Using the Quorum Flag to Prevent Split-brain

If you do not use I/O fencing, you can use the quorum flag to prevent split-brain in situations where the cluster splits with an unequal number of nodes in each sub-cluster.

In the event of a link failure, when a new cluster membership is created, GAB checks the number of systems in the new membership. If the number of systems is less than half the previous membership and if the quorum flag is set, VCS panics all nodes in the sub-cluster, thereby preventing split-brain.

Behavior Scenarios with the Quorum Flag Enabled

• If a 7-node cluster splits into two sub-clusters of 4 and 3 nodes respectively, the 3-node cluster panics.
• If a 4-node cluster splits into two sub-clusters of 2 nodes each, a split-brain condition occurs even if the quorum flag is set.
• If a 4-node cluster splits into four single-node sub-clusters, all sub-clusters panic.

Enabling and Disabling the Quorum Flag

Use the gabconfig command to enable and disable the quorum flag. You can add or remove the quorum flag from the file /etc/gabtab manually. Your settings take effect on system reboot.

  You must set this flag on all nodes in the cluster.

  Do not set this flag if you use I/O fencing (UseFence=SCSI3) in the cluster.

  To set (enable) the Quorum flag

1. Run the gabconfig command with the -q option:
     gabconfig -q
2. Verify that the -q flag appears in the file /etc/gabtab.

  To disable the Quorum flag

1. Run the gabconfig command with the -d option:
     gabconfig -d
2. Verify that the -q flag does not appear in the file /etc/gabtab.

  To verfiy changes to the GAB configuration