1.       定位出segment header的数据块

select segment_name, header_file, header_block
         from dba_segments
         where segment_name = 'EMPLOYEES';

2.    dump

  alter system dump datafile 5 block 83;

3. 如果是回滚段的话，还可用以下命令dump ，_SYSSMU1$为段名

   alter system dump undo header '_SYSSMU1$';

3. 在udump下找出对应的trace文件。内容如下

*** 2009-01-19 14:26:34.348

*** SERVICE NAME:(SYS$USERS) 2009-01-19 14:26:34.302

*** SESSION ID:(133.57) 2009-01-19 14:26:34.302

Start dump data blocks tsn: 6 file#: 5 minblk 83 maxblk 83

buffer tsn: 6 rdba: 0x01400053 (5/83)

scn: 0x0000.000831f3 seq: 0x01 flg: 0x04 tail: 0x31f32301

frmt: 0x02 chkval: 0x63bd type: 0x23=PAGETABLE SEGMENT HEADER

Hex dump of block: st=0, typ_found=1

Dump of memory from 0x07E52200 to 0x07E54200

。。。。。。。

  Extent Control Header

  -----------------------------------------------------------------

  Extent Header:: spare1: 0      spare2: 0      #extents: 1      #blocks: 8    

                  last map  0x00000000  #maps: 0      offset: 2716 

      Highwater::  0x01400059  ext#: 0      blk#: 8      ext size: 8    

  #blocks in seg. hdr's freelists: 0    

  #blocks below: 5    

  mapblk  0x00000000  offset: 0    

                   Unlocked

  --------------------------------------------------------

  Low HighWater Mark :

      Highwater::  0x01400059  ext#: 0      blk#: 8      ext size: 8    

  #blocks in seg. hdr's freelists: 0    

  #blocks below: 5    

  mapblk  0x00000000  offset: 0    

  Level 1 BMB for High HWM block: 0x01400051

  Level 1 BMB for Low HWM block: 0x01400051

  --------------------------------------------------------

  Segment Type: 1 nl2: 1      blksz: 8192   fbsz: 0     

  L2 Array start offset:  0x00001434

  First Level 3 BMB:  0x00000000

  L2 Hint for inserts:  0x01400052

  Last Level 1 BMB:  0x01400051

  Last Level II BMB:  0x01400052

  Last Level III BMB:  0x00000000

     Map Header:: next  0x00000000  #extents: 1    obj#: 51258  flag: 0x10000000

  Inc # 0

  Extent Map

  -----------------------------------------------------------------

   0x01400051  length: 8    

  Auxillary Map

  --------------------------------------------------------

   Extent 0     :  L1 dba:  0x01400051 Data dba:  0x01400054

  --------------------------------------------------------

   Second Level Bitmap block DBAs

   --------------------------------------------------------

   DBA 1:   0x01400052

End dump data blocks tsn: 6 file#: 5 minblk 83 maxblk 83