[ntp:questions] The libntp resumee...

Unruh unruh-spam at physics.ubc.ca
Tue Sep 9 20:23:06 UTC 2008


Uwe Klein <uwe_klein_habertwedt at t-online.de> writes:

>Unruh wrote:
>> Uwe Klein <uwe_klein_habertwedt at t-online.de> writes:
>> 
>> 
>>>Unruh wrote:
>>>
>>>>Yes, that is with reference to the road. Car three should thus completely
>>>>ignore the other two cars and use his speedometer. 
>>>>
>>>>Ie, put up a GPS receiver with a PPS and use that as your time source, and
>>>>ignore all the other ntp time sources, except perhaps as sanity checks (eg
>>>>if you r speedometer breaks you should get to know about it by occasionally
>>>>looking at the other cars)
>> 
>> 
>>>A)One GPS to each box or
>>>B) a single GPS with PPS line to all boxes?
>> 
>> 
>> Whichever you want. Up to you.
>> 
>> 
>>>A:
>>>Doesn't that impact reliability?
>> 
>> 
>>>You add the failure probability of a GPS-unit to each Box
>>>where one failure will make the whole system fail.
>> 
>> 
>> So, that is why ntp has backup servers. You have a single failure point
>> anyway-- the network. It goes down, and nothing can get the time.


>That actually is _three_ different scenarios.

>time over the network:

>network fails
>	1: time
>	2: the system as a whole

>	failure of network infrastructure
>	thus does not add to the probability of the complete system failing.

>time over PPS/GPS 1 unit with signaling to each box:

>network fails
>	2: the system as a whole

>GPS fails
>	1: time
>		-> the system as a whole

>	This adds up to a higher failure rate/probability.

>time over PPS/GPS unit per box:

>network fails
>	2: the system as a whole

>GPS fails
>	1: time
>		-> the system as a whole

>	This adds up to a higher failure rate/probability.

>	With the added disadvantage that GPS failure overall
>	is single failure times number of boxes.


So, put a GPS connected to each box. That will be a stratum 0 source and
will be selected by ntp. If that fails, have each of the other machines as
a backup. They will be stratum 1 source. Then have the system go out onto
the world wide net to pool.ntp. Those will be stratum 2 or lower. Each
backs up the otehr. Thus each machine will gets its time from GPS (usec
precision) It that fails, they get it from the local machines ( 10s of usec
precision) If that all fails they get it from the net ( ms precision) It
that all fails, you are SOOL. You probably have other worries anyway.

How many belts and braces you want is entirely up to you. 

I would have one GPS on  one machine. Everything gets their time from that,
unless it fails in which case pool.ntp would act as a backup. But it is
entirely up to you. 

>uwe




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