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Can we configure RMAN backup for very large database to use all the nodes instead of the node where it is scheduled through crontab?

 YES

RMAN and Oracle Net in Real Application Clusters

In any RMAN connection made through a net service name, each net service name must specify one and only one instance. This rule applies to all RMAN connections whether they are made from the command line or through the CONNECT clause in ALLOCATE CHANNEL or CONFIGURE CHANNEL commands. Therefore, you cannot specify a net service name that uses Oracle Net features to distribute RMAN connections to more than one instance.

Channel Connections to Cluster Instances

When making backups in a Real Application Clusters configuration, each allocated channel can connect to a different instance in the cluster, and each channel connection must resolve to one and only one instance. For example, configure automatic channels as follows:

CONFIGURE DEFAULT DEVICE TYPE TO sbt;
CONFIGURE DEVICE TYPE sbt PARALLELISM 3;
CONFIGURE CHANNEL 1 DEVICE TYPE sbt CONNECT = 'SYS/oracle@node1';
CONFIGURE CHANNEL 2 DEVICE TYPE sbt CONNECT = 'SYS/oracle@node2';
CONFIGURE CHANNEL 3 DEVICE TYPE sbt CONNECT = 'SYS/oracle@node3';
If the instance to which one of the channels is connected does not have the database open, then the database must not be open by any instance. In other words, either all channels must be connected to open instances, or all channels must be connected to instances that are not open. For example, if the node1 instance has the database mounted while the node2 and node3 instances have the database open, then the backup fails.

Node Affinity Awareness

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.
When deciding which channel to use to back up a particular datafile, RMAN gives preference to channels allocated at the nodes that have affinity to the datafiles you want to back up. For example, if you have a three-node cluster, and if node 1 has faster read/write access to datafiles 7, 8, and 9 than the other nodes, then node 1 has greater node affinity to those files than nodes 2 and 3.

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:

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);
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