在手动段空间管理（Manual Segment Space Management）中，段中只有一个HWM，但是在Oracle9iRelease1才添加的自动段空间管理（Automatic Segment Space Management）中，又有了一个低HWM的概念出来。为什么有了HWM还又有一个低HWM呢，这个是因为自动段空间管理的特性造成的。在手段段空间管理中，当数据插入以后，如果是插入到新的数据块中，数据块就会被自动格式化等待数据访问。而在自动段空间管理中，数据插入到新的数据块以后，数据块并没有被格式化，而是在第一次在第一次访问这个数据块的时候才格式化这个块。所以我们又需要一条水位线，用来标示已经被格式化的块。这条水位线就叫做低HWM。
未格式化，意思就是这个块，已经是属于这个段了，但是还保留着原来的样子没动
格式化就是把块中的数据清除掉，并把块头改为这个对象的
MSSM表空间中的段，只有一个高水位，高水位下的块都是格式化了的
但是ASSM表空间中的段，有两个高水位：低高水位和高高水位
低高水位下的块全部是格式化了的
但是低高水位和高高水位之间的块，则可能是格式化了的，也可能是没有
全表扫描时，通常都是读至低高水位线，然后根据位图去读低高与高高之间格式化过的块，避开未格式化的块
案例分析：
1、分析表
09:46:07 SCOTT@ test1>analyze table emp2 compute statistics
09:48:00 2 ;
Table analyzed.
2、查看table中的hwm
09:48:01 SCOTT@ test1>select table_name,num_rows,blocks,empty_blocks from user_tables
09:48:47 2 where table_name='EMP2';
TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
------------------------------ ---------- ---------- ------------
EMP2 71680 499 13
3、分析segment header block
10:09:18 SYS@ test1>select segment_name,header_block from dba_segments
10:09:32 2 where segment_name='EMP2';
SEGMENT_NAME HEADER_BLOCK
-------------------- ------------
EMP2 178
10:07:34 SYS@ test1>alter system dump datafile 4 block 178;
System altered.
[oracle@rh5 ~]$ ls -lt /u01/app/oracle/diag/rdbms/test1/test1/trace|more
total 3272
-rw-r----- 1 oracle oinstall 6083 May 6 10:07 test1_ora_3212.trc
4、查看header block中信息
[oracle@rh5 ~]$ more /u01/app/oracle/diag/rdbms/test1/test1/trace/test1_ora_3212.trc
Start dump data blocks tsn: 4 file#:4 minblk 178 maxblk 178
Block dump from cache:
Dump of buffer cache at level 4 for tsn=4, rdba=16777394
Block dump from disk:
buffer tsn: 4 rdba: 0x010000b2 (4/178)
scn: 0x0000.000a2333 seq: 0x13 flg: 0x04 tail: 0x23332313
frmt: 0x02 chkval: 0x980b type: 0x23=PAGETABLE SEGMENT HEADER
Hex dump of block: st=0, typ_found=1
Dump of memory from 0x00918200 to 0x0091A200
918200 0000A223 010000B2 000A2333 04130000 [#.......3#......]
918210 0000980B 00000000 00000000 00000000 [................]
918220 00000000 00000001 00000008 00000A9C [................]
918230 00000001 00000080 00000080 01000300 [................]
918240 00000000 00000001 00000000 000001F3 [................]
918250 001C0014 00000021 00000001 00000011 [....!...........]
918260 00000080 00000080 01000280 00000000 [................]
918270 00000011 00000000 00000180 01000201 [................]
918280 01000281 00000000 00000000 00000000 [................]
918290 00000000 00000000 00000000 00000000 [................]
Repeat 3 times
9182D0 00000001 00002000 00000000 00001434 [..... ......4...]
9182E0 00000000 010000B1 00000001 01000281 [................]
9182F0 010000B1 00000000 00000000 00000000 [................]
918300 00000000 00000000 00000001 00000000 [................]
918310 000032E0 12000000 010000B0 00000008 [.2..............]
918320 00000000 00000000 00000000 00000000 [................]
Repeat 152 times
918CB0 010000B0 010000B3 00000000 00000000 [................]
