数据文件第一个数据块到底有没有纳入数据块的数据块计算中,也就是我们通常所说的rdba(file#,block),是否真的是从数据文件的第一个数据块开始计算的？下面通过实验验证
相关信息和准备工作

SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
PL/SQL Release 9.2.0.4.0 - Production
CORE    9.2.0.3.0       Production
TNS for Linux: Version 9.2.0.4.0 - Production
NLSRTL Version 9.2.0.4.0 - Production
SQL> select to_char(sysdate,'yyyy-mm-dd hh24:mi:ss') "www.xifenfei.com" from dual;
www.xifenfei.com
-------------------
2012-05-29 19:39:48
SQL> select name,block_size from v$datafile where file#=9;
NAME                                                         BLOCK_SIZE
------------------------------------------------------------ ----------
/u01/oracle/oradata/xifenfei/users01.dbf                           8192
--dd出来数据文件第一和第二个数据块
[oracle@xifenfei ~]$ dd if=/u01/oracle/oradata/xifenfei/users01.dbf of=user.01 bs=8192 count=1
1+0 records in
1+0 records out
[oracle@xifenfei ~]$ dd if=/u01/oracle/oradata/xifenfei/users01.dbf of=user.02 bs=8192 count=1 skip=1
1+0 records in
1+0 records out
[oracle@xifenfei ~]$ ll user.*
-rw-r--r--  1 oracle oinstall 8192 May 26 04:43 user.01
-rw-r--r--  1 oracle oinstall 8192 May 26 04:44 user.02

bbed验证

[oracle@xifenfei ~]$ bbed password=blockedit blocksize=8192  listfile=/home/oracle/bbed_new.file mode=edit
BBED: Release 2.0.0.0.0 - Limited Production on Sat May 26 04:56:49 2012
Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.
************* !!! For Oracle Internal Use only !!! ***************
BBED> info
 File#  Name                                                        Size(blks)
 -----  ----                                                        ----------
     1  /u01/oracle/oradata/xifenfei/users01.dbf                             0
     2  /home/oracle/user.01                                                 0
     3  /home/oracle/user.02                                                 0
--users01.dbf(完整数据文件,第一个数据块)
BBED> set file 1
        FILE#           1
BBED> set block 1
        BLOCK#          1
BBED> d /v count 128
 File: /u01/oracle/oradata/xifenfei/users01.dbf (1)
 Block: 1       Offsets:    0 to  127  Dba:0x00400001
-------------------------------------------------------
 0b020000 01004002 00000000 00000104 l ......@.........
 7f4b0000 00002009 00000008 cdb41453 l .K.... ........S
 58494645 4e464549 c7010000 800c0000 l XIFENFEI........
 00200000 09000300 00000000 00000000 l . ..............
 00000000 00000000 00000000 00000000 l ................
 00000000 00000000 00000000 00000000 l ................
 00000000 47180000 00000000 cf4d851e l ....G........M..
 8f40512e 78ab0200 00000000 00000000 l .@Q.x...........
 <16 bytes per line>
--直接设置file 2错误(后续提供其他方法)
BBED> set file 2
BBED-00307: incorrect blocksize (8192) or truncated file
--查看users01.dbf(第二个数据块)
BBED> set file 3
        FILE#           3
BBED> set block 1
        BLOCK#          1
BBED> d /v count 128
 File: /home/oracle/user.02 (3)
 Block: 1       Offsets:    0 to  127  Dba:0x00c00001
-------------------------------------------------------
 0b020000 01004002 00000000 00000104 l ......@.........
 7f4b0000 00002009 00000008 cdb41453 l .K.... ........S
 58494645 4e464549 c7010000 800c0000 l XIFENFEI........
 00200000 09000300 00000000 00000000 l . ..............
 00000000 00000000 00000000 00000000 l ................
 00000000 00000000 00000000 00000000 l ................
 00000000 47180000 00000000 cf4d851e l ....G........M..
 8f40512e 78ab0200 00000000 00000000 l .@Q.x...........
 <16 bytes per line>
--查看users01.dbf(真正第一个数据块)
BBED> set filename 'user.01'
        FILENAME        user.01
BBED> d /v count 128
 File: user.01 (0)
 Block: 1       Offsets:    0 to  127  Dba:0x00000000
-------------------------------------------------------
 00020000 00200000 800c0000 5d5c5b5a l ..... ......]\[Z
 00000000 86280000 00000000 00000000 l .....(..........
 00000000 00000000 00000000 00000000 l ................
 00000000 00000000 00000000 00000000 l ................
 00000000 00000000 00000000 00000000 l ................
 00000000 00000000 00000000 00000000 l ................
 00000000 00000000 00000000 00000000 l ................
 00000000 00000000 00000000 00000000 l ................
 <16 bytes per line>

