On Wed, 2009-11-18 at 11:08 +0000, Karl Podesta wrote: > On Wed, Nov 18, 2009 at 06:32:25AM +0100, Fabio M. Di Nitto wrote: > > > Apologies if a similar question has been asked in the past, any inputs, > > > thoughts, or pointers welcome. > > > > Ideally you would find a way to plug the storage into the 2 nodes that > > do not have it now, and then run qdisk on top. > > > > At that point you can also benefit from "global" failover of the > > applications across all the nodes. > > > > Fabio > > Thanks for the reply and pointers, indeed the 4 nodes attached to storage > with qdisk sounds best... I believe in the particular scenario above, > 2 of the nodes don't have any HBA cards / attachment to storage. Maybe > an IP tiebreaker would have to be introduced if storage connections could > not be obtained and the cluster was to split into two. > > I wonder how common that type of quorum disk setup would be these days, > I gather most would use GFS in this scenario with 4 nodes, eliminating > the need for any specific failover of an ext3 disk mount etc., and merely > failing over the services accross all cluster nodes instead. We don't have an IP tiebreaker in the traditional sense. I wrote a demo IP tiebreaker which works for 2 node clusters, but it does not work in 4 node clusters since there is no coordination about whether other nodes in a partition can "see" the tiebreaker in the demo application. You can use a tweaked version of Carl's weighted voting scheme to be able to sustain 2 node failures 1/2 the time in a 4 node cluster: node# 1 2 3 4 votes 1 3 5 4 Votes = 13 Quorum = 7 Any 1 node can fail: Nodes 1 2 3 = 9 votes Nodes 2 3 4 = 12 votes Nodes 1 3 4 = 10 votes Nodes 1 2 4 = 8 votes Half of the time, 2 nodes can fail (ex: if you were worried about a random partition between 2 racks): Nodes 2 3 = 8 votes Nodes 3 4 = 9 votes Nodes 2 4 = 7 votes Obviously in the other half of the possible failure permutations, 2 nodes failing would mean loss of quorum: Nodes 1 2 = 4 votes -> NO QUORUM Nodes 1 3 = 6 votes -> NO QUORUM Nodes 1 4 = 5 votes -> NO QUORUM If you do this, put your critical applications on nodes 1 and 2. In the event of a failure, nodes 3 and 4 can pick up the load without losing quorum. Well, in theory ;) -- Lon -- Linux-cluster mailing list Linux-cluster@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/linux-cluster
