[ntp:questions] question regarding NTP configuration for clusters, and "cluster time" stability
Richard B. Gilbert
rgilbert88 at comcast.net
Sat Nov 21 03:55:22 UTC 2009
> "Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
>> E-Mail Sent to this address will be added to the BlackLists wrote:
>>> rotordyn at yahoo.com wrote:
>>>> Look at it this way: We produce a system (in the form
>>>> of a cluster) that works today, but that can drift away
>>>> from UTC since it currently doesn't accept any external
>>>> time reference. That drift isn't a huge issue, but over
>>>> the multi-year lifespan of the hardware, can be significant.
>>>> If we allow the use of external NTP servers, we now have
>>>> opened up what was a closed system, and must prevent that
>>>> new input source from causing instability. ...
>>>> An alternate approach would be to make our cluster
>>>> software resilient to intra-node time variations, but it
>>>> was deemed simpler to use NTP to have the cluster accept
>>>> an external UTC source.
>> There are solutions that do not require that you get time from the internet!
>> A GPS timing receiver, for example, requires only an unobstructed view
>> of most of the sky. The Pulse Per Second (PPS) output is generally
>> accurate to within +/- 50 nanoseconds. You lose a great deal of
> Well, no, on the cheapest option, the GPS18xLVC it is about 500ns, and
> furthermore, the computer almost certainly cannot respond to the interrupt on a
> 50ns basis-- tests I did suggest more like 1-2usec (or worse if it happens to come
> in during a time when interrupts are switched off).
>> accuracy getting the time into your computer. Determining just how much
>> you lose is difficult but you can assume that you know the time to
>> within, say, 100 microseconds.
> You can reliably get a few microseconds 1 or 2)
>> If you connect the GPS to a computer running NTPD, the machine becomes a
>> stratum one server, meaning that the computer is getting time directly
>>from an atomic clock on board one of the NAVSTAR satellites. As of the
>> last time I heard there were 27 satellites in orbit and your GPS
>> receiver should be able see at least four and be able to determine your
>> latitude, longitude, and elevation to within about 50 feet and the time
>> to within 100 nanoseconds.
> The receiver may. If you tell it where you are, then it can get the time with many
> fewer sattelites.
If you know your *exact* location you need only a single satellite. If
not, you will need four.
The cellular phone companies depend on extremely precise timing which
they get from GPS. When a new base station is installed, they do a site
survey that takes about 30 days and gets an extremely accurate location.
Once they know their position, a single satellite is sufficient to give
them the time.
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