[ntp:questions] question regarding NTP configuration for clusters, and "cluster time" stability

rotordyn@yahoo.com rotordyn1 at gmail.com
Mon Sep 14 19:09:12 UTC 2009

I have a question that seems somewhat similiar to one that was just
but there are a couple of differences, so I figured I'd ask mine as
Apologies for the long post, but I'm trying to skip the "more info
phase. :-)

I have a product that is comprised of a cluster of Linux nodes, with
cluster ranging in size from 4 to over 100 nodes. To date, we've used
the version of NTP included in the OS (SLES 10) to maintain internal
time synchronization in the cluster, but without associations to any
external NTP servers nor any hardware based time sources. While
this has worked satisfactorily, it does allow for a gradual drift
UTC over time, so we'd like to extend the product to eliminate this.

What this means in terms of requirements is that we still must
maintain a stabile internal "cluster time" with sub-second tolerance.
This should be trivial for NTP to maintain, as that is a rather loose
tolerance compared to many others I've seen discussed. The requirement
to match true UTC is even looser, as all we're trying to do is enable
the use of an external reference to stop what can be a perpetual
Just to give it a number though, let's say we'd like it to be within
60 seconds of UTC.

The topology of our cluster has two tiers. All of the nodes are
over a private network, and some subset of the nodes also have
connections to the LAN where it is deployed. The subset is always at
least 2 nodes, and can be as high as 25% of the total number of nodes.

Prior to extending the product to allow use of an external (to the
NTP server or servers, those nodes with external connections were
configured as peer servers to the internal cluster, with all other
pure clients.

After adding support for external NTP servers, we kept something like
the same config: The nodes with external connections were still
servers to the internal network, and were peers of each other. But now
they were also clients of one or more external servers. I understand
requiring three or more would be better, and we can do that, but we
have to ensure stability of the internal cluster time even if a
reduced set
of servers (including the null set) were reachable.

Our configuration did not work, because we were able to cause
in the internal cluster time with perturbations in the external
server. And we
have to guarantee stabililty even with bad inputs.

What happened was that some (but not all) of those externally
nodes deemed the external server a false ticker, and stopped believing
But some of the other externally connected nodes did not, and as a
there was time divergence between members of this group. It is this
that I'm referring to when I speak of a lack of stability.

So before I go into configuration details, is there a known "best way"
handle the sort of requirements I described? It sounds like orphan
might provide functionality I'm looking for, but I figured in parallel
emperical experimentation, I'd pursue the analytical approach and ask
people who know more than me. :)


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