11g RMAN UNDO backup optimization

Question

Hi all?
I have tested 11g RMAN UNDO backup optimization
1st I fill the undo tablespace by sql manipulations and not commiting
2nd backed undo_ts up by RMAN (size 24m)
3rd I made a commit
Then backed undo tablespace again but backup_size didn’t change     (24m)
Then I made some more manipulations and backed undo_ts again. This time backup_size reduced. (11m)
Then I restarted db and backed up undo_ts again. This time backup size became what I expected (600K)
The question is why 11g rman undo tablespace backup size didn’t reduce after commit?
according to 11g undo optimization it had to
SQL> select sum(bytes) from dba_free_space where tablespace_name = 'UNDOTBS2';
SUM(BYTES)
13172736
SQL> begin
for i in 1..100000 loop
insert into testundo values(i);
end loop;
end;
2 3 4 5 6
PL/SQL procedure successfully completed.
SQL> SQL> update testundo set
id=2 where id>0;
2
update testundo set
ERROR at line 1:
ORA-30036: unable to extend segment by 8 in undo tablespace 'UNDOTBS2'
SQL> select sum(bytes) from dba_free_space where tablespace_name = 'UNDOTBS2';
SUM(BYTES)RMAN> backup datafile 6;
RMAN> list backup of datafile 6;
List of Backup Sets
===================
BS Key Type LV Size Device Type Elapsed Time Completion Time
10 Full *24.54M* DISK 00:00:04 10-JUN-10
BP Key: 10 Status: AVAILABLE Compressed: NO Tag: TAG20100610T142437
Piece Name: /home/oracle/flash_recovery_area/11GR1/backupset/2010_06_10/o1_mf_nnndf_TAG20100610T142437_611ctr1f_.bkp
List of Datafiles in backup set 10
File LV Type Ckp SCN Ckp Time Name
6 Full 577669 10-JUN-10 /home/oracle/oradata/11GR1/datafile/undotbs2.dbf
SQL> commit;
Commit complete.RMAN> backup datafile 6 format 'after commit.backup';
RMAN> list backup of datafile 6;
List of Backup Sets
===================
BS Key Type LV Size Device Type Elapsed Time Completion Time
11 Full *24.54M* DISK 00:00:02 10-JUN-10
BP Key: 11 Status: AVAILABLE Compressed: NO Tag: TAG20100610T142541
Piece Name: /home/oracle/product/11/Db_1/dbs/after commit.backup
List of Datafiles in backup set 11
File LV Type Ckp SCN Ckp Time Name
6 Full 577705 10-JUN-10 /home/oracle/oradata/11GR1/datafile/undotbs2.dbf
SQL> alter system archive log current;
System altered.
SQL> commit;
Commit complete.
SQL> select count(*) from testundo;
COUNT(*)
100000
SQL> delete from testundo;
100000 rows deleted.
SQL> commit;
Commit complete.
SQL> insert into testundo values(1);
1 row created.
SQL> commit;
Commit complete.
SQL> alter system flush buffer_cache;
System altered.RMAN> backup datafile 6;
RMAN> list backup of datafile 6;
List of Backup Sets
===================
BS Key Type LV Size Device Type Elapsed Time Completion Time
13 Full *11.03M* DISK 00:00:01 10-JUN-10
BP Key: 13 Status: AVAILABLE Compressed: NO Tag: TAG20100610T143359
Piece Name: /home/oracle/flash_recovery_area/11GR1/backupset/2010_06_10/o1_mf_nnndf_TAG20100610T143359_611dd8sz_.bkp
List of Datafiles in backup set 13
File LV Type Ckp SCN Ckp Time Name
6 Full 578410 10-JUN-10 /home/oracle/oradata/11GR1/datafile/undotbs2.dbf
RMAN>
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 393375744 bytes
Fixed Size 1300156 bytes
Variable Size 352323908 bytes
Database Buffers 33554432 bytes
Redo Buffers 6197248 bytes
Database mounted.
Database opened.
SQL> RMAN> backup datafile 6;
RMAN> list backup of datafile 6;
List of Backup Sets
===================
BS Key Type LV Size Device Type Elapsed Time Completion Time
14 Full *600.00K* DISK 00:00:02 10-JUN-10
BP Key: 14 Status: AVAILABLE Compressed: NO Tag: TAG20100610T152843
Piece Name: /home/oracle/flash_recovery_area/11GR1/backupset/2010_06_10/o1_mf_nnndf_TAG20100610T152843_611hlwmv_.bkp
List of Datafiles in backup set 14
File LV Type Ckp SCN Ckp Time Name
6 Full 580347 10-JUN-10 /home/oracle/oradata/11GR1/datafile/undotbs2.dbf
Thanks in advance
Turkel

Answer 1

Hi Turkel,
The space used for undo is also related to the undo retention setting.
As it seems you do a test update and proceed with backups on:
- 14:24:37 (-> 25M)
- 14:25:41 (-> 25M)
- 14:33:59 (-> 11M)
- 15:28:43 (-> 600K)
The first two backups probably are still within the undo retention period for the update.
The third backup shows a partly empty undo (is your setting 900?).
The last falls outside the retention period for the update resulting in the small backup size.
Regards,
Tycho