Retention Policy in rman REDUNDANCY or RECOVERY WINDOW , which one is effective ?

What is  Retention Policy in RMAN ?
Setting the RETENTION POLICY in RMAN keep track about the backup files and tells the report whether they are obsoleted and whether it is needed to backup the database or datafile.
Notes:- 

• If you have flash recovery configured then the database automatically deletes unnecessary files from the flash recovery area based on its internal disk quota rules. The disk quota rules are distinct from the backup retention policy rules, but the database will never delete files in violation of the retention policy to satisfy the disk quota.If you don't have flash recovery configured then the database does not delete any file even they are obsolete.

How to set retention policy in RMAN ? 

The configuration parameter RETENTION POLICY can be set by CONFIGURE RETENTION POLICY  TO .... 

What is the use of crosschek and obsolete command ?

A backup becomes obsolete based on retention policy, that it is not needed for recovery.A backup becomes expired only when RMAN perform CROSSCHECK and can't find the file. (A most common is file is deleted by OS). Obsolete means "not needed," whereas expired means "not found."

Issue the following RMAN command to crosscheck backup of an Oracle Database,

RMAN> crosscheck backup of database;

using channel ORA_DISK_1

crosschecked backup piece: found to be ‘EXPIRED’

backup piece handle=D:\STAGE\RACDEVD1_503\DATAFILE_BACKUP recid=79 stamp=475839783

crosschecked backup piece: found to be ‘EXPIRED’

backup piece handle=D:\STAGE\RACDEVD1_504\DATAFILE_BACKUP recid=137 stamp=475839784

crosschecked backup piece: found to be ‘AVAILABLE’

backup piece handle=D:\DGSTAGE\DPLCU5L5_1_1 recid=132 stamp=475839755

crosschecked backup piece: found to be ‘AVAILABLE’

backup piece handle=D:\DGSTAGE\DQLCU62F_1_1 recid=133 stamp=475839756Crosschecked 4 objects

As we see the RMAN crosscheck command compares the RMAN  catalog entries with the actual OS files and reports to locate "expired" or "obsolete" RMAN catalog entries.

Once a RMAN crosscheck identified expired, deleted  entries you can run an RMAN delete command to remove these entries to synchronize the RMAN catalog / controlfile  with the real database files:
RMAN> delete expired backup;
Identify which backups are obsolete and no longer needed for recovery

RMAN> REPORT OBSOLETE;
Delete obsolete backup information from RMAN repository.
RMAN> DELETE OBSOLETE;

How can we set retention policy in RMAN ?

There is two mutually exclusive options for setting a retention policy , redundancy and recovery window.

To set Retention policy to  redundancy 2 copies,

RMAN> CONFIGURE RETENTION POLICY TO REDUNDANCY 2;

set Retention Policy to  Recovery Window of 7 days,

RMAN>CONFIGURE RETENTION POLICY TO RECOVERY WINDOW OF 7 DAYS;

Whenever retention policy is set to redundancy of 2 copies the rman at least retains latest 2 copies of each datafile. If I took 3rd backup of datafile 3 then 1st backup of datafile 3 become obsolete. 

If you specify a recovery window of 7 days. That means rman will not obsolete any backup needed for recovery to any point in the last seven days. Check. But what's the subtle detail?

Suppose you take a full backup on 1 Sep, then start taking incremental backups going forward. Now we're down to 10 Sep which puts that full backup 3 days earlier than the recovery window. However, it is still the most recent full backup and so is needed to recover into any time during the 7 day recovery window. Therefore, it will NOT be marked obsolete.

Think about what this does in the textbook backup routine of a full backup every 7 days, with incremental backups the other 6 days. We take a full backup on day 1. We take incremental backups on days 2 - 7. We take a full backup again on Day 8, and incrementals on days 9 - 14. 

Day  =  1  2  3  4  5  6  7  8  9 10 11 12 13 14

Bkup =  F  I   I   I  I   I   I  F   I   I   I   I   I    I

On day 14, our recovery window reaches back to day 7, and the only way to recover to day 7 is to use the full backup from day 1 and the incrementals from 2 through 7. So even though we have a 7 day recovery window, we actually have 14 days worth of backups that are yet to go obsolete. On day 15, we can obsolete and drop everything prior to the full backup on Day 8. Thus the Recovery Window retention policy doesn't just keep the last N days of the backup. It calculates the point-of-recovery (POR) value, accounting the current date and the number of the days mentioned in the retention policy. 

The default retention policy is REDUNDANCY = 1

you can disable the retention policy by setting,

RMAN>CONFIGURE RETENTION POLICY TO NONE;

If the retention policy is configured to NONE, then REPORT OBSOLETE and DELETE OBSOLETE do not consider any backups to be obsolete

Conclusion :- 
When having a rman retention policy based on REDUNDANCY is a bad idea ,if you intend to incomplete recover your database to a previous time in the past it's really a good idea to switch to a retention policy based on a "RECOVERY WINDOW" instead.