[ntp:questions] ntpd questions - FreeBSD 5.5

David Shoulders dhs at bendcable.com
Sat Jul 25 23:44:54 UTC 2009


I have some actual data on my server's sudden leaps into instability. 
The two queries below were run about 2 minutes apart (presumably on 
either side of a poll).

(For reference, I've put my original question and a follow-up at the end 
of this post.)

(The "cron/ntpd -q" system was not working any better (huge resets), so 
I've been running ntpd full-time for the last 5 days.  It went off into 
gaga-land once a couple of days ago, so I had to restart it with a 
reasonable drift value.)

> ntpq> peers
>      remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> -rainforest.neor 33.247.251.47    3 u  894 1024  377   17.418   31.986  15.040
> +clock-a.develoo 207.171.30.106   2 u  888 1024  377   57.953   30.674   2.065
> +mtnlion.com     139.78.135.14    2 u  858 1024  377   15.182   23.067   2.919
> +enigma.wiredgoa 209.81.9.7       2 u  859 1024  377   45.946   22.677   1.881
> *time-sj.stsn.ne 192.5.41.40      2 u  851 1024  377   53.168   31.283   7.098
> ntpq> q

> 103 [zorg:/etc]# ntpq -c peers zorg
>      remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> +rainforest.neor 33.247.251.47    3 u  101 1024  377   19.168  -586.83 618.823
> *clock-a.develoo 207.171.30.106   2 u   96 1024  377   64.535  -587.66 618.341
> +mtnlion.com     139.78.135.14    2 u   62 1024  377   15.002  -615.87 638.938
> +enigma.wiredgoa 209.81.9.7       2 u   65 1024  377   47.882  -614.02 636.702
> +time-sj.stsn.ne 192.5.41.40      2 u   58 1024  377   50.959  -610.66 641.951
> 104 [zorg:/etc]#

Anyone have a notion what might be going on?  The big offsets are 
presumably just because the server's clock was off by about half a 
second, but the huge jitter values don't make sense to me.  Aren't they 
the jitter in round-trip time?  That wouldn't be affected by my clock 
being off.  I'm sure this is my ignorance, being an ntpd tyro, but I'd 
like to understand...

TIA

- David

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
ORIGINAL QUESTION

I have a little file server running in my basement, and since it's the
only machine running all the time, I set it up to run ntpd and provide
clock settings to my other machines.

The machine is running FreeBSD 5.5.  I installed it some years ago, and
have had no reason to upgrade it.

Initially, I ran ntpd for a day or two to establish a drift value, then
killed it and set up a crontab entry to run "ntpd -q" every 6 hours.
This worked perfectly for 2 or 3 years.  Corrections were always small
numbers of msec.

Then, a few days ago, a disk failed.  I replaced the disk and restored,
and everything was fine -- except that I had lost the drift file.  So, I
started ntpd, let it run overnight, and looked at the drift file.  It
had an obviously bogus number.  The clock corrections were very large
and not getting smaller.  So I put a reasonable number in ntp.drift
(based on my vague memory of the old good value -- about 100), restarted
ntpd and let it run a few hours.  It seemed to be converging, so I
stopped it and reinstated the crontab/ntpd -q routine -- this time every
3 hours.

12 out of 19 corrections were around 20-30 msec, but the others were
off-the-wall -- hundreds of msec.  So I did some arithmetic (on the
reasonable corrections only) and adjusted the drift value.  Since then,
most of the corrections have been less than 10 msec, but I'm still
getting some crazy ones -- like 1.7 seconds!

The wild corrections are all in the same direction (-), so I don't think
the time derived from the servers is wrong.  It seems as if the clock in
the PC must be taking off on wild excursions occasionally.  Is this
possible?  How could replacing a disk have brought this on?  What am I
missing?

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
FOLLOW-UP on why I've been using cron/nptd -q

I don't have a critical need for accuracy, so I didn't want to add any 
more load to the time servers than necessary.  I thought that a few hits 
4 times a day would be a smaller load than running the daemon all the 
time.  Now that my computer's clock seems to be running inconsistently, 
the load from the ntpd running continuously would be even higher, right?

BTW, new data on my little mystery:  my computer's clock seems to gain 
time (like 2 seconds in 3 hours) during times when I'm using my other 
systems -- never when idling.  This even though the computer is just 
providing nfs service and occasional backups.  Shouldn't an increase in 
average load SLOW the clock (by masking more interrupts)?
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-




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