I have seen many users asking how to find “Master node” in Oracle RAC, let me clear some of their doubts.

There are two types of Masters in Oracle RAC, one is Mater node at Oracle Clusterware level and other is Master node for specific resource or block or object.

The node which gets the active state during startup is authorized to be a master node by Cluster Synchronization Service.

Run the below command to find which node is master at Clusterware level
$cat $ORA_CRS_HOME/log/`hostname`/cssd/ocssd* |grep master
or
$ for x in `ls -tr $ORA_CRS_HOME/log/`hostname`/cssd/ocssd* `; do grep -i "master node" $x ; done | tail -1

The OCR Automatic backups are taken only by master node. If the Master fails, the OCR backups will be created on the new Master. The Master node which has OCR backups goes down due to failure then we cannot be recover the OCR that’s why Oracle recommends taking backups using “ocrconfig” and also integrating OCR backups with backup strategy.

Run the below command to find which node is OCR Master and taking automatic backups.
$ocrconfig –showbackup
testrac02 2010/08/30 16:29:52 /oracle/crs/cdata/crs
testrac02 2010/08/30 16:29:52 /oracle/crs/cdata/crs
testrac02 2010/08/30 12:29:49 /oracle/crs/cdata/crs
testrac02 2010/08/30 08:29:46 /oracle/crs/cdata/crs
testrac02 2010/08/29 00:29:23 /oracle/crs/cdata/crs

The block level masters are used by Cache fusion while transferring the block. Any node can become the master node of a particular block and you can also see which node acting as master in V$GES_RESOURCE table (MASTER_NODE column)

You can manually remaster an object with oradebug command:
SQL> oradebug lkdebug -m pkey "object_id"

Regards
Satishbabu Gunukula
http://www.oracleracexpert.com