Oracle automatically selects the optimum degree of parallelism for instance and crash recovery based on availability of CPU’s.
Oracle Database automatically makes parallel the following three stages of recovery:
Restoring Data Files When restoring data files, the number of channels you allocate in the RMAN recover script effectively sets the parallelism that RMAN uses. For example, if you allocate five channels, you can have up to five parallel streams restoring data files.
Applying Incremental Backups Similarly, when you are applying incremental backups, the number of channels you allocate determines the potential parallelism.
Applying Archived Redo Logs With RMAN, the application of archived redo logs is performed in parallel. Oracle Database automatically selects the optimum degree of parallelism based on available CPU resources.
Disable :
sql>show parameter recovery_parallelism (=0)
Enable : set to 1