[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


Unruh wrote:
> "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.
>>> <http://lopsa.org/node/1480>
>>>
> 
>> 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.

<snip>




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