最近发现10g的RMAN备份保留REDUNDANCY策略和9i相比发生了改变。
在Oracle9i中，备份保留策略的REDUNDANCY的值，指的是备份冗余的个数。也就是说，如果REDUNDANCY设置为1，那么Oracle会保留2个备份。
但是在10g以后，REDUNDANCY的值，就是最终备份保留的值，手头没有10g的环境，用11g的rman做了一个例子：
solaris*orcl-/home/oracle$ rman target /
Recovery Manager: Release 11.2.0.3.0 - Production on Sun Jul 8 19:04:43 2012
Copyright (c) 1982, 2011, Oracle and/or its affiliates. All rights reserved.
connected to target database: ORCL (DBID=1299676637)
RMAN> show retention policy;
using target database control file instead of recovery catalog
RMAN configuration parameters for database with db_unique_name ORCL are:
CONFIGURE RETENTION POLICY TO REDUNDANCY 1; # default
RMAN> backup tablespace ts_32k;
Starting backup at 08-JUL-12
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=180 device type=DISK
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datbafile(s) in backup set
input datafile file number=00005 name=/u01/app/oracle/oradata/ORCL/datafile/o1_mf_ts_32k_7w1w3zmb_.dbf
channel ORA_DISK_1: starting piece 1 at 08-JUL-12
channel ORA_DISK_1: finished piece 1 at 08-JUL-12
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2012_07_08/o1_mf_nnndf_TAG20120708T190559_7zltdqxy_.bkp tag=TAG20120708T190559 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 08-JUL-12
RMAN> backup tablespace ts_32k;
Starting backup at 08-JUL-12
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datbafile(s) in backup set
input datafile file number=00005 name=/u01/app/oracle/oradata/ORCL/datafile/o1_mf_ts_32k_7w1w3zmb_.dbf
channel ORA_DISK_1: starting piece 1 at 08-JUL-12
channel ORA_DISK_1: finished piece 1 at 08-JUL-12
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2012_07_08/o1_mf_nnndf_TAG20120708T190609_7zltf22b_.bkp tag=TAG20120708T190609 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:02
Finished backup at 08-JUL-12
RMAN> list backup of tablespace ts_32k;
List of Backup Sets
===================
BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
20 Full 2.69M DISK 00:00:01 08-JUL-12
BP Key: 20 Status: AVAILABLE Compressed: NO Tag: TAG20120708T190559
Piece Name: /u01/app/oracle/flash_recovery_area/ORCL/backupset/2012_07_08/o1_mf_nnndf_TAG20120708T190559_7zltdqxy_.bkp
List of Datafiles in backup set 20
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
5 Full 28932281 08-JUL-12 /u01/app/oracle/oradata/ORCL/datafile/o1_mf_ts_32k_7w1w3zmb_.dbf
BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
21 Full 2.69M DISK 00:00:01 08-JUL-12
BP Key: 21 Status: AVAILABLE Compressed: NO Tag: TAG20120708T190609
Piece Name: /u01/app/oracle/flash_recovery_area/ORCL/backupset/2012_07_08/o1_mf_nnndf_TAG20120708T190609_7zltf22b_.bkp
List of Datafiles in backup set 21
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
5 Full 28932300 08-JUL-12 /u01/app/oracle/oradata/ORCL/datafile/o1_mf_ts_32k_7w1w3zmb_.dbf
RMAN> delete obsolete;
RMAN retention policy will be 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。
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。