[20160831]关于数据块Checksum.txt

--以前我学习bbed时做过一些测试,将'AAAA'替换成'BBBB',你可以发现数据块的Checksum并没有发生变化,当时并没有仔细探究,
--现在想起来计算Checksum算法应该相对简单,就是做异或操作.

--比如上面的字符'AAAA'如果2个字符按位做异或操作,变成00000000,这个正好巧合,如果修改成'CCCC',做相同的异或操作结果
--也是00000000.
--如果按照这个推测修改为'CDCD',这样做异或操作的结果也是00000000. 还是通过测试来说明问题:

1.环境
SCOTT@book> @ &r/ver1
PORT_STRING         VERSION        BANNER
------------------- -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx 11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

2.建立测试环境:

create table tx  (id number,name varchar2(20));
insert into tx values (1,'AAAA');
commit;

SCOTT@book> select rowid , tx.* from tx;
ROWID                      ID NAME
------------------ ---------- --------------------
AAAVq1AAEAAAAeMAAA          1 AAAA

SCOTT@book> @ &r/rowid AAAVq1AAEAAAAeMAAA
    OBJECT       FILE      BLOCK        ROW ROWID_DBA            DBA                  TEXT
---------- ---------- ---------- ---------- -------------------- -------------------- ----------------------------------------
     88757          4       1932          0  0x100078C           4,1932               alter system dump datafile 4 block 1932
                                                                                      ;
SCOTT@book> @ &r/bbvi 4 1932
BVI_COMMAND
------------------------------------------------------
bvi -b 15826944 -s 8192 /mnt/ramdisk/book/users01.dbf

SCOTT@book> alter system checkpoint;
System altered.

3.使用bvi修改'AAAA'=>'CCCC'看看.
--//注:我个人喜欢使用bvi修改,这样修改快一些.再使用bbed计算checksum.如果你喜欢也可以使用bbed操作.
--//再修改前看看checksum.

BBED> set dba  4,1932
        DBA             0x0100078c (16779148 4,1932)

BBED> p kcbh.chkval_kcbh
ub2 chkval_kcbh                             @16       0x9b53

--//修改'AAAA'=>'CCCC',注意bbed查看最好退出再进入.

BBED> set dba  4,1932
        DBA             0x0100078c (16779148 4,1932)

BBED> sum
Check value for File 4, Block 1932:
current = 0x9b53, required = 0x9b53

BBED> p kcbh.chkval_kcbh
ub2 chkval_kcbh                             @16       0x9b53

--//以发现checksum=0x9b53
--//使用bvi修改'CCCC'=>'CDCD'看看.注意bbed查看最好退出再进入.

BBED> set dba  4,1932
        DBA             0x0100078c (16779148 4,1932)

BBED> sum
Check value for File 4, Block 1932:
current = 0x9b53, required = 0x9b53

BBED> p kcbh.chkval_kcbh
ub2 chkval_kcbh                             @16       0x9b53

--//如果修改为'CCDD'应该检查和就不一样了。

BBED> set dba  4,1932
        DBA             0x0100078c (16779148 4,1932)

BBED> sum
Check value for File 4, Block 1932:
current = 0x9b53, required = 0x9c54

--//可以发现这样修改出现了不一致。因为CC与DD做异或，CD与CD做异或两者结果不同。
--//修改回来'AAAA'.

4.由此我们可以"制造"出定制的检查和。比如我想实现检查和0x0000.只要在freedata区域找0x0000，换成0x9b53就可以实现检查和为0x0000。
--//使用bvi在freedata区域找0x0000，换成0x9b53。
BBED> set dba  4,1932
        DBA             0x0100078c (16779148 4,1932)

BBED> sum
Check value for File 4, Block 1932:
current = 0x9b53, required = 0x0000

--//可以发现,如果我应用sum apply，就可以现实checksum=0x0000.
BBED> sum apply ;
Warning: contents of previous BIFILE will be lost. Proceed? (Y/N) y
Check value for File 4, Block 1932:
current = 0x0000, required = 0x0000

BBED> p kcbh.chkval_kcbh
ub2 chkval_kcbh                             @16       0x0000

SCOTT@book> alter system flush buffer_cache;
System altered.