918CC0 00000000 00000000 00000000 00000000 [................]
Repeat 151 times
919640 00000000 00000000 010000B1 00000000 [................]
919650 00000000 00000000 00000000 00000000 [................]
Repeat 185 times
91A1F0 00000000 00000000 00000000 23332313 [.............#3#]
Extent Control Header
-----------------------------------------------------------------
Extent Header:: spare1: 0 spare2: 0 #extents: 1 #blocks: 8
last map 0x00000000 #maps: 0 offset: 2716
Highwater:: 0x01000300 ext#: 1 blk#: 128 ext size: 128
#blocks in seg. hdr's freelists: 0
#blocks below: 499
mapblk 0x00000000 offset: 1
Disk Lock:: Locked by xid: 0x0014.01c.00000021
--------------------------------------------------------
Low HighWater Mark :
Highwater:: 0x01000280 ext#: 17 blk#: 128 ext size: 128
#blocks in seg. hdr's freelists: 0
#blocks below: 384
mapblk 0x00000000 offset: 17
Level 1 BMB for High HWM block: 0x01000281
Level 1 BMB for Low HWM block: 0x01000201
--------------------------------------------------------
Segment Type: 1 nl2: 1 blksz: 8192 fbsz: 0
L2 Array start offset: 0x00001434
First Level 3 BMB: 0x00000000
L2 Hint for inserts: 0x010000b1
Last Level 1 BMB: 0x01000281
Last Level II BMB: 0x010000b1
Last Level III BMB: 0x00000000
Map Header:: next 0x00000000 #extents: 1 obj#: 13024 flag: 0x12000000
Inc # 0
Extent Map
-----------------------------------------------------------------
0x010000b0 length: 8
Auxillary Map
--------------------------------------------------------
Extent 0 : L1 dba: 0x010000b0 Data dba: 0x010000b3
--------------------------------------------------------
5、验证hwm的变化
09:48:55 SCOTT@ test1>delete from emp2 where rownum < 70001;
70000 rows deleted.
09:55:20 SCOTT@ test1>commit;
Commit complete.
09:55:22 SCOTT@ test1>alter table emp2 move;
Table altered.
09:56:45 SCOTT@ test1>select table_name,num_rows,blocks,empty_blocks from user_tables
09:56:58 2 where table_name='EMP2';
TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
------------------------------ ---------- ---------- ------------
EMP2 1680 13 3
10:09:02 SYS@ test1>col segment_name for a20
10:09:18 SYS@ test1>select segment_name,header_block from dba_segments
10:09:32 2 where segment_name='EMP2';
SEGMENT_NAME HEADER_BLOCK
-------------------- ------------
EMP2 362
10:12:58 SYS@ test1>alter system dump datafile 4 block 362;
System altered.
[oracle@rh5 ~]$ ls -lt /u01/app/oracle/diag/rdbms/test1/test1/trace|more
total 3284
-rw-r----- 1 oracle oinstall 4514 May 6 10:13 test1_ora_3300.trc
[oracle@rh5 ~]$ more /u01/app/oracle/diag/rdbms/test1/test1/trace/test1_ora_3300.trc
Trace file /u01/app/oracle/diag/rdbms/test1/test1/trace/test1_ora_3300.trc
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
ORACLE_HOME = /u01/app/oracle/product/11.2.0/db_1
System name: Linux
Node name: rh5
Release: 2.6.18-194.el5
Version: #1 SMP Tue Mar 16 21:52:43 EDT 2010
Machine: i686
Instance name: test1
Redo thread mounted by this instance: 1
Oracle process number: 25
Unix process pid: 3300, image: oracle@rh5 (TNS V1-V3)
*** 2016-05-06 10:13:07.142
*** SESSION ID:(31.2) 2016-05-06 10:13:07.142
*** CLIENT ID:() 2016-05-06 10:13:07.142
*** SERVICE NAME:(SYS$USERS) 2016-05-06 10:13:07.142
*** MODULE NAME:(sqlplus@rh5 (TNS V1-V3)) 2016-05-06 10:13:07.142
*** ACTION NAME:() 2016-05-06 10:13:07.142
Start dump data blocks tsn: 4 file#:4 minblk 362 maxblk 362
Block dump from cache:
Dump of buffer cache at level 4 for tsn=4, rdba=16777578
Block dump from disk:
buffer tsn: 4 rdba: 0x0100016a (4/362)
scn: 0x0000.000a2350 seq: 0x01 flg: 0x04 tail: 0x23502301
frmt: 0x02 chkval: 0x9801 type: 0x23=PAGETABLE SEGMENT HEADER
Hex dump of block: st=0, typ_found=1
Dump of memory from 0x05DCB600 to 0x05DCD600
5DCB600 0000A223 0100016A 000A2350 04010000 [#...j...P#......]
