[ntp:questions] Isolated Network Drift Problem

Cal Webster cwebster at ec.rr.com
Tue Nov 25 17:02:48 UTC 2008

On Tue, 2008-11-25 at 09:25 +0100, Maarten Wiltink wrote:
> [...]
> > What's the best way to determine which of our NTP servers provides the
> > best local clock?
> First order: reset drift (delete all their drift files), synchronise their
> watches, let them run for a few days, and see which one has drifted least.
> Correct for drift.
> This depends on how well you can put them all in the same starting state
> by hand, and on the time source you use to measure drift at the end. You
> can correct for the former by waiting longer. You _cannot_ outwit your
> dependency on the latter.
> Second order: after the previous procedure, they should all drift very
> little, and no one significantly more than any other. The one that stays
> closest to that time source you're comparing against has 'the best local
> clock'. This depends mostly on temperature stability.

So, to recap:

##>> Delete the NTP drift files.

##>> "Synchronizing" their clocks:

1. Stop NTP daemon
2. Consult http://wwp.greenwichmeantime.com/time-zone/usa/eastern-time/
3. Construct a "date" command with the next minute after what's observed
in step 2 and zero seconds.
4. Copy the date command and watch the count-up to the next minute on
5. Paste the date command into the terminal window exactly when the
counter turns to the next minute on greenwichmeantime.com.
6. Execute "hwclock --systohc" to set the hardware clock.
7. Start NTP daemon

Repeat this for each NTP server

##>> Wait 4 days (Thanksgiving day weekend)

##>> Record value in NTP drift files

[root at fluid root]# cat /etc/ntp/drift

##?? Is this a good source to measure the drift?

##>> Correct for the drift:

ntptime -f 20.196
adjtime -f (result of some formula)

##?? Not sure about the signage here. From what I've read, the value
stored in the drift file is the frequency offset - the value required to
bring the clock back to normal.

##?? Should this be done with ntpd stopped or running? From the man page
it seems this command communicates with the running kernel. Is there a
kernel parameter I can set in /etc/sysctl.conf to make this setting
persist? The only thing I can see related to the clock in /proc is

##?? Is this what you had in mind?



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