通过这个对比可以知道:当我们直接使用bbed查看数据块内容的时候,自动屏蔽了数据文件上真正的第一个数据块.其实block 1是数据文件上的第二个数据块

hexdump验证

--users01.dbf(完整文件)
[oracle@xifenfei ~]$ hexdump -C /u01/oracle/oradata/xifenfei/users01.dbf|head -20
00000000  00 02 00 00 00 20 00 00  80 0c 00 00 5d 5c 5b 5a  |..... ......]\[Z|
00000010  00 00 00 00 86 28 00 00  00 00 00 00 00 00 00 00  |.....(..........|
00000020  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00002000  0b 02 00 00 01 00 40 02  00 00 00 00 00 00 01 04  |......@.........|
00002010  7f 4b 00 00 00 00 20 09  00 00 00 08 cd b4 14 53  |.K.... ........S|
00002020  58 49 46 45 4e 46 45 49  c7 01 00 00 80 0c 00 00  |XIFENFEI........|
00002030  00 20 00 00 09 00 03 00  00 00 00 00 00 00 00 00  |. ..............|
00002040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00002060  00 00 00 00 47 18 00 00  00 00 00 00 cf 4d 85 1e  |....G........M..|
00002070  8f 40 51 2e 78 ab 02 00  00 00 00 00 00 00 00 00  |.@Q.x...........|
00002080  00 00 00 00 00 00 00 00  00 00 04 00 58 0d 0b c0  |............X...|
00002090  2c 0b 00 00 5a ea be 2e  01 00 aa bd 15 00 00 00  |,...Z...........|
000020a0  02 00 00 00 10 00 ff bf  02 00 00 00 00 00 00 00  |................|
000020b0  5a 00 00 00 4f ea be 2e  59 00 00 00 00 00 00 00  |Z...O...Y.......|
000020c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000020f0  00 00 00 00 09 00 00 00  05 00 55 53 45 52 53 00  |..........USERS.|
00002100  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
--users01.dbf(第一个数据块文件)
[oracle@xifenfei ~]$ hexdump -C user.01
00000000  00 02 00 00 00 20 00 00  80 0c 00 00 5d 5c 5b 5a  |..... ......]\[Z|
00000010  00 00 00 00 86 28 00 00  00 00 00 00 00 00 00 00  |.....(..........|
00000020  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00002000
--users01.dbf(第二个数据块文件)
[oracle@xifenfei ~]$ hexdump -C user.02|head -20
00000000  0b 02 00 00 01 00 40 02  00 00 00 00 00 00 01 04  |......@.........|
00000010  7f 4b 00 00 00 00 20 09  00 00 00 08 cd b4 14 53  |.K.... ........S|
00000020  58 49 46 45 4e 46 45 49  c7 01 00 00 80 0c 00 00  |XIFENFEI........|
00000030  00 20 00 00 09 00 03 00  00 00 00 00 00 00 00 00  |. ..............|
00000040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000060  00 00 00 00 47 18 00 00  00 00 00 00 cf 4d 85 1e  |....G........M..|
00000070  8f 40 51 2e 78 ab 02 00  00 00 00 00 00 00 00 00  |.@Q.x...........|
00000080  00 00 00 00 00 00 00 00  00 00 04 00 58 0d 0b c0  |............X...|
00000090  2c 0b 00 00 5a ea be 2e  01 00 aa bd 15 00 00 00  |,...Z...........|
000000a0  02 00 00 00 10 00 ff bf  02 00 00 00 00 00 00 00  |................|
000000b0  5a 00 00 00 4f ea be 2e  59 00 00 00 00 00 00 00  |Z...O...Y.......|
000000c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000000f0  00 00 00 00 09 00 00 00  05 00 55 53 45 52 53 00  |..........USERS.|
00000100  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000110  00 00 00 00 00 00 00 00  09 00 00 00 00 00 00 00  |................|
00000120  00 00 00 00 f9 e9 be 2e  00 00 00 00 00 00 00 00  |................|
00000130  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

通过hexdump对三个文件的对比可以知道users01.dbf的头两个数据文件确实是由第一和第二个数据块组成.然后结合上面bbed dump出来的结果.可以再次证明数据文件第一个数据块,不能被bbed识别(从第二个数据文件开始)

实验总结
我们的数据文件其实是从文件的第二个数据块开始记录起(该数据块为block 1).也就是说系统的数据块和oracle中的rdba标示的数据块不是一致.而是系统数据块比oracle数据块多1.
因这个原因解释了以前的一个疑问:Oracle数据文件大小的限制为什么指定数据文件最大值为(2^22-1*block_size),而不是根据rowid的2^22*block_size
关于类此问题在windows验证请见:在UltraEdit中定位数据文件内容