What happens internally , when you backup database using rman ..

if you give ' Backup database ' from RMAN, it will first get all the datafiles information from the controlfile.

 Then it will divide all the datafiles among the allocated channels. (Roughly equal size of work as per the datafile size)  .Then it takes the backup in 2 steps

Step1:

The channel will read all the Blocks of the entire datafile to find out all the formatted blocks to backup

Note:

  RMAN do not take backup of the unformatted blocks

Step2:

  In the second step it takes back up of the formatted blocks

Example:

• This is the best advantage of using RMAN as it only takes back up of the required blocks 
•   Lets say in a datafile of 100 MB size, there may be only 10 MB of use full data and rest 90 MB is free then RMAN will only take backup of those 10 MB