[ntp:questions] suggestion

Richard B. Gilbert rgilbert88 at comcast.net
Mon Oct 26 23:28:09 UTC 2009


gizero wrote:
> Hi group users,
> 
> I'm writing this, since I am extremely confused about how to configure
> network time sync for an embedded system I'm working on.
> 
> Possible time sources for the box are:
> 1. onboard RTC (used at boot to start-up system clock)
> 2. NTP server (optional and configurable: used as soon as reachable
> for synchronisation)
> 3. local GPS connected to serial line (an alternative to NTP: used as
> soon as it fix ad send timestamps)
> 
> When first thinking about the time sync problem, I was pleased to
> learn that ntpd was able to handle the time data from the GPS as a
> special time source, together with gpsd daemon. So I thought of using
> it instead of any sort of cronjob calling ntpdate to step the clock.
> 
> Then I learned - not so deeply, I have to say - that ntpd was a lot
> more than I expected in term of algorithms guessing the best source
> among many.
> 
> My question is: should I expect to be able to set-up my configuration
> that allow a single NTP server to be defined and, optionally, the GPS,
> when available? Or am I out of the track?
> 

Yes.  You can use a single server.  You must take into consideration the 
fact that you are totally dependent on that single server for the 
correct time!  If the server fails you may get some very odd value of time.

The minimum number of servers for a robust configuration is four.  If 
one of the servers becomes "insane" the other three will out vote it.
Two is the worst possible config for it is written that a man with two 
clocks can never be certain what time it is!




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