我们知道，Oracle的最小IO单位是Block。在Oracle中，有多种block存在，比如file header block,segment header block,undo header block,data block,等等。那么，Oracle是根据什么来区别不同的block类型的呢？
每个Block的第一个字节就代表了该Block的类型。随着版本的进化，不断的有新的block类型产生，也有些旧的block类型消失。如果将每种block的内部存储结果弄清楚了，花点时间和耐心，要再写个dul也不是不可能的事情。
这两天抽空用c写了几行demo，基本上可以将普通的block dump出来，跟oracle自己提供的dump功能相似，只不过是可以脱机直接dump datafile的^_^
下表是从Ixora抄来的资料：
| Code | Block Type | Versions |
|---|---|---|
| 1 | Undo header block (limited extents) | all, but superseded by type 14 from 8.0 |
| 2 | Undo data block | all |
| 3 | Save undo header block (limited extents) | all, but superseded by type 15 from 8.0 |
| 4 | Save undo data block | all |
| 5 | Data segment header block (limited extents, no freelist groups) | all, but superseded by type 16 from 8.0 |
| 6 | Table/cluster/index segment data block | all |
| 7 | Temporary segment data block | all |
| 8 | Sort key block | all |
| 9 | Sort run block | all |
| 10 | Free list block | all, but superseded by type 22 from 8.0 |
| 11 | Data file header block | all |
| 12 | Data segment header block (limited extents, with freelist groups) | all, but superseded by type 17 from 8.0 |
| 13 | Compatibility segment | all |
| 14 | Undo header block (unlimited extents) | from 7.3 |
| 15 | Save undo header block (unlimited extents) | from 7.3 |
| 16 | Data segment header block (unlimited extents, no freelist groups) | from 7.3 |
| 17 | Data segment header block (unlimited extents, with freelist groups) | from 7.3 |
| 18 | Extent map block | from 7.3 |
| 19 | Backup set piece header | from 8.0 |
| 20 | Backup set directory block | from 8.0 |
| 21 | Controlfile block | from 8.0 |
| 22 | Free list block with free block count | from 8.0 |
| 23 | LOB segment header block | from 8.0 |
| 24 | LOB segment freelist block | from 8.0 |
| 25 | LOB segment space usage index block | from 8.0 |
| 26 | LOB segment space usage bitmap block | from 8.0 |
| 27 | LOB segment chunk data block | from 8.0 |
| 28 | don't know | |
| 29 | LMT space map header block | from 8.1 |
| 30 | LMT space map block | from 8.1 |
| 31 | don't know | |
| 32 | ASSM level 1 bitmap block | from 9.0 |
| 33 | ASSM level 2 bitmap block | from 9.0 |
| 34 | ASSM level 3 bitmap block | from 9.0 |
| 35 | ASSM segment header block | from 9.0 |
| 36 | ASSM extent map block | from 9.0 |
| 37 | don't know | |
| 38 | System managed undo header block | from 9.0 |