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Re: [Linux-cluster] Howto define two-node cluster in enterpriseenvironment



On Mon, 10 Jan 2011 21:21:45 +0100, "Kit Gerrits" <kitgerrits gmail com> wrote:
Hello fellow administrator,

If you have a SAN...
Why can't you have the SAN publish the same LUN to the two cluster nodes
simultaneously?

You can, but you minimally need to guarantee (not believe or think, but guarantee!) that both nodes do not

a) write to the same sectors, file systems or LVM volumes at the same time (this is actually a whole lot more difficult to do than most people think) - including boot sectors, partition tables, LVM metadata, etc, etc,

b) think they're exclusively accessing the LUN I.e. there must be something on the nodes - an application, OS tool or something else - that understands that there is more than one reader & writer to a LUN and thus synchronizes this.

It is only used as a raw device, so there should be no ugly filesystem
side-effects.

File systems only serve to make this a lot more obvious to the end user or administrator since it's integrity tends to get shot fairly quickly and there are integrity checks in place. On raw devices, you get the "benefit" of ignorance about the fact that your data is corrupt, unless b) above is true.

Hth,

// Thomas



Regards,

Kit

-----Original Message-----
From: linux-cluster-bounces redhat com
[mailto:linux-cluster-bounces redhat com] On Behalf Of Andreas Bleischwitz
Sent: maandag 10 januari 2011 11:25
To: linux-cluster redhat com
Subject: [Linux-cluster] Howto define two-node cluster in
enterpriseenvironment

Hello list,

I recently ran into some questions regarding a two-node cluster in an
enterprise environment, where single-point-of-failures were tried to be
eliminated whenever possible.

The situation is the following:
Two-node cluster, SAN-based shared storage - multipathed; host-based
mirrored, bonded NICS, Quorum device as tie-breaker.

Problem:
The quorum device is the single-point-of-failure as the SAN-device could
fail and hence the quorum-disc wouldn't be accessible.
The quorum-disc can't be host-based mirrored, as this would require cmirror
- which depends on a quorate cluster.
One solution: use storage-based mirroring - with extra costs, limited to no
support with mixed storage vendors.
Another solution: Use a third - no service - node which has to have the same SAN-connections as the other two nodes out of cluster reasons. This node
will idle most of the time and therefore be very uneconomic.

How are such situations usually solved using RHCS? There must be a way of
configuring a two-nodecluster without having a SPOF defined.

HP had a quorum-host with their no longer maintained Service Guard, which
could do quorum for more than on cluster at once.

Any suggestions appreciated.

Best regrads,

Andreas Bleischwitz

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