步骤8: 多插入一些信息看看变化情况.
SQL> select extent_id, block_id from dba_extents where owner='HARRY' and segment_name='T4';

EXTENT_ID BLOCK_ID
---------- ----------
0 481

SQL> declare
2 a number;
3 i number;
4 begin
5 for i in 1.. 510 loop
6 a := i+2;
7 insert into t4
8 select 'i'||'i'||'i', 'harry'||'i' from dual;
9 end loop;
10 end;
11 /

PL/SQL procedure successfully completed.

SQL> commit;

SQL> select extent_id, block_id, blocks from dba_extents where owner='HARRY' and segment_name='T4';

EXTENT_ID BLOCK_ID BLOCKS
---------- ---------- ----------
0 481 8
1 969 8
2 977 8
3 985 8
4 993 8
5 1001 8
6 1009 8
7 1017 8
8 1025 8
9 1161 8
10 1169 8

EXTENT_ID BLOCK_ID BLOCKS
---------- ---------- ----------
11 1177 8
12 1185 8
13 1193 8
14 1201 8
15 1209 8
16 1289 128
17 1417 128

我们可以看到 Oracle 首先会按8个blocks为一个extent扩展16次,　然后会再按128个blocks为一个extent来分配. 现在再去看看段头中信息的变化.

SQL> alter system dump datafile 5 block 483;

截取dump文件中部分信息

Extent Control Header
-----------------------------------------------------------------
Extent Header:: spare1: 0 spare2: 0 #extents: 18 #blocks: 384
last map 0x00000000 #maps: 0 offset: 2716
Highwater:: 0x014005c9 ext#: 17 blk#: 64 ext size: 128
--------------------------------------------------------
Low HighWater Mark :
Highwater:: 0x01400589 ext#: 16 blk#: 128 ext size: 128

Extent Map
-----------------------------------------------------------------
0x014001e1 length: 8
0x014003c9 length: 8
0x014003d1 length: 8
0x014003d9 length: 8
0x014003e1 length: 8
0x014003e9 length: 8
0x014003f1 length: 8
0x014003f9 length: 8
0x01400401 length: 8
0x01400489 length: 8
0x01400491 length: 8
0x01400499 length: 8
0x014004a1 length: 8
0x014004a9 length: 8
0x014004b1 length: 8
0x014004b9 length: 8
0x01400509 length: 128
0x01400589 length: 128

Auxillary Map
--------------------------------------------------------
Extent 0 : L1 dba: 0x014001e1 Data dba: 0x014001e4
Extent 1 : L1 dba: 0x014001e1 Data dba: 0x014003c9
Extent 2 : L1 dba: 0x014003d1 Data dba: 0x014003d2
Extent 3 : L1 dba: 0x014003d1 Data dba: 0x014003d9
Extent 4 : L1 dba: 0x014003e1 Data dba: 0x014003e2
Extent 5 : L1 dba: 0x014003e1 Data dba: 0x014003e9
Extent 6 : L1 dba: 0x014003f1 Data dba: 0x014003f2
Extent 7 : L1 dba: 0x014003f1 Data dba: 0x014003f9
Extent 8 : L1 dba: 0x01400401 Data dba: 0x01400402
Extent 9 : L1 dba: 0x01400401 Data dba: 0x01400489
Extent 10 : L1 dba: 0x01400491 Data dba: 0x01400492
Extent 11 : L1 dba: 0x01400491 Data dba: 0x01400499
Extent 12 : L1 dba: 0x014004a1 Data dba: 0x014004a2
Extent 13 : L1 dba: 0x014004a1 Data dba: 0x014004a9
Extent 14 : L1 dba: 0x014004b1 Data dba: 0x014004b2
Extent 15 : L1 dba: 0x014004b1 Data dba: 0x014004b9
Extent 16 : L1 dba: 0x01400509 Data dba: 0x0140050b
Extent 17 : L1 dba: 0x01400589 Data dba: 0x0140058b

