[Linux-cluster] 4 node cluster even split

[rhurst at bidmc.harvard.edu]

I don't know if this could help you, but let me share our configuration
as an example.  I have overridden the default votes on 2 nodes that are
vital to application implementation of an 11-node cluster overall.  If I
cold start the cluster, those 2 nodes running alone are enough to
quorate, because they each carry 5 votes each for a total of 10 votes.
The remaining 9 nodes carry the default 1 vote each, so the cluster
expected votes are 19.

10 = (19 / 2) + 1

If I lose 1 of those 2 network director nodes, you lose 5 votes but you
remain quorate, unless you lose 5 more regular nodes along with it.  If
I lose BOTH network director nodes (10 votes), I don't care about
quorum, because my application is dead anyways (no network directors
managing client connections!).  But, we have a contingency plan to
"promote" one of the failover nodes as a network director by running the
correct services and adjusting its vote count to 5 for extra redundancy.

It would be nice to see other implementations that vary from the typical
1 vote per node cluster.


On Tue, 2007-04-10 at 12:43 +0100, Patrick Caulfield wrote:

> Janne Peltonen wrote:
> > Hi!
> > 
> > I've been wondering...
> > 
> > If I were to build a 4 node cluster, what would happen (with default
> > quorum settings) if I lost two nodes at once (say, by losing one of my
> > two blade racks)?  Would the remaining two nodes be able to continue, or
> > would they consider quorum dissolved and shut down the remaining
> > services? I only have three nodes for testing purposes, so I haven't
> > been able to look at this yet.
> 
> Under normal circumstances you need (n/2)+1 nodes to keep quorum. So if you lose
> two nodes out of four then the services will stop. To prevent this you can use
> the qdisk program to keep the cluster quorate in such circumstances.
> 

Robert Hurst, Sr. Caché Administrator
Beth Israel Deaconess Medical Center
1135 Tremont Street, REN-7
Boston, Massachusetts   02120-2140
617-754-8754 ∙ Fax: 617-754-8730 ∙ Cell: 401-787-3154
Any technology distinguishable from magic is insufficiently advanced.
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