much the 
same way, regardless of whether you're looking for millisecond or 
microsecond resolution.

	Millisecond resolution is easy enough to achieve with NTP, and 
indeed I would say that it's hard for NTP to be that far out -- if 
you're off by that much, that's usually pretty easy to detect and 
you'll get your clock marked as a falseticker.


	If you set up NTP correctly within your local network, and you 
have good hardware with a good OS that is properly configured, I 
would think that you should be able to easily achieve accuracy levels 
in the single-digit microsecond range.  Even the double-digit 
microsecond range would be more than enough for your needs.

	Assuming you get NTP to work at all, I would think that it should 
actually be difficult to have accuracy that is only as good as the 
triple-digit microsecond range.

-- 
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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