[ntp:questions] Query about NTP accuracy

Andy Yates andyy1234 at gmail.com
Fri May 22 23:13:13 UTC 2009


Hal Murray wrote:
> In article <4a15e001$0$18238$da0feed9 at news.zen.co.uk>,
>  Andy Yates <andyy1234 at gmail.com> writes:
>> Does anybody have any figures that shows the effect on accuracy of an
>> NTP v3 client using a stratum 1 server rather than a stratum 2 or 3
>> server? It's all in a GE LAN based scenario, commercial stratum 1
>> servers connected to GPS and stratum 2 and 3 servers are typically
>> dedicated Linux boxes.
> 
>> However I'm been pressed to supply an SLA for accuracy. My argument is
>> that although you can get your stratum one server to synchronize to
>> microseconds of UTP, as soon as the client uses NTP v3 over the LAN,
>> even a GE LAN, then the accuracy degrades and putting well designed well
>> specified stratum between the boxes is not going to decrease accuracy
>> sufficiently to warrant purchasing many stratum one appliances.
> 
> What sort of accuracy are you interested in?  1 ms?  10 ms?  100 ms?

Hi Hal

Its up to us to specify what we think the SLA should be - the guide is
"as accurate as possible"!
> 
> How stable is your temperature?  (Both the room and the CPU load.)

Temperature will be very stable, the DC is the very well specified and
scrupulously engineered - no cables blocking air flow etc. Generally
speaking the CPU is over specified.

> 
> What is the load on the LAN between the clients and servers?
> (Delay is not a problem.  Variation in delay is a problem.)
The NTP will be on a separate management LAN to the production traffic
so not subject to the variances that application load has on the network.
> 
> 
> I suggest you measure it.  Start with your current system.
> 
> Setup a box as a ntpd system and tell it to use several target boxes
> as servers and turn on logging.  peerstats will tell you the difference
> between your local clock and the target system.
> 
I'll look at the current NTP infrastructure however its completely
different to the new. The old has 3 geographically diverse GPS receivers
plus a GPS and radio source on the roof of the data centre and we use
network components to provide the intermediate strata between stratum
one and client - and have not really had many issues after almost 10
years of use. However, requirements are changing and we will probably be
using dedicated stratum 2/3 servers as required.

Regards
Andy




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