Today’s tutorial will be on the infamous Quorum disk. When I first setup my GFS2 shared Cluster of 3 nodes, I was quite impressed with the fact that 3 nodes were sharing the same file system. Now that everything was up and running, I wanted to see what would happen if I brought down, 2 out of the 3 nodes in the cluster. I turned off 1st node and all was well, I was still able to access my GFS2 mount on the other 2 nodes. Then I decided to reboot the 2nd node, and guess what happened???? QUORUM DISSOLVED!!! Now on my final node the GFS2 file system was still mounted but I could not touch a file or run a ls on the mount… It just hung there!!
Well I knew this was not going to be acceptable…. Since if I still have 1 node available, the node should still be able to use the GFS2 mount. So I did some research about this quorum disk and what it can do for me. Let me tell you, this was exactly what I was looking for. 1st let me start out my explaining what a quorum is ( relating to clustering ). A quorum is the minimal number of votes that is needed in a cluster, usually the majority. So if you have 3 nodes in a cluster, that means you have a total of 3 votes in the cluster and you will need a minimum of 2 votes to remain in a quorate state. Which means you can lose 1 node in the cluster and the other nodes are still functional. But if you lose 2 nodes, your quorum will be dissolved. Which means even though your GFS2 file system is still mounted on your final node, it will not be accessible to you.
The quorum disk will help this particular situation… You ask how???? Well here it is… Qdisk needs at a minimum of a 10MB disk partition shared across the cluster.
Qdiskd runs on each node in the cluster, periodically checking its
own health and then placing its state information into its assigned
portion of the shared disk. On each node qdiskd then looks at the state of
the other nodes in the cluster as posted in their area of the qdisk
partition. When all the nodes that are running qdiskd are in a healthy state, the quorum of the cluster is increased by the value of the shared Quorum Disk.
The Value of the Quorum Disk should be n-1 ( Number of nodes – 1 ). In this case the Value should be 2 ( 3 -1 ).
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