但是alter tablespace XXX begin backup带来的另一个问题是会导致产生多余的日志，通过一个小小的试验就可以证明这一点。
SQL> select name,value from v$sysstat where name='redo size';
NAME VALUE
--------------------------------------------------- ----------
redo size 43408
SQL> update test set a=a;
1 row updated.
SQL> commit;
Commit complete.
SQL> select name,value from v$sysstat where name='redo size';
NAME VALUE
--------------------------------------------------------------
redo size 44060
SQL> ALTER SYSTEM DUMP LOGFILE '/netappredo/redo05.log';
System altered.一个update的动作产生44060－43408＝652bytes的redo,把表空间置为backup mode:
SQL> alter tablespace test begin backup;
Tablespace altered.
SQL> select name,value from v$sysstat where name='redo size';
NAME VALUE
------------------------------------------------------------------
redo size 44732
SQL> update test set a=a;
1 row updated.
SQL> commit;
Commit complete.
SQL> select name,value from v$sysstat where name='redo size';
NAME VALUE
-------------------------------------------------------------------
redo size 53560
SQL> alter tablespace test end backup;
Tablespace altered.一个update的动作产生53560－44732＝8828bytes的redo,看看到底是记了些什么？
SQL> ALTER SYSTEM DUMP LOGFILE '/netappredo/redo05.log';
System altered.
REDO RECORD - Thread:2 RBA: 0x00004e.000000b0.0128 LEN: 0x01b0 VLD: 0x01
SCN: 0x0000.19ed24f7 SUBSCN: 1 06/29/2004 15:05:32
CHANGE #1 TYP:0 CLS:29 AFN:33 DBA:0x08400029
SCN:0x0000.19ed24f2 SEQ: 1 OP:5.2
...... (改动向量1，记载对undo header事务表的修改)
CHANGE #2 TYP:0 CLS:30 AFN:33 DBA:0x0840002e
SCN:0x0000.19ed24f0 SEQ: 1 OP:5.1
...... (改动向量2，记载对undo block的修改)
CHANGE #3 TYP:2 CLS: 1 AFN:51 DBA:0x0cc0000f
SCN:0x0000.19ed24e8 SEQ: 1 OP:11.5
KTB Redo (改动向量3，记载对数据块的修改，
也就是在数据块上执行update test set a=a）
op: 0x11 ver: 0x01
op: F xid: 0x0007.001.00014ece uba: 0x0840002e.0859.38
Block cleanout record, scn: 0x0000.19ed24f7 ver: 0x01 opt: 0x02,
entries follow...
itli: 1 flg: 2 scn: 0x0000.19ed24e8
KDO Op code: URP row dependencies Disabled
xtype: XA bdba: 0x0cc0000f hdba: 0x0cc0000b
itli: 2 ispac: 0 maxfr: 4858
tabn: 0 slot: 0(0x0) flag: 0x2c lock: 2 ckix: 0
ncol: 1 nnew: 1 size: 0
col 0: [ 2] c1 02
CHANGE #4 MEDIA RECOVERY MARKER SCN:0x0000.00000000 SEQ: 0 OP:5.20
......(改动向量4，一些标记）更多内容请看PCdog.com--MySQL数据备份 备份恢复 数据库备份与恢复专题
