The degree of parallelism for a specific device type controls the number of server sessions
that will be used for I/O for a specific device type. You use the configure device type ...
parallelism command to specify the number of automatic channels to be assigned for both
types of device types—disk and tape. The default degree of parallelism is 1. It’s best to allocate only one channel for each physical device on the server. That is, if you have only a single disk drive, don’t set the degree of parallelism (default is 1).
You can use the show device type command to see the current parallelism settings:
RMAN> show device type;
RMAN configuration parameters are:
CONFIGURE DEVICE TYPE DISK PARALLELISM 1 BACKUP TYPE TO BACKUPSET;