在使用RMAN进行数据库备份时，数据文件中从未使用过的数据块不会被备份，从Oracle 10.2.0.3开始，RMAN也能够跳过那些使用过，但是已经被释放的区间（Extent），只备份那些真正存有数据的区，从而可以缩减备份集的大小。
在Oracle 10.2.0.3之前的版本中，测试结果如下
SQL> create tablespace eygle datafile '/opt/oracle/oradata/test97/eygle01.dbf' size 50M;
SQL> alter user eygle default tablespace eygle;
SQL> connect eygle/eygle
SQL> create table eygle as select * from dba_objects;
SQL> insert into eygle select * from dba_objects;
SQL> insert into eygle select * from dba_objects;
SQL> insert into eygle select * from dba_objects;
SQL> insert into eygle select * from dba_objects;
SQL> commit;
SQL> col segment_name for a30
SQL> select segment_name,bytes/1024/1024 from dba_segments where segment_name='EYGLE';
SEGMENT_NAME BYTES/1024/1024
------------------------------ ---------------
EYGLE 5
这个对象占用的区间情况：
SQL> select segment_name,block_id,blocks from dba_extents where segment_name='EYGLE';
SEGMENT_NAME BLOCK_ID BLOCKS
------------------------------ ---------- ----------
EYGLE 9 8
EYGLE 17 8
EYGLE 25 8
EYGLE 33 8
EYGLE 41 8
EYGLE 49 8
EYGLE 57 8
EYGLE 65 8
EYGLE 73 8
EYGLE 81 8
EYGLE 89 8
EYGLE 97 8
EYGLE 105 8
EYGLE 113 8
EYGLE 121 8
EYGLE 129 8
EYGLE 137 128
EYGLE 265 128
EYGLE 393 128
EYGLE 521 128
我们备份一下这个数据文件：
RMAN> backup datafile 9 format '/opt/oracle/obak/%U';
RMAN> exit
这个备份文件大约有5M左右：
$ ls -l obak/*
-rw-r----- 1 oracle dba 5341184 Sep 7 16:57 obak/01irbtb1_1_1
进行一下Move操作：
SQL> alter table eygle move tablespace eygle;
Table altered.
该测试表的存储空间此时发生了变化：
SQL> select segment_name,block_id,blocks from dba_extents where segment_name='EYGLE';
SEGMENT_NAME BLOCK_ID BLOCKS
------------------------------ ---------- ----------
EYGLE 649 8
EYGLE 657 8
EYGLE 665 8
EYGLE 673 8
EYGLE 681 8
EYGLE 689 8
EYGLE 697 8
EYGLE 705 8
EYGLE 713 8
EYGLE 721 8
EYGLE 729 8
EYGLE 737 8
EYGLE 745 8
EYGLE 753 8
EYGLE 761 8
EYGLE 769 8
EYGLE 777 128
EYGLE 905 128
EYGLE 1033 128
EYGLE 1161 128
EYGLE 1289 128
再作一次备份：
$ rman target /
Recovery Manager: Release 10.2.0.1.0 - Production on Fri Sep 7 16:57:59 2007
Copyright (c) 1982, 2005, Oracle. All rights reserved.
connected to target database: TEST97 (DBID=765011863)
RMAN> backup datafile 9 format '/opt/oracle/obak/%U';
此时的备份集大小变为10M，也就是说Move之前和之后的空间都被RMAN备份了下来：
$ ls -l obak/*
-rw-r----- 1 oracle dba 5341184 Sep 7 16:57 obak/01irbtb1_1_1
-rw-r----- 1 oracle dba 10608640 Sep 7 16:58 obak/02irbtd5_1_1
再来看一下RMAN的报告:
RMAN> list backup of tablespace eygle;
using target database control file instead of recovery catalog
List of Backup Sets
===================
BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
1 Full 5.09M DISK 00:00:07 07-SEP-07
BP Key: 1 Status: AVAILABLE Compressed: NO Tag: TAG20070907T165703
Piece Name: /opt/oracle/obak/01irbtb1_1_1
List of Datafiles in backup set 1
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
9 Full 3167484 07-SEP-07 /opt/oracle/oradata/test97/eygle01.dbf
BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
2 Full 10.11M DISK 00:00:07 07-SEP-07
BP Key: 2 Status: AVAILABLE Compressed: NO Tag: TAG20070907T165813
Piece Name: /opt/oracle/obak/02irbtd5_1_1
List of Datafiles in backup set 2
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
9 Full 3167631 07-SEP-07 /opt/oracle/oradata/test97/eygle01.dbf
这显然不是我们期望的结果，如果释放的空间能够跳过，那将是一个理想的状态。看看Oracle11g中Oracle的行为。
首先创建测试用户和测试表：
Connected to:
