Oracle 数据块(data block)的结构和解析

　　 First of all let’s create a new tablespace and a table:
　　SQL> create tablespace my_tbs datafile '/u01/oradata/chenlin/my_tbs.dbf' size 200m ;
　　 Tablespace created.
　　 then:
　　SQL> create table my_tab tablespace my_tbs as select * from dba_objects  ;
　　 Table created.
　　 Now let’s get more information about this table from DBA_SEGMENTS view:
　　SQL> select header_file, header_block, bytes, blocks, extents from dba_segments where segment_name='MY_TAB';
　　


HEADER_FILE   HEADER_BLOCK       BYTES        BLOCKS       EXTENTS
-----------    ------------           --------      ----------      ----------
12                         17               4194304           1024        19
　　 
　　From the output we can say that the header block of the table is stored in the datafile 12, block 17. 
　　The size of the table 4194304 bytes (4.2 Mb) and it contains 1024 blocks and 19 extents.
　　After getting block size of the database, let’s do some math:
　　

　　SQL> show parameter db_block_size
　　


NAME                                 TYPE        VALUE
--------------                  ----------- --------------------------
db_block_size                     integer     4096
　　 
　　BLOCKS (dba_segments) * DB_BLOCK_SIZE (parameter file) = BYTES (dba_segments)
　　so : 1024 * 4096 =  4194304 
　　

　　Now let’s query DBA_EXTENTS view to get some information on extents and data blocks of this table: 
　　SQL> col segment_name format a20
　　SQL> select segment_name, extent_id, block_id,  blocks, bytes from dba_extents where segment_name='MY_TAB';
　　 SEGMENT_NAME          EXTENT_ID     BLOCK_ID       BLOCKS      BYTES

--------------------  ----------   ----------    ----------   -------
MY_TAB                              0         17         16        65536
MY_TAB                              1         33         16        65536
MY_TAB                              2         49         16        65536
MY_TAB                              3         65         16        65536
MY_TAB                              4         81         16        65536
MY_TAB                              5         97         16        65536
MY_TAB                              6        113        16        65536
MY_TAB                              7        129        16        65536
MY_TAB                              8        145        16        65536
MY_TAB                              9        161        16        65536
MY_TAB                             10       177        16        65536
MY_TAB                             11       193        16        65536
.............................
　　 
　　19 rows selected.
　　 we do this test by follow  info :



SEGMENT_NAME          EXTENT_ID   BLOCK_ID     BLOCKS      BYTES
--------------------    ---------         ---------    ----------      ----------
MY_TAB                             0                    17              16         65536
MY_TAB                             1                    33              16         65536

　　 
　　Here we see that the first extent (0) contains 16 blocks which id range between 17 and 33 ,
　　so: 17 (block_id) + 16 (number of blocks) = 33 (block_id of the next extent)
　　And now let’s start dumping the header block of the table. For this we need data file and header block number.
　　It can be taken from the DBA_SEGMENTS
　　

　　SQL> select header_file, header_block from dba_segments where segment_name='MY_TAB';
　　


HEADER_FILE     HEADER_BLOCK
-----------           ------------
12                            17
　　 
　　See more about dba_segments:
　　http://docs.oracle.com/cd/B19306_01/server.102/b14237/statviews_4097.htm#i1626854
　　

　　Now, dump the data block 12 of the datafile 17:
　　notice :you should rm  your udump file   by force  first  , and  relogin  again 
　　

　　 SQL> alter system dump datafile 12 block 17;    
　　

　　 System altered.
　　

　　If you use  SecureCRT  , you can use the command  : rz  or sz to download or upload files 
　　

　　Clone a new session and change to oracle user  by using : su - oracle 
　　

　　[oracle@chance udump]$ ll
　　total 4
　　-rw-r-----  1 oracle oinstall 2106 May  6 22:15 chenlin_ora_7831.trc
　　

　　[oracle@chance udump]$sz chenlin_ora_7831.trc 
　　rz
　　Starting zmodem transfer.  Press Ctrl+C to cancel.
　　Transferring chenlin_ora_7831.trc...
　　 100%       2 KB    2 KB/s 00:00:01       0 Errors
　　

　　Let's find  the file chenlin_ora_7831.trc  at  C:\Users\Administrator\Downloads  and see the infomation  in it :
　　 #---------------------------------begin--------------------------
　　/u01/admin/chenlin/udump/chenlin_ora_7831.trc
　　Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
　　With the Partitioning, OLAP and Oracle Data Mining options
　　JServer Release 9.2.0.4.0 - Production
　　ORACLE_HOME = /u01/oracle
　　System name:Linux
　　Node name:chance
　　Release:2.6.9-89.EL
　　Version:#1 Mon Jun 22 12:19:40 EDT 2009
　　Machine:i686
　　Instance name: chenlin
　　Redo thread mounted by this instance: 1
　　Oracle process number: 14
　　Unix process pid: 7831, image: oracle@chance (TNS V1-V3)
　　*** SESSION ID:(11.17) 2012-05-06 22:15:42.270
　　Start dump data blocks tsn: 14 file#: 12 minblk 17 maxblk 17
　　buffer tsn: 14 rdba: 0x03000011 (12/17)
　　scn: 0x0000.000a40da seq: 0x01 flg: 0x04 tail: 0x40da1001
　　frmt: 0x02 chkval: 0x28a8 type: 0x10=DATA SEGMENT HEADER - UNLIMITED
　　

