Graphing convergence of the system clock
ntpd behaves differently depending on the difference between the system time and the reference clock time(s), various settings etc. You can watch the convergence of the system clock to the references using a cron job to log the data, and gnuplot to graph it.
By default ntpd has a socket interface (or UDP maybe?) which permits querying its status (and maybe even changing some settings) remotely. You can disable that in /etc/ntp.conf:
restrict default nomodify nopeer noquery restrict 127.0.0.1
(the first line disables some features by default, the second re-enables everything for clients connecting from localhost) But if it was not disabled you can use it to log the offset of a remote host's clock.
[localhost][02:02:12 PM] ntpdc -sn
remote local st poll reach delay offset disp
=======================================================================
*67.192.108.35 192.168.2.114 5 64 377 0.08722 -0.056842 0.14363
10.90.1.35 192.168.2.114 16 1024 0 0.00000 0.000000 3.99217
~
[localhost][02:02:14 PM] ntpdc -sn remotehost.local
remote local st poll reach delay offset disp
=======================================================================
*67.192.108.35 192.168.2.115 5 128 367 0.08578 -0.072788 0.14655
So just create a cron job which does this once per minute:
echo `date +%s | cut -c5-12` " " `ntpdc -s remotehost.local | tail -n1 | cut -c54-64` >> remotehost-ntp.log
then to graph the data, create a file with all the gnuplot commands:
set style line 1 lt rgb "blue" lw 1 pt 0 set xlabel "time,min" set ylabel "offset,s" set title "Convergence of clock via ntpd after startup" #~ set terminal x11 set terminal png size 1024,400 set output "remotehost-ntp.png" plot "remotehost-ntp.log" using (($1-791000) / 60):($2) with linespoints ls 1 title 'hostname'
and run it like this
gnuplot -persist ntp-gnuplot
The -persist is for the set terminal x11 case, to make the window stay up until you close it.