Re: The libntp resumee...

Uwe Klein <uwe_klein_habertwedt@xxxxxxxxxxx> writes:

Unruh wrote:
Uwe Klein <uwe_klein_habertwedt@xxxxxxxxxxx> 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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