Oracle RAC cluster fencing tips Oracle Tips by Steve Karam

Question:  What is RAC cluster fencing?  Traditionally, "fencing" has described the segmentation of RAM and CPU, but the term fencing appears to be different in Real Application Clusters?

Answer:  Nodes in a RAC cluster can fall victim to conditions called Split Brain and Amnesia.  These conditions usually occur from a temporary network disconnect.  Because of the disconnect, the "sick" node thinks it is the only node in the cluster, and forms its own "sub cluster" consisting only of itself.

In this case, the cluster needs to correct the issue.  Traditional clusters use a process called STONITH (Shoot The Other Node In The Head) in order to correct the issue; this simply means the healthy nodes kill the sick node.  Oracle's Clusterware does not do this; instead, it simply gives the message "Please Reboot" to the sick node.  The node bounces itself and rejoins the cluster.

There are other methods of fencing that are utilized by different hardware/software vendors.  When using Veritas Storage Foundation for RAC (VxSF RAC), you can implement I/O fencing instead of node fencing.  This means that instead of asking a server to reboot, you simply close it off from shared storage.