总体上来讲,Oracle的坏块可以分为两种情景:物理损坏和逻辑损坏。物理损坏是由于存储等原因造成的,致使Oracle在处理数据块时发
总体上来讲,Oracle的坏块可以分为两种情景:物理损坏和逻辑损坏。物理损坏是由于存储等原因造成的,致使Oracle在处理数据块时发现块的checksum不一致。逻辑损坏多是由于Oracle的bug或者内存错误引起,通过检测数据块的checksum并不会发现什么问题,但是在逻辑上这些块已经发生了损坏。
Oracle通过两个参数来控制对物理损坏和逻辑损坏的检测:
SQL> show parameter db_block_check
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_block_checking string FALSE
db_block_checksum string TRUE
db_block_checking 是当block发生任何变化的时候进行逻辑上的完整性和正确性检查。该参数能够避免内存中数据块的损坏。块的检查将对系统会有1%到10%的性能影响。取决于对db_block_checking参数的设置。频繁的DML将使得块检查带来更多的开销。在系统负荷允许的情形下建议设置为full。该参数对SYSTEM表空间始终是处于“打开”状态,,而不管该参数是否设置为OFF。下面是该参数的设置参考。FALSE和TRUE是为了老版本的兼容。
Property Description
--------------- ------------
Parameter type String
Syntax DB_BLOCK_CHECKING = { FALSE| OFF| LOW | MEDIUM | TRUE| FULL} -->OFF(=FALSE),FULL(=TRUE)
Defaultvalue FALSE
Modifiable ALTER SYSTEM
Basic No
这里有一点需要注意,即只有在对数据块发生修改时,db_block_checking才会发生作用。如
SQL> show parameter db_block_check
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_block_checking string FALSE
db_block_checksum string TRUE
SQL> alter system flush buffer_cache;
System altered.
SQL> select * from scott.s2;//查询时没有问题
T1 T2
---------- --------------------
51809 lllll
51888 SC
51574 PK_DEPT
51573 DEPT
1575EMP, 5
51576 PK_EMP
51578 SALGRADE
7 rows selected.
SQL> update scott.s2 set t2='aaaa' where t1='51809';//修改也没有问题
1 row updated.
SQL> commit;
Commit complete.
SQL> alter system flush buffer_cache;
System altered.
SQL> alter system set db_block_checking=true;
System altered.
SQL> select * from scott.s2;//查询是没有问题的
T1 T2
---------- --------------------
51809 aaaa
51888 SC
51574 PK_DEPT
51573 DEPT
1575EMP, 5
51576 PK_EMP
51578 SALGRADE
7 rows selected.
SQL> update scott.s2 set t2='bbbb' where t1 = '51809';//修改时触发了逻辑检测
update scott.s2 set t2='bbbb' where t1 = '51809'
*
ERROR at line 1:
ORA-00607: Internal error occurred while making a change to a data block
ORA-00600: internal error code, arguments: [kddummy_blkchk], [5], [20], [6113],
[], [], [], []
SQL> select * from scott.s2;//发现逻辑错误后,oracle将块标记为坏块,此时数据块无法访问了
select * from scott.s2
*
ERROR at line 1:
ORA-01578: ORACLE data block corrupted (file # 5, block # 20)
ORA-01110: data file 5: '/home/app/oraten/oradata/oraten/tbs201.dbf'
db_block_checksum 用于DBWn和direct loader数据块写入到磁盘时,基于块内的所有字节计算得出一个校验值并将其写入块头。在该参数设置为typical和full时,当读入时候重新计算校验和写出时候的校验对比,如果不同则认为是块损坏。如果设置为FULL模式,则基于update/delete应用程序语句级别的改变发生后,校验值会被重新计算并写入。同时对于日志块,在写入之前,同样会生产校验值并写入到块头。该参数主要是防止IO硬件和IO子系统的错误。如果设置为OFF则只对系统表空间有效。下面是该参数的设置参考。FALSE和TRUE是为了老版本的兼容。
Property Description
--------------- ------------
Parameter type String
Syntax DB_BLOCK_CHECKSUM = { OFF| FALSE| TYPICAL | TRUE| FULL} -->OFF(=FALSE),FULL(=TRUE)
Defaultvalue TYPICAL
Modifiable ALTER SESSION,ALTER SYSTEM
Basic No

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