RMAN> configure retention policy to redundancy 2;
Let’s say you make five backups of a specific datafile, one on each day of the workweek,
starting on a Monday. Thus, on Friday, you end up with five different backups of that datafile.
However, the first three days backups of the datafile are deemed obsolete by RMAN, since
you’ve set your backup redundancy value at 2. That is, only the Thursday and Friday backups are considered nonobsolete backups.
RMAN>configure retention policy of recovery window of 2 days.
Well in case of recovery window setting backup is counted as in number of days instead of count. So even if the backup is obsolete then also oracle will keep this backup as to ensure that database will be fully recovered.
Even if you fire report obsolete then also RMAN will display only those backup copies which can be deleted and will cause no harm.
Example: Click on the below link to find out the perfect example of difference between the above two’s: