Oracle Rac database Rman Backup using multiple Instances -- Node affinity

Recently we had requirement to share rman backup across  nodes .   Came across Oracle article that facilities requirement using  Node affinity

In some cluster database configurations, some nodes of the cluster have faster access to some datafiles than to other datafiles. RMAN automatically detects this affinity, which is known as node affinity awareness.

To use node affinity, configure RMAN channels on the nodes of the cluster that have affinity to the datafiles you want to back up. For example, use the syntax:

-- setup for a parallel backup

RMAN> CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO COMPRESSED BACKUPSET PARALLELISM 4;

RMAN> CONFIGURE CHANNEL 1 DEVICE TYPE DISK CONNECT 'sys/passwd@inst1';

RMAN> CONFIGURE CHANNEL 2 DEVICE TYPE DISK CONNECT 'sys/passwd@inst1';

RMAN> CONFIGURE CHANNEL 3 DEVICE TYPE DISK CONNECT 'sys/passwd@inst2';

RMAN> CONFIGURE CHANNEL 4 DEVICE TYPE DISK CONNECT 'sys/passwd@inst2' ;

CONFIGURE DEVICE TYPE sbt PARALLELISM 3;

CONFIGURE DEFAULT DEVICE TYPE TO sbt; 

CONFIGURE CHANNEL 1 DEVICE TYPE sbt CONNECT 'user1/password1@node1';

CONFIGURE CHANNEL 2 DEVICE TYPE sbt CONNECT 'user2/password2@node2';

CONFIGURE CHANNEL 3 DEVICE TYPE sbt CONNECT 'user3/password3@node3';

You can manually override the automatic node affinity by specifying which channels should back up which datafiles. For example:

BACKUP

  # channel 1 gets datafile 1

  (DATAFILE 1 CHANNEL ORA_SBT_TAPE_1)

  # channel 2 gets datafiles 2-4

  (DATAFILE 2,3,4 CHANNEL ORA_SBT_TAPE_2)

  # channel 3 gets datafiles 5-10

  (DATAFILE 5,6,7,8,9,10 CHANNEL ORA_SBT_TAPE_3); 

References : 

https://docs.oracle.com/cd/B10500_01/rac.920/a96596/backup.htm