怎么计算出正确的checksum值？

1)创建一个测试表

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SQL> create table test (id int, name varchar2(10));

Table created.

SQL> insert into test values(1,'AAAAA');

1 row created.

SQL> commit;

Commit complete.

SQL> select dbms_rowid.ROWID_RELATIVE_FNO(rowid) file#,dbms_rowid.ROWID_BLOCK_NUMBER(rowid) blk# from test;

FILE# BLK#

---------- ----------

4 284

通过BBED查看当前的CHECKSUM值

[oracle@MySQL ~]$ bbed

BBED: Release 2.0.0.0.0 - Limited Production on Fri May 25 00:54:11 2018

************* !!! For Oracle Internal Use only !!! ***************

BBED> set file 4 block 284

FILE# 4

BLOCK# 284

BBED> sum

Check value for File 4, Block 284:

current = 0xf9ff, required = 0xf9ff

把284号数据块dump出来。

[oracle@Mysql ~]$ dd if=/u01/app/oracle/oradata/dsidb/users01.dbf of=/tmp/test.dd count=1 skip=284 bs=8192

1+0 records in

1+0 records out

8192 bytes (8.2 kB) copied, 0.000435816 s, 18.8 MB/s

我们使用UE编辑器打开test.dd数据块

然后把C1 02改成C1 03

然后把test.dd数据块copy回去

[oracle@Mysql tmp]$ dd if=/tmp/test.dd of=/u01/app/oracle/oradata/dsidb/users01.dbf count=1 seek=284 bs=8192 conv=notrunc

1+0 records in

1+0 records out

8192 bytes (8.2 kB) copied, 0.000339993 s, 24.1 MB/s

然后重启一下数据库

SQL> startup force;

ORACLE instance started.

Total System Global Area 588746752 bytes

Fixed Size 2230592 bytes

Variable Size 201328320 bytes

Database Buffers 377487360 bytes

Redo Buffers 7700480 bytes

Database mounted.

Database opened.

SQL> conn scott/oracle

Connected.

SQL> select * from test;

select * from test

ERROR at line 1:

ORA-01578: ORACLE data block corrupted (file # 4, block # 284)

ORA-01110: data file 4: '/u01/app/oracle/oradata/dsidb/users01.dbf'

可以看到数据库查询表test报错，我们再看一下数据库日志。

2018-05-25 01:16:29.248000 +08:00

Errors in file /u01/app/oracle/diag/rdbms/dsidb/dsidb/trace/dsidb_ora_10666.trc (incident=102183):

ORA-01578: ORACLE data block corrupted (file # 4, block # 284)

ORA-01110: data file 4: '/u01/app/oracle/oradata/dsidb/users01.dbf'

Incident details in: /u01/app/oracle/diag/rdbms/dsidb/dsidb/incident/incdir_102183/dsidb_ora_10666_i102183.trc

Sweep [inc][102182]: completed

Hex dump of (file 4, block 284) in trace file /u01/app/oracle/diag/rdbms/dsidb/dsidb/incident/incdir_102182/dsidb_m000_10668_i102182_a.trc

Corrupt block relative dba: 0x0100011c (file 4, block 284)

Bad check value found during validation

Data in bad block:

type: 6 format: 2 rdba: 0x0100011c

last change scn: 0x0000.003ffe2d seq: 0x1 flg: 0x06

spare1: 0x0 spare2: 0x0 spare3: 0x0

consistency value in tail: 0xfe2d0601

check value in block header: 0xf9ff

computed block checksum: 0x100

可以看到文件头上的check value值为0xf9ff，计算的check sum值为0x100

然后我们再使用BBED去sum一下这个数据块，可以看到，当前check value的值为f9ff，而需要的值为f8ff

[oracle@Mysql ~]$ bbed

BBED: Release 2.0.0.0.0 - Limited Production on Fri May 25 01:18:44 2018

************* !!! For Oracle Internal Use only !!! ***************

BBED> set file 4 block 284

FILE# 4

BLOCK# 284

BBED> sum

Check value for File 4, Block 284:

current = 0xf9ff, required = 0xf8ff

我们根据 0xf9ff与0x100计算一下当前block正常的checksum值应该是多少。

F9FF= 1111 1001 1111 1111

100= 0000 0001 0000 0000

根据异或算法原理，这里很容易可以看出oracle计算出来的正确的checksum值应该是： 1111 1000 1111 1111，也就是f8ff

好了，我们这里如法炮制再改一次上述block的checksum值，即将上述block的checksum值改为f8 ff

我们先verify一下

BBED> verify

DBVERIFY - Verification starting

FILE = /u01/app/oracle/oradata/dsidb/users01.dbf

BLOCK = 284

Block 284 is corrupt

Corrupt block relative dba: 0x0100011c (file 0, block 284)

Bad check value found during verification

Data in bad block:

type: 6 format: 2 rdba: 0x0100011c

last change scn: 0x0000.003ffe2d seq: 0x1 flg: 0x06

spare1: 0x0 spare2: 0x0 spare3: 0x0

consistency value in tail: 0xfe2d0601

check value in block header: 0xf9ff

computed block checksum: 0x100

DBVERIFY - Verification complete

Total Blocks Examined : 1

Total Blocks Processed (Data) : 0

Total Blocks Failing (Data) : 0

Total Blocks Processed (Index): 0

Total Blocks Failing (Index): 0

Total Blocks Empty : 0

Total Blocks Marked Corrupt : 1

Total Blocks Influx : 0

BBED> sum

Check value for File 4, Block 284:

current = 0xf9ff, required = 0xf8ff

BBED> sum

Check value for File 4, Block 284:

current = 0xf9ff, required = 0xf8ff

BBED> modify /x 0xf8ff offset 16

Warning: contents of previous BIFILE will be lost. Proceed? (Y/N) y

File: /u01/app/oracle/oradata/dsidb/users01.dbf (4)

