Oracle 事务一致性之块清理（block cleanout）
block cleanout原理方面不做过多解释，主要记录测试过程，备忘。
对测试表格做dml操作，记录其scn值
SQL> update zhoul set name='aaa';

3 rows updated.

SQL> commit;

Commit complete.

SQL> alter system checkpoint;

System altered.

SQL> select current_scn scn from v$database;

               SCN
------------------
    10995251665103

SQL> update zhoul set name='bbb';

3 rows updated.

SQL> commit;

Commit complete.

SQL>  alter system checkpoint;

System altered.

SQL> select current_scn scn from v$database;

               SCN
------------------
    10995251665322
   



SQL> select * from zhoul;

         I NAME
---------- --------------------
         1 bbb
         2 bbb
         3 bbb

dump datafile block可以看出其block所有行都处于提交状态。       
        
  Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0023.02f.0000004e  0x0280073e.0010.2b  --U-    3  fsc 0x0000.0811d995
0x02   0x0029.02d.0000004d  0x028008c5.0010.37  C---    0  scn 0x0a00.0811d8b7

物理上修改block，修改flag表示，此block有事务存在，锁住的行数为2行。
BBED> modify 0x0200


BBED> dump offset 88
File: /oradata/mcstar/zhoul01.dbf (0)
Block: 15511            Offsets:   88 to  599           Dba:0x00000000
------------------------------------
b7d81108 00000000 00000000 00010300 ffff1800 001f621f 621f0000 0300141f
0a1f001f 311fff1e cb1e5c1e 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000

<32 bytes per line>

修改此事务槽的ckpt值为A000811D8CF。
BBED> modify 0xcf
File: /oradata/mcstar/zhoul01.dbf (0)
Block: 15511            Offsets:   88