在 Extent Map区域描述了每个extent的第一个block的地址和这个extent中包含block的个数.
在Auxlillary Map区域描述了每个extent对应的L1 位图的物理地址(L1 dba) 和每个extent用于存放数据( 不包括存Metadata的块 )开始的block地址( Data dba ). 并且可以注意到,由8个block组成的extent每两个共有一个L1 dba( 如extent 0和extent 1, extent 2 和 extent 3 ). 但从extent 16(包含128个block)开始, 需使用两个blocks来管理( 如0x01400509和0x0140050a管理extent 16 ). 并且, 从上面的dump信息中可以看到High highwater mark是指向0x014005c9 (1481)这个块, low highwater mark是指向的0x01400589(1417)这个块上, 即extent 17的开始block . Dump 1417块, 截取dump文件中部分信息如下:

--------------------------------------------------------
DBA Ranges :
--------------------------------------------------------
0x01400589 Length: 64 Offset: 0

0:Metadata 1:Metadata 2:unformatted 3:unformatted
4:unformatted 5:unformatted 6:unformatted 7:unformatted
8:unformatted 9:unformatted 10:unformatted 11:unformatted
12:unformatted 13:unformatted 14:unformatted 15:unformatted
16:75-100% free 17:75-100% free 18:FULL 19:75-100% free
20:75-100% free 21:75-100% free 22:FULL 23:75-100% free
24:75-100% free 25:75-100% free 26:FULL 27:75-100% free
28:75-100% free 29:75-100% free 30:FULL 31:75-100% free
32:75-100% free 33:75-100% free 34:FULL 35:75-100% free
36:75-100% free 37:75-100% free 38:FULL 39:75-100% free
40:75-100% free 41:75-100% free 42:FULL 43:75-100% free
44:75-100% free 45:75-100% free 46:FULL 47:75-100% free
48:75-100% free 49:75-100% free 50:FULL 51:75-100% free
52:75-100% free 53:75-100% free 54:FULL 55:75-100% free
56:75-100% free 57:75-100% free 58:FULL 59:75-100% free
60:75-100% free 61:75-100% free 62:25-50% free 63:75-100% free
--------------------------------------------------------
End dump data blocks tsn: 5 file#: 5 minblk 1417 maxblk 1417

可以看到这个L1位图管理了64个blocks, 从块1417到1480. 其中有些blocks是已经被使用过或是free的, 而其他一些块还是unformatted状态的, 这也印证了前面关于low Highwater mark和high highwater mark 的描述. Extent 17一共由128个blocks构成, 1417是这个extent中的一个L1位图块, 管理了64个blocks. 剩下的64个blocks是由另外一个L1位图块1418来管理. Dump 1418这个块.

--------------------------------------------------------
DBA Ranges :
--------------------------------------------------------
0x014005c9 Length: 64 Offset: 0

0:unformatted 1:unformatted 2:unformatted 3:unformatted
4:unformatted 5:unformatted 6:unformatted 7:unformatted
8:unformatted 9:unformatted 10:unformatted 11:unformatted
12:unformatted 13:unformatted 14:unformatted 15:unformatted
16:unformatted 17:unformatted 18:unformatted 19:unformatted
20:unformatted 21:unformatted 22:unformatted 23:unformatted
24:unformatted 25:unformatted 26:unformatted 27:unformatted
28:unformatted 29:unformatted 30:unformatted 31:unformatted
32:unformatted 33:unformatted 34:unformatted 35:unformatted
36:unformatted 37:unformatted 38:unformatted 39:unformatted
40:unformatted 41:unformatted 42:unformatted 43:unformatted
44:unformatted 45:unformatted 46:unformatted 47:unformatted
48:unformatted 49:unformatted 50:unformatted 51:unformatted
52:unformatted 53:unformatted 54:unformatted 55:unformatted
56:unformatted 57:unformatted 58:unformatted 59:unformatted
60:unformatted 61:unformatted 62:unformatted 63:unformatted
--------------------------------------------------------
End dump data blocks tsn: 5 file#: 5 minblk 1418 maxblk 1418

可以看到后面这些块目前都没有使用, 都为unformatted状态,原因就在于当前的high highwater mark是在1481上.