[ntp:questions] NTP in a Linux cluster

Richard B. Gilbert rgilbert88 at comcast.net
Mon Sep 7 19:06:28 UTC 2009

Lorcan wrote:
> Folks,
> Could any NTP experts suggest how I should best configure NTP in
> a loosely-coupled Linux cluster, where intra-cluster synchronization
> is
> the top priority?
> I have done some reading about NTP, but can't seem to find an
> authoritative
> guide to using NTP in a cluster environment.  My company sells systems
> that
> run on small clusters of Linux servers - typically from 2 to 16
> servers ("nodes"),
> each running RedHat Linux.  All nodes in a cluster are equal.  We
> don't use
> any third-party clustering software, just the standard OS and our own
> applications.
> The main priorities are:
> 1)  Time must be kept closely synchronized between all nodes in the
> cluster.
That is, perhaps, the easiest of your requirements to satisfy.
> 2)  If one or more nodes become unavailable, synchronization must
> still be
>      maintained.
> 3)  Time must never go backwards, or jump - all changes must be by
> "slewing"

This may be difficult or impossible.  It happens rarely, but a 
computer's clock can get really messed up.  You can command NTPD to NOT 
"step" the clock but doing so may mean that a really messed up clock may 
take several days to straighten itself out!

> 4)  Time should track one or more external NTP servers as closely as
> possible,
>       while observing (1) to (3) above.

Five to ten milliseconds is usually the best you can get over the 
internet.  If you configure three different servers to use three 
independent sets of upstream servers, those servers will have similar 
but not identical times.

> The key requirement here is the as-close-as-possible synchronization
> between
> nodes in the cluster; that is far more important than closely tracking
> the external NTP server(s).

I would suggest a hardware reference clock; e.g. a GPS Timing receiver.
There are other sorts but a GPS timing receiver can be had for $100 US 
or perhaps a little less.  Such a rock solid beat is easy for NTPD to 
march to.  You can spend a lot more and some of the more expensive 
equipment may offer features you will find useful.

There are receivers that can get time from the reference signal 
broadcast by a CDMA cell phone base station.  This timing is derived 
from the NavStar GPS satellite system.

You can get some very nice but very expensive equipment from Symmetricom 
  or Meinberg Funkuhren.

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