[ntp:questions] Severe time drift for Undisciplined Local Clock

Steve Kostecke kostecke at ntp.isc.org
Thu Mar 29 19:06:43 UTC 2007

On 2007-03-28, deon at nulan.co.za <deon at nulan.co.za> wrote:

> We have a setup for a group of Linux machines (2.6.8 kernel,FC3) which  
> is NOT connected to any outside network. One of the machines has been  
> made the Time Server. It uses as it's reference the on-board hardware  
> clock (computer motherboard hardware clock, Real Time Clock (RTC)).

The average quartz wrist watch is a better clock than the on-board RTC.

> In NTP terminology this referred to as the Undisciplined local clock.

Note the "undisciplined" part.

> These machines all perform the same function and has some I/O
> cards that causes a heavy interrupt load. The Linux System Clock
> is synchronised to the hardware clock yet experiences upto 1 hour
> timedrift per month!

One way to avoid this drift is to use a dedicated time server system
(i.e. one that does nothing but run ntpd). You would need to spend some
time calibrating the clock to achieve minimal drift.

Another solution is to use a GPS receiver as a ref-clock. A number of
us have had good results with the Garmin GPS-18LVC (currently $65.79 +
shipping from Provantage).

> In any case here is my /etc/ntp.conf file:

My comments are unrelated to your clock stability problems. But ...

> restrict default nomodify notrap
> restrict
> server prefer

You only have one time source listed so 'prefer' makes no sense.

> fudge stratum 0

It is not necessary to fudge the Undicsiplined Local Clock to such a low
stratum. Doing this does not make the time "better".

> driftfile /var/lib/ntp/drift

> broadcastdelay 0.008
> restrict <ipaddres.255>

Those last two lines aren't doing anything for you.

Steve Kostecke <kostecke at ntp.isc.org>
NTP Public Services Project - http://ntp.isc.org/

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