A snapshot controlfile is a read consistent copy of a database controlfile which RMAN creates because the controlfile is changed constantly.
As of Oracle 11.2 (184.108.40.206 to more precise) the snapshot controlfile must be on a location shared by all RAC nodes in a cluster (it must be accessable by all nodes on which instances for an RAC database can run). If you do not do so, you most likely will receive an ORA-00245 (ORA-00245: control file backup operation failed) when running an RMAN backup.
Although this is an requirement as of 11.2, when you create a RAC database using dbca the default location will still be the $ORACLE_HOME/dbs which in most cases is a non-shared location. Possible shared locations: cluster filesystems like OCFS and GPFS, ACFS and ASM.
You can change the snapshot controlfile location with RMAN. Because this setting is stored into the database controlfile, you only have to configure it on one instance. Use the following command to change the snapshot controlfile name:
CONFIGURE SNAPSHOT CONTROLFILE NAME TO '<shared snapshot controlfile>';
To set the snapshot controlfile location for you RAC database to an location within ASM (on an ODA which uses an RECO diskgroup for the FRA) for example use:
CONFIGURE SNAPSHOT CONTROLFILE NAME TO '+RECO/DBTS1/CONTROLFILE/snapcf_dbts1.f';
You can show the current setting for the snapshot controlfile using:
SHOW SNAPSHOT CONTROLFILE NAME;
My Oracle Support (MOS) documents describing this new requirement: