假設每5天進行一次備份，第一次是1號，第二次是6號，第三次是11號。

那么請問，REDUNDANCY 1 的情況下，備份腳本有刪除過期備份的命令。 這種情況下，在9號時，有一個需求，將DB 使用備份恢復全庫至4號？ 是否可行？備份文件是否還在。

測試： 如下測試是測試腳本中，先刪除歷史過期的備份，之后再進行備份的測試，后續根據信息，進行總結。
delete backup;
RMAN> list backup;

RMAN> show all;

using target database control file instead of recovery catalog
RMAN configuration parameters for database with db_unique_name TT are:
CONFIGURE RETENTION POLICY TO REDUNDANCY 1; # default
CONFIGURE BACKUP OPTIMIZATION OFF; # default
CONFIGURE DEFAULT DEVICE TYPE TO DISK; # default
CONFIGURE CONTROLFILE AUTOBACKUP OFF; # default
CONFIGURE CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO '%F'; # default
CONFIGURE DEVICE TYPE DISK PARALLELISM 1 BACKUP TYPE TO BACKUPSET; # default
CONFIGURE DATAFILE BACKUP COPIES FOR DEVICE TYPE DISK TO 1; # default
CONFIGURE ARCHIVELOG BACKUP COPIES FOR DEVICE TYPE DISK TO 1; # default
CONFIGURE MAXSETSIZE TO UNLIMITED; # default
CONFIGURE ENCRYPTION FOR DATABASE OFF; # default
CONFIGURE ENCRYPTION ALGORITHM 'AES128'; # default
CONFIGURE COMPRESSION ALGORITHM 'BASIC' AS OF RELEASE 'DEFAULT' OPTIMIZE FOR LOAD TRUE ; # default
CONFIGURE ARCHIVELOG DELETION POLICY TO APPLIED ON STANDBY;
CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/u01/app/oracle/product/11.2.0/dbhome_1/dbs/snapcf_tt.f'; # default


delete noprompt obsolete redundancy 1;

RMAN> backup  database format '/u01/db_%d_%T_%U';

Starting backup at 24-SEP-20
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00004 name=/11.2.0.4/app/oracle/tt/users01.dbf
input datafile file number=00003 name=/11.2.0.4/app/oracle/tt/undotbs01.dbf
input datafile file number=00001 name=/11.2.0.4/app/oracle/tt/system01.dbf
input datafile file number=00002 name=/11.2.0.4/app/oracle/tt/sysaux01.dbf
input datafile file number=00005 name=/11.2.0.4/app/oracle/tt/example01.dbf
input datafile file number=00006 name=/11.2.0.4/app/oracle/tt/test1.dbf
input datafile file number=00007 name=/11.2.0.4/app/oracle/tt/test2.dbf
input datafile file number=00008 name=/11.2.0.4/app/oracle/tt/cc.dbf
channel ORA_DISK_1: starting piece 1 at 24-SEP-20
channel ORA_DISK_1: finished piece 1 at 24-SEP-20
piece handle=/u01/db_TT11204_20200924_41vb8jge_1_1 tag=TAG20200924T231734 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:45
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
including current control file in backup set
including current SPFILE in backup set
channel ORA_DISK_1: starting piece 1 at 24-SEP-20
channel ORA_DISK_1: finished piece 1 at 24-SEP-20
piece handle=/u01/db_TT11204_20200924_42vb8jhr_1_1 tag=TAG20200924T231734 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 24-SEP-20
RMAN-08591: WARNING: invalid archived log deletion policy

report obsolete; 無記錄，因為當前之有一次備份
--delete noprompt obsolete redundancy 1;

backup  datafile 1 format '/u01/%U.bak';
report obsolete;
RMAN retention policy will be applied to the command
RMAN retention policy is set to redundancy 1
Report of obsolete backups and copies
Type                 Key    Completion Time    Filename/Handle
-------------------- ------ ------------------ --------------------
Control File Copy     16     08-SEP-19          /11.2.0.4/app/oracle/product/11.2.0/db_1/dbs/snapcf_tt11204.f
Backup Set           4      13-MAY-19         
  Backup Piece       4      13-MAY-19          /home/oracle/work/dg/stdby.ctl
Control File Copy     29     17-SEP-19          /u01/app/oracle/product/11.2.0/dbhome_1/dbs/snapcf_tt.f
Backup Set           12     24-SEP-20         
  Backup Piece       12     24-SEP-20          /u01/db_TT11204_20200924_42vb8jhr_1_1
  
delete noprompt obsolete redundancy 1;

也就是說正常情況下先刪除過期的備份

第一次備份后，保留一次備份;
第二次備份后，保留二次備份;

第三次備份呢？
先刪除最初的第一次的備份集,保留第二次備份的集合;第三次備份成功后，還是保留2次備份?。。?



反過來，先備份，后清理歷史備份；
第一次備份，保留一次備份；
第二次備份成功后，將刪除第一次備份，還是保留一次備份
第三次同理 

根據上述結果： 如果清理歷史備份腳本在備份腳本之前，那么在9號的時候，可以使用1號的備份及歸檔，恢復至4號；
反之則無有效備份，無法回退！  【本篇文檔不考慮歸檔日志備份丟失情況，假設存在】

總結

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