Oracle Study之--Oracle High Water Mark


       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