[CentOS] Date drift and ntpd

Thu Aug 12 21:41:17 UTC 2010
Warren Young <warren at etr-usa.com>

On 8/12/2010 5:07 AM, Jason Pyeron wrote:
> [root at devserver21 ~]# cat /etc/ntp.conf | grep -v ^# | grep -v ^$
> restrict default nomodify notrap noquery
> restrict
> server
> server
> server

Some HOWTOs tell you that more time servers is better, on a standard 
knee-jerk redundancy theory, but they're ignoring two things.

First, you already have a fallback: the system's built-in clock.  It's 
perfectly fine to run on that while you ride out your time server's 

Second, ntpd, internally, is built on a phase-locked loop, which is 
supposed to stabilize its time corrections in the face of jitter and 
other bad things out in the real world.  Like anything based on a 
negative feedback loop, however, it can be destablized with certain 
inputs.  Giving ntpd two or more servers is a pretty good way to 
destabilize its PLL in the real, non-ideal world we find on the modern 

To anyone considering flaming me, please read this first:


At minimum, read the section "One server is enough".  The bit on PLLs 
about halfway down is also directly relevant.