　　 Extent Control Header
　　  -----------------------------------------------------------------
　　  Extent Header:: spare1: 0      spare2: 0      #extents: 19     #blocks: 1023  
　　  last map  0x00000000  #maps: 0      offset: 2080  
　　  Highwater::  0x03000351  ext#: 18     blk#: 64     ext size: 256   
　　

　　  #blocks in seg. hdr's freelists: 0     
　　  #blocks below: 831   
　　  mapblk  0x00000000  offset: 18    
　　  Unlocked
　　  Map Header:: next  0x00000000  #extents: 19   obj#: 30558  flag: 0x40000000
　　

　　  Extent Map
　　  -----------------------------------------------------------------
　　   0x03000012  length: 15    
　　   0x03000021  length: 16    
　　   0x03000031  length: 16    
　　   0x03000041  length: 16    
　　   0x03000051  length: 16    
　　   0x03000061  length: 16    
　　   0x030000c1  length: 16    
　　   0x030000d1  length: 16    
　　   0x030000e1  length: 16    
　　   0x030000f1  length: 16    
　　   0x03000101  length: 16    
　　   0x03000111  length: 256   
　　   0x03000211  length: 256   
　　   0x03000311  length: 256   
　　   nfl = 1, nfb = 1 typ = 1 nxf = 0 ccnt = 0
　　  SEG LST:: flg: UNUSED lhd: 0x00000000 ltl: 0x00000000 
　　 End dump data blocks tsn: 14 file#: 12 minblk 17 maxblk 17
　　

　　#---------------------------------------end------------------------------------------
　　

　　* Start dump data blocks tsn: 14 file#: 12 minblk 17 maxblk 17  
　　This means that the following lines is the output of datafile 17, block 17. Range of blocks can be given to dump:
　　

　　 SQL> alter system dump datafile 1 block min 1 block max 17 
　　* rdba: 0x03000011
　　

　　rdba is the Data Block Address which is internal representation of the block address.The first 10 bits contains file number, and the rest 22 bits contains block id.
　　

　　if we want to convert 0x03000011 to decimal, we would get 12/17 which is shown between brackets.
　　

　　Moreover, DBMS_UTILITY.DATA_BLOCK_ADDRESS_FILE and DBMS_UTILITY.DATA_BLOCK_ADDRESS_BLOCK can be used to get the file and block number based on RDA value. But before using these functions, rda should be converted to the decimal value.
　　

　　So here, we convert the rda value (0x03000011) to decimal and got  50331665.
　　

　　In order to convert hexadecimal value to the decimal online, you can use the following link:http://www.statman.info/conversions/hexadecimal.html
　　

　　 SQL>  select DBMS_UTILITY.DATA_BLOCK_ADDRESS_FILE(50331665) as fileNo , DBMS_UTILITY.DATA_BLOCK_ADDRESS_BLOCK(50331665) as blockNo from dual;
　　


FILENO                      BLOCKNO
----------                  ----------
12                            17
　　 
　　 By using DBMS_UTILITY.MAKE_DATA_BLOCK_ADDRESS function and passing file and block value, we can get RDA value:
　　SQL> select DBMS_UTILITY.MAKE_DATA_BLOCK_ADDRESS(12,17) from dual;
　　

　　


DBMS_UTILITY.MAKE_DATA_BLOCK_ADDRESS(12,17)
-------------------------------------------
50331665
　　 
　　

　　

　　* scn: 0x0000.000a40dais the SCN value of the block which defines when it was last modified. We can compare it with each transaction SCN value inside the dump file. The SCN value of the data block is 0×0000.02eeaf02. If second part of this value (02eeaf02) is converted to the decimal, then we got – 49196802. If we query the current_scn value we got:
　　

　　The SCN value of the data block is 0×0000.02eeaf02.
　　