5DCB610 00009801 00000000 00000000 00000000 [................]
5DCB620 00000000 00000002 00000010 00000A9C [................]
5DCB630 00000001 00000005 00000008 0100017D [............}...]
5DCB640 00000000 00000001 00000000 0000000D [................]
5DCB650 00000000 00000000 00000000 00000001 [................]
5DCB660 00000005 00000008 0100017D 00000000 [........}.......]
5DCB670 00000001 00000000 0000000D 01000168 [............h...]
5DCB680 01000168 00000000 00000000 00000000 [h...............]
5DCB690 00000000 00000000 00000000 00000000 [................]
Repeat 3 times
5DCB6D0 00000001 00002000 00000000 00001434 [..... ......4...]
5DCB6E0 00000000 01000169 00000001 01000168 [....i.......h...]
5DCB6F0 01000169 00000000 00000000 00000000 [i...............]
5DCB700 00000000 00000000 00000002 00000000 [................]
5DCB710 000033F8 10000000 01000168 00000008 [.3......h.......]
5DCB720 01000178 00000008 00000000 00000000 [x...............]
5DCB730 00000000 00000000 00000000 00000000 [................]
Repeat 151 times
5DCC0B0 01000168 0100016B 01000168 01000178 [h...k...h...x...]
5DCC0C0 00000000 00000000 00000000 00000000 [................]
Repeat 151 times
5DCCA40 00000000 00000000 01000169 00000000 [........i.......]
5DCCA50 00000000 00000000 00000000 00000000 [................]
Repeat 185 times
5DCD5F0 00000000 00000000 00000000 23502301 [.............#P#]
Extent Control Header
-----------------------------------------------------------------
Extent Header:: spare1: 0 spare2: 0 #extents: 2 #blocks: 16
last map 0x00000000 #maps: 0 offset: 2716
Highwater:: 0x0100017d ext#: 1 blk#: 5 ext size: 8 /／HWM在 header block中发生了变化
#blocks in seg. hdr's freelists: 0
#blocks below: 13
mapblk 0x00000000 offset: 1
Unlocked
--------------------------------------------------------
Low HighWater Mark :
Highwater:: 0x0100017d ext#: 1 blk#: 5 ext size: 8
#blocks in seg. hdr's freelists: 0
#blocks below: 13
mapblk 0x00000000 offset: 1
Level 1 BMB for High HWM block: 0x01000168
Level 1 BMB for Low HWM block: 0x01000168
--------------------------------------------------------
Segment Type: 1 nl2: 1 blksz: 8192 fbsz: 0
L2 Array start offset: 0x00001434
First Level 3 BMB: 0x00000000
L2 Hint for inserts: 0x01000169
Last Level 1 BMB: 0x01000168
Last Level II BMB: 0x01000169
Last Level III BMB: 0x00000000
Map Header:: next 0x00000000 #extents: 2 obj#: 13304 flag: 0x10000000
Inc # 0
Extent Map
-----------------------------------------------------------------
0x01000168 length: 8
0x01000178 length: 8
Auxillary Map
--------------------------------------------------------
Extent 0 : L1 dba: 0x01000168 Data dba: 0x0100016b
Extent 1 : L1 dba: 0x01000168 Data dba: 0x01000178
--------------------------------------------------------
Second Level Bitmap block DBAs
--------------------------------------------------------
DBA 1: 0x01000169
End dump data blocks tsn: 4 file#: 4 minblk 362 maxblk 362
