[CentOS] Date drift and ntpd

Jason Pyeron jpyeron at pdinc.us
Thu Aug 12 17:43:44 EDT 2010


 

> -----Original Message-----
> From: centos-bounces at centos.org 
> [mailto:centos-bounces at 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 at 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...



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