Jason Pyeron wrote:
-----Original Message----- From: centos-bounces@centos.org [mailto:centos-bounces@centos.org] On Behalf Of Warren Young Sent: Thursday, August 12, 2010 17:41 To: CentOS mailing list Subject: Re: [CentOS] Date drift and ntpd
On 8/12/2010 5:07 AM, Jason Pyeron wrote:
[root@devserver21 ~]# cat /etc/ntp.conf | grep -v ^# | grep -v ^$ restrict default nomodify notrap noquery restrict 127.0.0.1 server 192.168.1.67 server 192.168.1.66 server 192.168.1.65
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 downtime.
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 Internet.
To anyone considering flaming me, please read this first:
http://queue.acm.org/detail.cfm?id=1773943
At minimum, read the section "One server is enough". The bit on PLLs about halfway down is also directly relevant.
Okay, I only have one timeserver, but the ntp clients cowardly refuse to use less than 3. Back to the man page...
One server should be fine - you must have something else wrong, like your authoritative server not being a low stratum number - or not convinced itself that its time is correct.