Oracle表空间用统一区大小注意点

今天在做测试用EXTENT MANAGEMENT LOCAL UNIFORM管理时,遇到看起来比较奇怪的问题,于是研究了一下。
首先创建3个表空间,注意其初始大小和UNIFORM SIZE。
SQL> create tablespace testblock datafile '/oradata/mcstar/testblock01.dbf' SIZE 200M
  2  EXTENT MANAGEMENT LOCAL UNIFORM SIZE 2m
  3   SEGMENT SPACE MANAGEMENT AUTO;

Tablespace created.

SQL> create tablespace testblock2 datafile '/oradata/mcstar/testblock02.dbf' SIZE 10M
  2  EXTENT MANAGEMENT LOCAL UNIFORM SIZE 1m
  3   SEGMENT SPACE MANAGEMENT AUTO;

Tablespace created.

SQL> create tablespace testblock3 datafile '/oradata/mcstar/testblock03.dbf' SIZE 10M
  2  EXTENT MANAGEMENT LOCAL UNIFORM SIZE 8m
  3   SEGMENT SPACE MANAGEMENT AUTO;

Tablespace created.
需要注意的是Oracle创建表空间时,自动扩展为关闭
SQL> select file_id,file_name,blocks-user_blocks,AUTOEXTENSIBLE from  dba_data_files order by 1;

FILE_ID FILE_NAME                                BLOCKS-USER_BLOCKS AUT
------- ---------------------------------------- ------------------ ---
      1 /oradata/mcstar/system01.dbf                              8 YES
      2 /oradata/mcstar/undotbs01.dbf                             8 NO
      3 /oradata/mcstar/sysaux01.dbf                              8 YES
      4 /oradata/mcstar/users01.dbf                               8 YES
      5 /oradata/mcstar/mcstar01.dbf                              8 NO
      6 /oradata/mcstar/xu01.dbf                                  8 NO
      7 /oradata/mcstar/zhoul01.dbf                               8 YES
      8 /oradata/mcstar/zhoul201.dbf                              8 YES
      9 /oradata/mcstar/testblock01.dbf                         256 NO
     10 /oradata/mcstar/undotbs02.dbf                             8 YES
     11 /oradata/mcstar/testblock02.dbf                         128 NO

FILE_ID FILE_NAME                                BLOCKS-USER_BLOCKS AUT
------- ---------------------------------------- ------------------ ---
     12 /oradata/mcstar/testblock03.dbf                         256 NO

12 rows selected.
问题来了,可以看到数据文件block和用户可用的block想减,值竟然不一样。
9号文件差值为256*8k/1024/1024=2M,11号文件差值为128*8k/1024/1024=1M,12号文件差值为256*8k/1024/1024=2M。
由前面知道9号文件的uniform size为2M,11号文件的uniform_size为1M,似乎和差值对的上,但12号文件的差值和uniform_size完全对不上
SQL> select file_id,file_name,blocks-user_blocks from dba_data_files
  2  order by 1;

FILE_ID FILE_NAME                                BLOCKS-USER_BLOCKS
------- ---------------------------------------- ------------------
      1 /oradata/mcstar/system01.dbf                              8
      2 /oradata/mcstar/undotbs01.dbf                             8
      3 /oradata/mcstar/sysaux01.dbf                              8
      4 /oradata/mcstar/users01.dbf                               8
      5 /oradata/mcstar/mcstar01.dbf                              8
      6 /oradata/mcstar/xu01.dbf                                  8
      7 /oradata/mcstar/zhoul01.dbf                               8
      8 /oradata/mcstar/zhoul201.dbf                              8
      9 /oradata/mcstar/testblock01.dbf                         256
     10 /oradata/mcstar/undotbs02.dbf                             8
     11 /oradata/mcstar/testblock02.dbf                         128

FILE_ID FILE_NAME                                BLOCKS-USER_BLOCKS
------- ---------------------------------------- ------------------
     12 /oradata/mcstar/testblock03.dbf                         256

12 rows selected.

继续研究,在表空间testblock3上创建表格,并插入一定的数据量。
SQL> create table testblock3  tablespace testblock3 as select * from obj$;

Table created.

SQL> select EXTENT_ID,BLOCK_ID,BLOCKS from dba_extents where file_id=12;

EXTENT_ID   BLOCK_ID     BLOCKS
---------- ---------- ----------
         0          9       1024
        
SQL> exec DBMS_STATS.GATHER_TABLE_STATS (ownname=>user,tabname=>'TESTBLOCK3',estimate_percent=>100);

PL/SQL procedure successfully completed.