Oracle Database 11g Enterprise Edition Release 11.1.0.6.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL> create user eygle identified by eygle default tablespace eygle;
SQL> grant connect,resource,dba to eygle;
SQL> connect eygle/eygle
SQL> create table eygle as select * from dba_objects;
SQL> create table eygle as select * from dba_objects;
SQL> create table eygle as select * from dba_objects;
SQL> create table eygle as select * from dba_objects;
SQL> create table eygle as select * from dba_objects;
SQL> commit;
当前空间使用大约7M：
SQL> col segment_name for a30
SQL> select segment_name,bytes/1024/1024 from dba_segments where segment_name='EYGLE';
SEGMENT_NAME BYTES/1024/1024
------------------------------ ---------------
EYGLE 7
SQL> select segment_name,block_id,blocks from dba_extents where segment_name='EYGLE';
SEGMENT_NAME BLOCK_ID BLOCKS
------------------------------ ---------- ----------
EYGLE 9 8
EYGLE 17 8
EYGLE 25 8
EYGLE 33 8
EYGLE 41 8
EYGLE 49 8
EYGLE 57 8
EYGLE 65 8
EYGLE 73 8
EYGLE 81 8
EYGLE 89 8
EYGLE 97 8
EYGLE 105 8
EYGLE 113 8
EYGLE 121 8
EYGLE 129 8
EYGLE 137 128
EYGLE 265 128
EYGLE 393 128
EYGLE 521 128
EYGLE 649 128
EYGLE 777 128
此时进行一次备份，备份大约占用了7M空间：
RMAN> backup datafile 5 format '/opt/oracle/obak/%U';
[oracle@test126 obak]$ ll
total 6928
-rw-r----- 1 oracle dba 6955008 Sep 7 16:46 03irbsmv_1_1
对数据表进行Move操作：
SQL> alter table eygle move tablespace eygle;
Table altered.
SQL> select segment_name,block_id,blocks from dba_extents where segment_name='EYGLE';
SEGMENT_NAME BLOCK_ID BLOCKS
------------------------------ ---------- ----------
EYGLE 905 8
EYGLE 913 8
EYGLE 921 8
EYGLE 929 8
EYGLE 937 8
EYGLE 945 8
EYGLE 953 8
EYGLE 961 8
EYGLE 969 8
EYGLE 977 8
EYGLE 985 8
EYGLE 993 8
EYGLE 1001 8
EYGLE 1009 8
EYGLE 1017 8
EYGLE 1025 8
EYGLE 1033 128
EYGLE 1161 128
EYGLE 1289 128
EYGLE 1417 128
EYGLE 1545 128
EYGLE 1673 128
再进行RMAN备份：
RMAN> backup datafile 5 format '/opt/oracle/obak/%U';
我们注意到新的备份大约占用了7M空间，和之前的备份大致相同：
[oracle@test126 obak]$ ll
total 13544
-rw-r----- 1 oracle dba 6955008 Sep 7 16:46 03irbsmv_1_1
-rw-r----- 1 oracle dba 6881280 Sep 7 16:48 04irbsqo_1_1
列举一下RMAN的备份集：
RMAN> list backup of tablespace eygle;
using target database control file instead of recovery catalog
List of Backup Sets
===================
BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
1 Full 104.00K DISK 00:00:01 07-SEP-07
BP Key: 1 Status: AVAILABLE Compressed: NO Tag: TAG20070907T163650
Piece Name: /opt/oracle/obak/01irbs52_1_1
List of Datafiles in backup set 1
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
5 Full 216444 07-SEP-07 /opt/oracle/oradata/wapdbs/eygle01.dbf
BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
2 Full 6.63M DISK 00:00:01 07-SEP-07
BP Key: 2 Status: AVAILABLE Compressed: NO Tag: TAG20070907T164623
Piece Name: /opt/oracle/obak/03irbsmv_1_1
List of Datafiles in backup set 2
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
5 Full 216842 07-SEP-07 /opt/oracle/oradata/wapdbs/eygle01.dbf
BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
3 Full 6.55M DISK 00:00:00 07-SEP-07
BP Key: 3 Status: AVAILABLE Compressed: NO Tag: TAG20070907T164823
Piece Name: /opt/oracle/obak/04irbsqo_1_1
List of Datafiles in backup set 3
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
5 Full 217011 07-SEP-07 /opt/oracle/oradata/wapdbs/eygle01.dbf
在Oracle11g中，Oracle能够真正的跳过哪些Free的空间，从而使得备份集可能大大缩小。
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