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Re: [Linux-cluster] qdisk WITHOUT fencing



The problem you are overlooking is that without a reliable way to prevent split-brain, you cannot ensure that the services you are trying to make resilient will handle failure without resource clashes.

If you have a suggestion on how to make that viable, I'm sure it will be listened to. But I cannot see how you can logically prevent resource clash (or worse, in case of a shared file system, data corruption) without a reliable fencing method.

If all you want to do is fail over some floating IPs, then fair enough, you might be able to get away to some extent without fencing (you can always manually get into the nodes via their fixed IPs to rectify any issues). For for anything more complex, I don't see how you can make do without reliable fencing.

Gordan

On 18/06/2010 00:31, Jankowski, Chris wrote:
Jim,

You hit architectural limitation of Linux Cluster, which is specific to Linux Cluster design, which other clusters tend not to have.

Linux Cluster assumes that you will *always* be able to execute fencing of *all* other nodes.  In fact, this is a stated *prerequisite* for correct operation of the cluster.

This is all very well when you have two PCs under your desk and a power switch.

However, this model completely fails when any network more complex then a power switch is present. Your network fails and you have a partitioned cluster that cannot fence. It all gets stuck. From a practical, operational point of view of an IT this is a disaster worse then not having a cluster.

Having come to Linux Cluster with a TruCluster background, I always had a problem with the STONITH approach used by Linux Cluster. I deem it harmful. But I see no inclination anywhere in the Linux Cluster world to remove it.

I believe that there is a major philosophical chasm dividing the design stance between the Linux Cluster and others. The Linux Cluster seems to be saying "A node is the centre of the world and can control it".  Other clusters take the opposite stance: "A node is a part of the world, cannot control it and may have a very limited visibility of the world in some circuumstances."

Regards,

Chris Jankowski



-----Original Message-----
From: linux-cluster-bounces redhat com [mailto:linux-cluster-bounces redhat com] On Behalf Of jimbob palmer
Sent: Friday, 18 June 2010 01:59
To: linux-cluster redhat com
Subject: [Linux-cluster] qdisk WITHOUT fencing

Dear distinguished linux-cluster members!

I have two data centers linked by physical fibre. Everything goes over this physical route: everything.

I would like to setup a high availability nfs server with drbd:
* drbd to replicate storage
* nfsd running
* floating ip

If the physical link between the two data centers is lost, I would like the primary data center to win.

I've setup a qdisk, and this works well: the node which can access the qdisk wins. i.e. the primary datacenter, which is the data center where the san holding the qdisk also lives, wins.

Unfortunately for me, I get pages and pages of errors about being unable to fence the secondary node.

The docs tell me that I absolutely must use power fencing, but in this case fencing makes no sense: it won't work when the link between the data centers is severed. The network, and the qdisk is the decider for who "wins".

So what should I do?

Many thanks in advance.

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