SQL>  select AVG_ROW_LEN,NUM_ROWS,SAMPLE_SIZE from dba_tables where lower(table_name)='testblock3';

AVG_ROW_LEN   NUM_ROWS SAMPLE_SIZE
----------- ---------- -----------
         78      60759       60759

SQL> alter database datafile '/oradata/mcstar/testblock03.dbf' resize 16m;

Database altered.

SQL> begin
  2  for i in (select * from testblock3) loop
  3  insert into testblock3 select * from testblock3 where rownum=1;
  4  commit;
  5  end loop;
  6  end;
  7  /
begin
*
ERROR at line 1:
ORA-01653: unable to extend table SYS.TESTBLOCK3 by 1024 in tablespace
TESTBLOCK3
ORA-06512: at line 3

根据平均行长度为78,数据文件存放业务数据行数可近似计算为((16*1024/8-8)*(8192-819-60))/78=2040*7313/78=191263,但现在目前只存有88252。
两者相差较大。
SQL> select count(*) from testblock3;

  COUNT(*)
----------
     88252

由于目前数据文件只有16M,减去1个数据文件头,7个extent map block,插入数据时并足以进行第二个extent扩展,所以目前表格中只有一个extent,
业务表格实际值,远小于理论值,也就解释的通了。
SQL> select EXTENT_ID,BLOCK_ID,BLOCKS from dba_extents where file_id=12;

EXTENT_ID   BLOCK_ID     BLOCKS
---------- ---------- ----------
         0          9       1024

通过bbed查看,可以看到业务数据好像写到了1033个块,但理论上Oracle业务数据只要写到1032个block即可。
        
BBED> dump block 1032
File: /oradata/mcstar/testblock03.dbf (0)
Block: 1032             Offsets:    0 to  511           Dba:0x00000000
------------------------------------------------------------------------
06a20000 07040003 7a259c0e 000a0106 83bc0000 01000000 22c00400 4e0e9c0e

BBED> dump block 1033
File: /oradata/mcstar/testblock03.dbf (0)
Block: 1033             Offsets:    0 to  511           Dba:0x00000000
------------------------------------------------------------------------
06a20000 08040003 7a259c0e 000a0106 c4420000 01b60000 22c00400 4e0e9c0e


BBED> dump block 1034
File: /oradata/mcstar/testblock03.dbf (0)
Block: 1034             Offsets:    0 to  511           Dba:0x00000000
------------------------------------------------------------------------
00a20000 09040000 00000000 00000105 09a30000 00000000 00000000 00000000

于是采用10046进行跟踪,可以看到业务数据存储到block 11032个为止。
SQL> alter system flush buffer_cache;

System altered.


SQL> alter session set db_file_multiblock_read_count=1;

Session altered.

SQL> ALTER SESSION SET EVENTS '10046 trace name context forever, level 1';

Session altered.

SQL> select count(*) from testblock3;

  COUNT(*)
----------
     88252

SQL>  ALTER SESSION SET EVENTS '10046 trace name context off';

Session altered.

WAIT #19: nam='db file sequential read' ela= 8 file#=12 block#=1030 blocks=1 obj#=311330 tim=1273413313089160
WAIT #19: nam='db file sequential read' ela= 8 file#=12 block#=1031 blocks=1 obj#=311330 tim=1273413313089194
WAIT #19: nam='db file sequential read' ela= 7 file#=12 block#= 1032 blocks=1 obj#=311330 tim=1273413313089226
通过以上分析。12号文件不能被用户使用的256个块,似乎和extent不能扩展有关。
对dba_extent视图查询可知,Oracle开始extent扩展是从第9号block开始(1号block为datafile_head,2-7号block为extent map)。
由于extent扩展至少需要向数据文件一次性申请1个extent大小(本例12号文件一开始为10M,第二次extent扩展时,由于不能获得8M,所以将浪费2M空间,需要注意的是这2M空间包含了datafile_head和extent map)
为验证上述猜想,继续测试,将数据文件resize至刚好能满足2个extent大小的大小。8M*2*1024*1024+8*8K*1024=16777216+65536=16842752
将数据文件扩展至16842752大小
SQL> alter database datafile '/oradata/mcstar/testblock03.dbf' resize 16842752;

Database altered.
继续插入业务数据
SQL> begin
  2  for i in (select * from testblock3) loop
  3  insert into testblock3 select * from testblock3 where rownum=1;
  4  commit;
  5  end loop;
  6  end;
  7  /

PL/SQL procedure successfully completed.