　　If second part of this value (000a40da) is converted to the decimal, then we got – 671962. If we query the current_scn value we got:
　　

　　SQL> select dbms_flashback.get_system_change_number from dual;
　　

　　


GET_SYSTEM_CHANGE_NUMBER
------------------------
683658
　　 
　　* tail is used to keep track of consistency information between the beginning and end of the block against the possibility of distribution of oracle blocks over multiple OS blocks
　　* frmt – is the block format which tells whether it’s Oracle 9 or Oracle 10 and higher block. 0×02 represents that it’s Oracle 8 and higher block. (0×01 represents that it’s Oracle 7 block)
　　* chkval – which is checksum written to the blocks when it is set and used by Oracle in part to check the consistency and validity of the block, and  you show set  the parameter db_block_checksum  to true in init.ora file
　　* type : Type defines which type of block it is. In this example it is “Data Segment Header”
　　* Extent Header: spare1: 0    spare2: 0      #extents: 19     #blocks: 1023  
　　Then in the Extent Control Header part we can get information about extents. So it has 19 extents and 1023 blocks.
　　* Highwater:  0x03000351  ext#: 18     blk#: 64     ext size: 256   
　　#blocks in seg. hdr's freelists: 0     
　　#blocks below: 831 
　　

　　Highwater mark  ( 50332497 ) is at extent 18, block 64. This extent has 256 blocks. And there’re 831 data block below it. Let’s do some math:
　　

　　As it has 831 blocks below and the number of block is 1023 (dump file didn’t count the header block), then there’re 1023-831=192 data blocks free (and above the hightwater mark).
　　

　　Moreover, as the highwater mark is at block 64  and the extent has  256  data blocks, then 256-64 = 192 data blocks are free,
　　

　　So this means that there’s 192*4096 = 786432 bytes (768Kb) free space left
　　* Map Header: next  0x00000000  #extents: 19   obj#: 30558  flag: 0x40000000
　　Next, we got object id. To find more details on that object, query sys.obj# view:
　　

　　SQL>  set lines 180
　　SQL> select obj#, owner#, name, status, ctime, type# from sys.obj$ where obj#=30558;
　　


OBJ#   OWNER#   NAME       STATUS      CTIME          TYPE#
------- -------    -------     ----------   ---------          ----
30558      0       MY_TAB               1   06-MAY-12          2
　　 
　　

　　

　　 SQL> select username from dba_users where user_id=0;
　　


                           USERNAME
------------------------------
SYS
　　 
　　

　　So we can see the owner id is 0 (SYS user) and type is 2 (it means table)

　　

　　In order to show the free space, we can use DBMS_SPACE.UNUSED_SPACE procedure. Let’s create  procedure : 
　　

SQL> create or replace   procedure show_space
( p_segname in varchar2,
p_owner   in varchar2 default user,
p_type    in varchar2 default 'TABLE',
p_partition in varchar2 default NULL )
authid current_user
as
l_free_blks                 number;
l_total_blocks              number;
l_total_bytes               number;
l_unused_blocks             number;
l_unused_bytes              number;
l_LastUsedExtFileId         number;
l_LastUsedExtBlockId        number;
l_LAST_USED_BLOCK           number;
procedure p( p_label in varchar2, p_num in number )
is
begin
dbms_output.put_line( rpad(p_label,40,'.') ||
p_num );
end;
begin
dbms_space.unused_space
( segment_owner     => p_owner,
segment_name      => p_segname,
segment_type      => p_type,
partition_name    => p_partition,
total_blocks      => l_total_blocks,
total_bytes       => l_total_bytes,
unused_blocks     => l_unused_blocks,
unused_bytes      => l_unused_bytes,
LAST_USED_EXTENT_FILE_ID => l_LastUsedExtFileId,
LAST_USED_EXTENT_BLOCK_ID => l_LastUsedExtBlockId,
LAST_USED_BLOCK => l_LAST_USED_BLOCK );
p( 'Total Blocks', l_total_blocks );
p( 'Total Bytes', l_total_bytes );
p( 'Total MBytes', trunc(l_total_bytes/1024/1024) );
p( 'Unused Blocks', l_unused_blocks );
p( 'Unused Bytes', l_unused_bytes );
p( 'Last Used Ext FileId', l_LastUsedExtFileId );
p( 'Last Used Ext BlockId', l_LastUsedExtBlockId );
p( 'Last Used Block', l_LAST_USED_BLOCK );
end  show_space;
/
　　 
　　
　　

　　SQL> set serveroutput on 
　　SQL> exec show_space('MY_TAB','SYS');
　　

      Total Blocks............................1024
Total Bytes.............................4194304
Total MBytes............................4
Unused Blocks...........................192
Unused Bytes............................786432
Last Used Ext FileId....................12
Last Used Ext BlockId...................785
Last Used Block.........................64
　　
 
　　

　　PL/SQL procedure successfully completed.
　　

　　As it has 831 blocks below and the number of block is 1023 (dump file didn’t count the header block), then there’re 1023-831=192 data blocks free (and above the hightwater mark)
　　

　　Moreover, as the highwater mark is at block 64  and the extent has  256  data blocks, then 256-64 = 192 data blocks are free,
　　

　　So this means that there’s 192*4096 = 786432 bytes (768Kb) free space left .
　　

　　As you see comparison of both values led us to the same results .
　　

　　So in this post you’ve learned how to dump a header block of the segment and how to read it. 
　　

　　ps: 如果你想真正知道oracle的block块里面真正有些什么，请耐心读完这篇文章～