SCOTT@book> select rowid , tx.* from tx;
ROWID                      ID NAME
------------------ ---------- --------------------
AAAVq1AAEAAAAeMAAA          1 AAAA

--//显示正常！

5.在由此产生一个问题，就是如果在bbed执行corrupt看看这个时候检查和是多少。

BBED> set dba  4,1932
        DBA             0x0100078c (16779148 4,1932)

BBED> corrupt
Warning: contents of previous BIFILE will be lost. Proceed? (Y/N) y
Block marked media corrupt.

BBED> p kcbh.chkval_kcbh
ub2 chkval_kcbh                             @16       0x0217

BBED> p seq_kcbh
ub1 seq_kcbh                                @14       0xff

BBED> p tailchk
ub4 tailchk                                 @8188     0x000006ff

--//执行corrupt仅仅导致seq_kcbh=0xff.
--//修复,正常这样修复很简单,就是设置seq_kcbh=0x01,tailchk=0x00000601.

BBED> assign seq_kcbh=0x01
ub1 seq_kcbh                                @14       0x01

BBED> assign tailchk=0x00000601
ub4 tailchk                                 @8188     0x00000601

BBED> sum ;
Check value for File 4, Block 1932:
current = 0x0217, required = 0x0217

BBED> sum apply;
Check value for File 4, Block 1932:
current = 0x0217, required = 0x0217

--//可以发现这样修改检查和也不会变化，实际上我们仅仅修改2处0xff=>0x01,这样检查和是不会变化的。
SCOTT@book> alter system flush buffer_cache;
System altered.

SCOTT@book> select rowid , tx.* from tx;
ROWID                      ID NAME
------------------ ---------- --------------------
AAAVq1AAEAAAAeMAAA          1 AAAA

6.最后探究检查和的计算。
--使用bvi将这个数据块保存为文件a.txt
$ xxd -c 2 a.txt  | cut -c10-13 > a1.txt

--想办法将上面的结果导入数据库的表中。我简单使用vim的替换功能。转化成sql语句。
SCOTT@book> create table ty( a varchar2(20));
Table created.

--oracle没有异或操作，有位与操作。可以通过如下实现。
BITXOR(x,y)   =   BITOR(x,y)   -   BITAND(x,y)   =   (x   +   y)   -   BITAND(x,   y)   *   2;

--通过递归写了一个sql语句，不考虑效率有点慢。感谢kelis2004的指点。
--链接：http://www.itpub.net/thread-2066614-1-1.html

SCOTT@book> alter table ty add (b  number);
Table altered.

SCOTT@book> commit ;
Commit complete.

SCOTT@book> update ty set b=TO_NUMBER (a, 'xxxxxxxxxxxxxxx');
4096 rows updated.

SCOTT@book> commit ;
Commit complete.

WITH t AS (SELECT ROWNUM ID, b FROM ty)
    ,prod (lastID, lastprod)
     AS (SELECT id, b
           FROM t
          WHERE id = 1
         UNION ALL
         SELECT ID, (b + lastprod) - BITAND (b, lastprod) * 2
           FROM prod, t
          WHERE t.id = lastID + 1)
SELECT *
  FROM prod
WHERE lastid = (SELECT MAX (ID) FROM t);
;

LASTID   LASTPROD
---------- ----------
      4096          0

--正好是0，说明算法正常，当然我写的sql效率不是很高哈哈。

--补充1点：

SCOTT@book> alter table ty add (c  number);
Table altered.

SCOTT@book> update ty set c=rownum;
4096 rows updated.

SCOTT@book> commit ;
Commit complete.

SCOTT@book> create unique index pk_ty on ty(c);
Index created.

WITH prod (lastID, lastprod)
     AS (SELECT c, b
           FROM ty
          WHERE c = 1
         UNION ALL
         SELECT c, (b + lastprod) - BITAND (b, lastprod) * 2
           FROM prod, ty
          WHERE ty.c = lastID + 1)
SELECT *
  FROM prod
WHERE lastid = (SELECT MAX (c) FROM ty);

LASTID   LASTPROD
---------- ----------
      4096          0