SQL> begin
  2  for i in (select * from testblock3) loop
  3  insert into testblock3 select * from testblock3 where rownum=1;
  4  commit;
  5  end loop;
  6  end;
  7  /
begin
*
ERROR at line 1:
ORA-01653: unable to extend table SYS.TESTBLOCK3 by 1024 in tablespace
TESTBLOCK3
ORA-06512: at line 3

可以看到表格testblock3扩展出第二个分区
SQL>  select EXTENT_ID,BLOCK_ID,BLOCKS from dba_extents where file_id=12;


EXTENT_ID   BLOCK_ID     BLOCKS
---------- ---------- ----------
         0          9       1024
         1       1033       1024
        
SQL>  select count(*) from testblock3;

  COUNT(*)
----------
    204172
采用10046跟踪,可以看到业务数据已经存放至2056为止,该数据文件得到了充分利用。
SQL> ALTER SESSION SET EVENTS '10046 trace name context forever, level 1';

Session altered.

SQL> alter session set db_file_multiblock_read_count=1;

Session altered.

SQL>  alter system flush buffer_cache;

System altered.

SQL>  select count(*) from testblock3;

  COUNT(*)
----------
    204172

SQL>  ALTER SESSION SET EVENTS '10046 trace name context off';

Session altered.

WAIT #1: nam='db file sequential read' ela= 11 file#=12 block#=2054 blocks=1 obj#=311330 tim=1273414177770633
WAIT #1: nam='db file sequential read' ela= 10 file#=12 block#=2055 blocks=1 obj#=311330 tim=1273414177770673
WAIT #1: nam='db file sequential read' ela= 10 file#=12 block#= 2056 blocks=1 obj#=311330 tim=1273414177770715
12号数据文件不可用空间也降至8个数据块。
SQL> select file_id,file_name,blocks-user_blocks from dba_data_files
order by 1;
  2 
FILE_ID FILE_NAME                                BLOCKS-USER_BLOCKS
------- ---------------------------------------- ------------------
      1 /oradata/mcstar/system01.dbf                              8
      2 /oradata/mcstar/undotbs01.dbf                             8
      3 /oradata/mcstar/sysaux01.dbf                              8
      4 /oradata/mcstar/users01.dbf                               8
      5 /oradata/mcstar/mcstar01.dbf                              8
      6 /oradata/mcstar/xu01.dbf                                  8
      7 /oradata/mcstar/zhoul01.dbf                               8
      8 /oradata/mcstar/zhoul201.dbf                              8
      9 /oradata/mcstar/testblock01.dbf                         256
     10 /oradata/mcstar/undotbs02.dbf                             8
     11 /oradata/mcstar/testblock02.dbf                         128

FILE_ID FILE_NAME                                BLOCKS-USER_BLOCKS
------- ---------------------------------------- ------------------
     12 /oradata/mcstar/testblock03.dbf                           8
    
12 rows selected.

通过以上测试表明,我们创建数据文件设置的大小只要能满足extent扩展的需要,就能最大程度的节省空间。如果不加考虑就设置大小,在数据文件自动扩展关闭的情况下,那最大程度将浪费 (uniform size-8*block_size)的空间大小。
SQL> create tablespace testblock4 datafile '/oradata/mcstar/testblock04.dbf' SIZE 16842752
  2  EXTENT MANAGEMENT LOCAL UNIFORM SIZE 8m
  3   SEGMENT SPACE MANAGEMENT AUTO;

Tablespace created.


SQL> select file_id,file_name,blocks-user_blocks from dba_data_files
order by 1;
  2 
FILE_ID FILE_NAME                                BLOCKS-USER_BLOCKS
------- ---------------------------------------- ------------------
      1 /oradata/mcstar/system01.dbf                              8
      2 /oradata/mcstar/undotbs01.dbf                             8
      3 /oradata/mcstar/sysaux01.dbf                              8
      4 /oradata/mcstar/users01.dbf                               8
      5 /oradata/mcstar/mcstar01.dbf                              8
      6 /oradata/mcstar/xu01.dbf                                  8
      7 /oradata/mcstar/zhoul01.dbf                               8
      8 /oradata/mcstar/zhoul201.dbf                              8
      9 /oradata/mcstar/testblock01.dbf                         256
     10 /oradata/mcstar/undotbs02.dbf                             8
     11 /oradata/mcstar/testblock02.dbf                         128

FILE_ID FILE_NAME                                BLOCKS-USER_BLOCKS
------- ---------------------------------------- ------------------
     12 /oradata/mcstar/testblock03.dbf                           8
     13 /oradata/mcstar/testblock04.dbf                           8

13 rows selected.

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