[ntp:questions] question regarding NTP configuration for clusters, and "cluster time" stability

Terje Mathisen Terje.Mathisen at tmsw.no
Tue Oct 6 18:10:30 UTC 2009


rotordyn at yahoo.com wrote:
> On Oct 6, 11:52 am, Terje Mathisen<Terje.Mathi... at tmsw.no>  wrote:
>
>> Having an external server fail but still reply (a "falseticker") is an
>> expected issue, it is solved by having at least 4 servers.
>
> Right. But even if we require 4, there is the unlikely but possible
> scenario of 3 going offline during the same period that the last one
> is ill-behaved.

At his point you need more servers:

4 allow for a single falseticker, then I believe you need 6 to handle 
two errors and 7 or 8 for three simultaneous falsetickers.

You should know though that nobody has ever seen this happen, over the 
entire history of the ntp protocol, unless all those servers have a 
common reference: If you make sure you have several independent stratum 
1 or 2 servers, then you can very safely bet that it simply won't happen.

You _can_ have a backhoe cutting all your internet connections at the 
same time, this is far more likely than multiple simultaneous errors all 
going in the same direction (and within a few ms from each other).

Terje
-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"




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