[RndTbl] strange NTP problem on one of 3 peers

Gilles Detillieux grdetil at scrc.umanitoba.ca
Wed Apr 25 10:53:41 CDT 2012

I have a weird problem with clock drift that just started to happen 
today on one of my Linux systems.  I was wondering if someone on the 
list has some NTP experience and could help me solve this puzzle.

I have a group of 3 systems operating as peers, and they've been keeping 
time well for years.  Yesterday I upgraded them from Scientific Linux 
5.7 to 5.8 (an RHEL 5.8 clone like CentOS 5.8), and rebooted them to the 
latest kernel on SL 5.8, 2.6.18-308.4.1.el5.  I rebooted 2 of them 
yesterday evening, and the last one I set an at job to reboot at 2:30 
am.  (It's our mail server so I didn't want to reboot it earlier.)  This 
morning, I noticed this last system's clock was 4-5 minutes behind the 
others.  I've stopped ntpd, reset the clock to the correct time, and 
restarted ntpd.  I've done this twice already this morning, and each 
time, the clock starts slowly drifting backwards.

The syslog entries from ntpd in /var/log/messages on the 2 other systems 
show fairly frequent occurrences of "synchronized to <IP>, stratum <n>", 
where n is usually 2 or 3.  But for the mail server with the drifting 
clock, the only ntp sync logged this week was at 21:03:03 yesterday. 
The last ones before that were April 10 & April 4, i.e. very 
irregularly.  The oldest log entries I have in /var/log/messages.4 show 
more regular syncs (at least 1-2 a day) up to March 31.  So it's 
possible this problem existed for a while and had nothing to do with the 
updates yesterday, but this is the first time the drift got so bad it 
drew attention to itself (some file modification times got out of sync 
between this server and another system).

I'd appreciate any ideas on how to tackle this problem.


Gilles R. Detillieux              E-mail: <grdetil at scrc.umanitoba.ca>
Spinal Cord Research Centre       WWW:    http://www.scrc.umanitoba.ca/
Dept. Physiology, U. of Manitoba  Winnipeg, MB  R3E 0J9  (Canada)

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