RMAN仅当以下情况下需要快照控制文件：1.同步恢复目录时2.对当前控制文件进行备份时。在RAC环境下，仅仅在实施RMAN备份的节点上需要快照控制文件;因此快照控制文件是不需要在所有实例上全局可用的。
You can specify a cluster file system file or a raw device destination for the location of your snapshot control file.
你同样可以使用共享文件系统或裸设备指定快照控制文件的位置。
Run the following RMAN command to determine the configured location of the snapshot control file:
SHOW SNAPSHOT CONTROLFILE NAME;
You can change the configured location of the snapshot control file. For example, on UNIX-based systems you can specify the snapshot control file location as $ORACLE_HOME/dbs/scf/snap_prod.cf by entering the following at the RMAN prompt:
CONFIGURE SNAPSHOT CONTROLFILE NAME TO '$ORACLE_HOME/dbs/scf/snap_prod.cf';
This command globally sets the configuration for the location of the snapshot control file throughout your cluster database. Therefore, ensure that the directory $ORACLE_HOME/dbs/scf exists on all nodes that perform backups.
如果快照控制文件配置在共享存储上时：
If one RMAN job is already backing up the control file while another needs to create a new snapshot control file, you may see the following message:
waiting for snapshot control file enqueue
Under normal circumstances, a job that must wait for the control file enqueue waits for a brief interval and then successfully retrieves the enqueue. Recovery Manager makes up to five attempts to get the enqueue and then fails the job. The conflict is usually caused when two jobs are both backing up the control file, and the job that first starts backing up the control file waits for service from the media manager.
这样看来，每个实例使用自己的快照控制文件可以避免以上问题。
配置实例：