[ntp:questions] question regarding NTP configuration for clusters, and "cluster time" stability
Richard B. Gilbert
rgilbert88 at comcast.net
Sat Nov 21 00:36:18 UTC 2009
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
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.
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.
A shortwave radio receiver should be able to pick up a time signal from
WWV, the NIST HF broadcast from Fort Collins, CO (2.5MHz, 5MHz, 10MHz,
20MHz, . . . . There are special purpose receivers that will
automatically select the best radio frequency from the five or six
frequencies that NIST uses and decode the timing signal. WWVH
broadcasts from Hawaii.
A VLF receiver should be able to pick up a signal from WWVB.
See the NIST web site for more information.
Other countries offer similar services.
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