How does RMAN channel take the backup ?

The work of each channel, whether of type disk or System Backup Tape (SBT), is subdivided into the following distinct phases:

1. Read Phase
   A channel reads blocks from disk into input I/O buffers.
   
2. Copy Phase
   A channel copies blocks from input buffers to output buffers and performs additional processing on the blocks.
   
3. Write Phase
   A channel writes the blocks from output buffers to storage media. The write phase can take either of the following mutually exclusive forms, depending on the type of backup media:
   
   • Write Phase for System Backup Tape (SBT)
   • Write Phase for Disk
     
     
      
     
     
     REF:
     Oracle® Database Backup and Recovery User's Guide
     11g Release 2 (11.2)
     E10642-05