RAC master node tips

Question: Why do we need master node in Oracle RAC ? What are the architectural difference between the master node and the other nodes in RAC?

Answer:  be able to keep track of the resources in a RAC cluster.  There is no true concept of a master node in Oracle RAC. Instead, each instance in the cluster becomes the resource master for a subset of resources. See these notes on RAC.

Instead, the RAC master node has the smallest node ID in the RAC cluster, and is usually the first node started in the cluster.  . Node-ids are assigned to the nodes in the same order as the nodes join the cluster. Hence, normally the node which joins the cluster first is the master node.  However, in cases

 In case of  node eviction, the RAC cluster is divided into two sub-clusters, each with it's own master node. The sub-cluster containing fewer no. of nodes is evicted. But, in case both the sub-clusters have same no. of nodes, the sub-cluster having the master node survives whereas the other sub-cluster is evicted.

You can run these OS queries to see the RAC master node:

root> cat $ORA_CRS_HOME/log/`hostname`/cssd/ocssd* |grep master

or

root> for x in `ls -tr $ORA_CRS_HOME/log/`hostname`/cssd/ocssd* `; do grep -i "master node" $x ; done | tail -1

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