Oracle11gR1的新特性，Rman備份UNDO表空間時排除已經提交的會話對應的數據，提高了Rman備份的效率。
官方文檔：http://docs.oracle.com/cd/B28359_01/server.111/b28279/chapter1.htm#AREANO02323
我們知道，UNDO表空間主要用於存儲前鏡像數據，這些數據在回滾以及恢復過程中可能被用到。但是一個生產數據庫的UNDO表空間可能會變得非常巨大，而備份完整的UNDO數據文件在恢復時一般可能用到的比例很小。
測試一下：
--數據庫版本 sys@ORCL>select * from v$version; BANNER ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production PL/SQL Release 11.2.0.4.0 - Production CORE 11.2.0.4.0 Production TNS for Linux: Version 11.2.0.4.0 - Production NLSRTL Version 11.2.0.4.0 - Production --創建環境 zx@ORCL>insert into t1 select * from dba_segments; 5887 rows created. zx@ORCL>insert into t1 select * from dba_segments; 5887 rows created. zx@ORCL>insert into t1 select * from dba_segments; 5887 rows created. zx@ORCL>insert into t1 select * from dba_segments; 5887 rows created. zx@ORCL>insert into t1 select * from dba_segments; 5887 rows created. zx@ORCL>insert into t1 select * from dba_segments; 5887 rows created. zx@ORCL>insert into t1 select * from dba_segments; 5887 rows created. zx@ORCL>insert into t1 select * from dba_segments; 5887 rows created. zx@ORCL>insert into t1 select * from dba_segments; 5887 rows created. zx@ORCL>commit; Commit complete. zx@ORCL>delete from t1; 288463 rows deleted. zx@ORCL>select status,sum(bytes)/1024/1024 from dba_undo_extents group by status; STATUS SUM(BYTES)/1024/1024 --------------------------- -------------------- UNEXPIRED 9.125 EXPIRED .4375 ACTIVE 89.125 zx@ORCL>commit; Commit complete. --兩次備份undo表空間文件 RMAN> backup datafile 5; Starting backup at 2016-12-22 13:09:27 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=00005 name=/u02/app/oracle/oradata/orcl/undotbs2_01.dbf channel ORA_DISK_1: starting piece 1 at 2016-12-22 13:09:27 channel ORA_DISK_1: finished piece 1 at 2016-12-22 13:09:28 piece handle=/u02/app/oracle/product/11.2.4/db1/dbs/3aro4007_1_1 tag=TAG20161222T130927 comment=NONE channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01 Finished backup at 2016-12-22 13:09:28 Starting Control File and SPFILE Autobackup at 2016-12-22 13:09:28 piece handle=/u02/app/oracle/product/11.2.4/db1/dbs/c-1444351641-20161222-0f comment=NONE Finished Control File and SPFILE Autobackup at 2016-12-22 13:09:31 --查看備份後的文件大小 RMAN> list backup of datafile 5; List of Backup Sets =================== --第一次備份文件大小99.27M BS Key Type LV Size Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 87 Full 99.27M DISK 00:00:03 2016-12-22 12:11:54 BP Key: 87 Status: AVAILABLE Compressed: NO Tag: TAG20161222T121151 Piece Name: /u02/app/oracle/product/11.2.4/db1/dbs/36ro3sk7_1_1 List of Datafiles in backup set 87 File LV Type Ckp SCN Ckp Time Name ---- -- ---- ---------- ------------------- ---- 5 Full 9042031 2016-12-22 12:11:51 /u02/app/oracle/oradata/orcl/undotbs2_01.dbf --第二次備份文件大小2.16M BS Key Type LV Size Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 89 Full 2.16M DISK 00:00:01 2016-12-22 12:34:42 BP Key: 89 Status: AVAILABLE Compressed: NO Tag: TAG20161222T123441 Piece Name: /u02/app/oracle/product/11.2.4/db1/dbs/38ro3tv1_1_1 List of Datafiles in backup set 89 File LV Type Ckp SCN Ckp Time Name ---- -- ---- ---------- ------------------- ---- 5 Full 9042576 2016-12-22 12:34:41 /u02/app/oracle/oradata/orcl/undotbs2_01.dbf --查看操作系統文件大小 [oracle@rhel6 release]$ ls -lh /u02/app/oracle/product/11.2.4/db1/dbs/36ro3sk7_1_1 -rw-r----- 1 oracle oinstall 100M Dec 22 12:11 /u02/app/oracle/product/11.2.4/db1/dbs/36ro3sk7_1_1 [oracle@rhel6 release]$ ls -lh /u02/app/oracle/product/11.2.4/db1/dbs/38ro3tv1_1_1 -rw-r----- 1 oracle oinstall 2.2M Dec 22 12:34 /u02/app/oracle/product/11.2.4/db1/dbs/38ro3tv1_1_1
這個新特性也有一些限制
- Compatible parameter must be set to 11.0 or higher
- Backup must use a disk or OSB channel
- For 'backup copy of <object>' or 'backup datafilecopy' the database must be open for undo optimization to be used.
- Not active for LEVEL 1 incremental backups, only for LEVEL 0 and FULL backups
MOS文檔：RMAN 11G : RMAN UNDO backup optimization (文檔 ID 406468.1)
A Complete Understanding of RMAN Compression (文檔 ID 563427.1)