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Ash_R's avatar
Ash_R
Level 3
9 years ago

Majority Based Fencing

Hi All,

I was configuring majority based fencing (introduced in 6.2) and had a question which is more logical than practical.

Suppose I have a 10 node cluster and my cluster breaks off at 3 points, leading to two 3 node subclusters and one 4 node sub-cluster.

In that case clearly the 4 node sub-cluster has the majority and should win the fencing race. But as a sub-cluster, the 4 node sub-cluster will think that there are totally 10 nodes and it is a 4 node sub-cluster, which implies it doesn't have the majority. In this case will it panic ? Or can it somehow determine that the other 6 nodes arent in 1 sub-cluster and that it has the majority ??

Thanks.

  • Majority based fencing allows a 1/2 or more subcluster to win.  This feature was created to allow availability within an appliance form factor to not need an external coordination point.  In the scenario you describe you have subdevided into 3 sub-clusters, each having at maximum of 4-nodes within them.  In this scenario, all three subclusters will panic.  If you used SCSI3 Coordinator disks or Coordination Point Servers (CPS) then your 4-node sub cluster would have the advantage in the fencing race.  Introducing Preferred Fencing would just muddy the waters.

     

    If you are looking to satisfy the 3 subcluster solution, I would advise you not to use Majority Fencing.

  • The sub-cluster that has the majority at the time of split will win the fencing race. In your scenario where there are 10 nodes the sub-clustrer majority will be 6. If a split-brain occurs than the sub-cluster with 6 or mode nodes will survive. But if the cluster splits into 3-3-4 at the same time than it may lead to a scenario where no nodes in the cluster are running as there is no sub-cluster that has the majority (6 nodes).
     

    If the cluster has to split in 3-7 and once the fencing race has occured and the majority sub-cluster won and another split in 3-4 occurs than the sub-cluster with 4 nodes would ultimately survive.

    Hope that answers your query.

     

    Regards,

    Sudhir
     

2 Replies

  • Majority based fencing allows a 1/2 or more subcluster to win.  This feature was created to allow availability within an appliance form factor to not need an external coordination point.  In the scenario you describe you have subdevided into 3 sub-clusters, each having at maximum of 4-nodes within them.  In this scenario, all three subclusters will panic.  If you used SCSI3 Coordinator disks or Coordination Point Servers (CPS) then your 4-node sub cluster would have the advantage in the fencing race.  Introducing Preferred Fencing would just muddy the waters.

     

    If you are looking to satisfy the 3 subcluster solution, I would advise you not to use Majority Fencing.

  • The sub-cluster that has the majority at the time of split will win the fencing race. In your scenario where there are 10 nodes the sub-clustrer majority will be 6. If a split-brain occurs than the sub-cluster with 6 or mode nodes will survive. But if the cluster splits into 3-3-4 at the same time than it may lead to a scenario where no nodes in the cluster are running as there is no sub-cluster that has the majority (6 nodes).
     

    If the cluster has to split in 3-7 and once the fencing race has occured and the majority sub-cluster won and another split in 3-4 occurs than the sub-cluster with 4 nodes would ultimately survive.

    Hope that answers your query.

     

    Regards,

    Sudhir