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Re: [Linux-cluster] GFS on SAN, does a quorum make sense?



On Thu, 2005-05-05 at 18:49 -0400, Dan B. Phung wrote:
> From my understanding,
> the quorum/voting procedure is to prevent split-brain scenarios where two
> nodes coming up for the first time might try to form two separate clusters
> of the same name, which will cause data corruption.  How would I prevent
> that, while still allowing any one node, even by itself, to access the
> storage media.

It's not to prevent two nodes: it's to prevent "less than a majority" of
the nodes (votes really) from forming their own cluster.

What you're trying to do is exactly what the algorithm is designed to
prevent :)

Consider a case where any one node can become quorate (by itself) in an
N-node cluster.  If you unplug the network cables on each node and start
up the cluster software on all N nodes, you'll end up with an N-way
split brain!  I think that is probably not a good thing.

You can do it manually by adjusting cman_tool's expected votes down to a
small number while doing a one-node boot, but please ensure the rest of
the cluster is down before doing so.

> Another use of the quorum is for distributed disks in the case of a node
> failure the I/O to that disk is fenced.  Is that correct?

Yes.

-- Lon


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