Block: 284 Offsets: 16 to 527 Dba:0x0100011c

------------------------------------------------------------------------

f8ff0000 01000000 fa2a0100 2cfe3f00 00000000 02003200 18010001 1d000900

a0000000 ba040002 4d001100 01200000 2dfe3f00 00000000 00000000 00000000

00000000 00000000 00000000 00000000 00000000 00010100 ffff1400 8c1f781f

781f0000 01008c1f ffff3200 a0046e04 6e040000 1000c01d 961c7b1b b9190418

50167a14 9812f110 4b0f830d ba0bde09 29087606 a0040000 00000000 00000000

00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000

<32 bytes per line>

BBED> sum apply

Check value for File 4, Block 284:

current = 0xf8ff, required = 0xf8ff

然后再次verify，可以看到，已经不报坏块了。

BBED> verify

DBVERIFY - Verification starting

FILE = /u01/app/oracle/oradata/dsidb/users01.dbf

BLOCK = 284

DBVERIFY - Verification complete

Total Blocks Examined : 1

Total Blocks Processed (Data) : 1

Total Blocks Failing (Data) : 0

Total Blocks Processed (Index): 0

Total Blocks Failing (Index): 0

Total Blocks Empty : 0

Total Blocks Marked Corrupt : 0

Total Blocks Influx : 0

BBED> sum

Check value for File 4, Block 284:

current = 0xf8ff, required = 0xf8ff

数据也可以正常返回了。

SQL> select * from test;

ID NAME

---------- ----------

2 AAAAA

2.重现数据块内空间计算错误？（详细的实验操作步骤,BBED工具verify如下命令提示）

BBED>verify

kdbchk:the amount of space used is not equal to block size

Total Blocks Failing(Data)

SQL> create table t2 (id int,name varchar2(10));

Table created.

SQL> insert into t2 values(1,'AAAAA');

1 row created.

SQL> commit;

Commit complete.

SQL> alter system flush buffer_cache;

System altered.

SQL> select dbms_rowid.rowid_relative_fno(rowid) file#,dbms_rowid.rowid_block_number(rowid) blk# from t2;

FILE# BLK#

---------- ----------

4 220

SQL> delete from t2;

1 row deleted.

SQL> alter system flush buffer_cache;

System altered.

BBED> set file 4 block 220

FILE# 4

BLOCK# 220

BBED> map

File: /u01/app/oracle/oradata/dsidb/users01.dbf (4)

Block: 220 Dba:0x010000dc

------------------------------------------------------------

KTB Data Block (Table/Cluster)

struct kcbh, 20 bytes @0

struct ktbbh, 72 bytes @20

struct kdbh, 14 bytes @100

struct kdbt[1], 4 bytes @114

sb2 kdbr[1] @118

ub1 freespace[8055] @120

ub1 rowdata[13] @8175

ub4 tailchk @8188

BBED> p rowdata

ub1 rowdata[0] @8175 0x3c

ub1 rowdata[1] @8176 0x02

ub1 rowdata[2] @8177 0x02

ub1 rowdata[3] @8178 0x02

ub1 rowdata[4] @8179 0xc1

ub1 rowdata[5] @8180 0x02

ub1 rowdata[6] @8181 0x06

ub1 rowdata[7] @8182 0x41

ub1 rowdata[8] @8183 0x41

ub1 rowdata[9] @8184 0x41

ub1 rowdata[10] @8185 0x41

ub1 rowdata[11] @8186 0x41

ub1 rowdata[12] @8187 0x0a

BBED> modify /x 2c offset 8175

File: /u01/app/oracle/oradata/dsidb/users01.dbf (4)

Block: 220 Offsets: 8175 to 8191 Dba:0x010000dc

------------------------------------------------------------------------

2c020202 c1020641 41414141 0a010621 59

<32 bytes per line>

BBED> sum apply

Check value for File 4, Block 220:

current = 0x1f3f, required = 0x1f3f

BBED> verify

DBVERIFY - Verification starting

FILE = /u01/app/oracle/oradata/dsidb/users01.dbf

BLOCK = 220

Block Checking: DBA = 16777436, Block Type = KTB-managed data block

data header at 0x24d9064

kdbchk: the amount of space used is not equal to block size

used=33 fsc=11 avsp=8055 dtl=8088

Block 220 failed with check code 6110

DBVERIFY - Verification complete

Total Blocks Examined : 1

Total Blocks Processed (Data) : 1

Total Blocks Failing (Data) : 1

Total Blocks Processed (Index): 0

Total Blocks Failing (Index): 0

Total Blocks Empty : 0

Total Blocks Marked Corrupt : 0

Total Blocks Influx : 0

Message 531 not found; product=RDBMS; facility=BBED

BBED> p kdbh

struct kdbh, 14 bytes @100

ub1 kdbhflag @100 0x00 (NONE)

sb1 kdbhntab @101 1

sb2 kdbhnrow @102 1

sb2 kdbhfrre @104 -1

sb2 kdbhfsbo @106 20

sb2 kdbhfseo @108 8075

sb2 kdbhavsp @110 8055

sb2 kdbhtosp @112 8068

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