e applied to the command
RMAN retention policy is set to redundancy 1
using channel ORA_DISK_1
Deleting the following obsolete backups and copies:
Type Key Completion Time Filename/Handle
Backup Set 20 08-JUL-12
Backup Piece 20 08-JUL-12 /u01/app/oracle/flash_recovery_area/ORCL/backupset/2012_07_08/o1_mf_nnndf_TAG20120708T190559_7zltdqxy_.bkp
Do you really want to delete the above objects (enter YES or NO) yes
deleted backup piece
backup piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2012_07_08/o1_mf_nnndf_TAG20120708T190559_7zltdqxy_.bkp RECID=20 STAMP=788123159
Deleted 1 objects
可以看到，从10g开始设置的REDUNDANCY的值，就是最终备份保留的个数。为了确认这个问题，特意查询了一下9i和10g的官方文档。www.2cto.com
9i的说法是：
The REDUNDANCY parameter specifies that any number of backups or copies beyond a specified number need not be retained.
而10g的文档中，该参数的描述变为：
A redundancy-based backup retention policy determines whether a backup is obsolete based on how many backups of a file are currently on disk.
Oracle改变功能的实现很常见，但是没有想到，对于这种细节的定义也会调整。对于熟悉9i备份策略的DBA需要留神，在设置10g以后的RMAN备份保留策略时，需要在9i的基础上增加1。