Oracle Study之Oracle High Water Mark的示例分析

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在手動段空間管理（Manual Segment Space Management）中，段中只有一個 HWM，但是在 Oracle9iRelease1 才添加的自動段空間管理（Automatic Segment Space Management）中，又有了一個低 HWM 的概念出來。為什么有了 HWM 還又有一個低 HWM 呢，這個是因為自動段空間管理的特性造成的。在手段段空間管理中，當數據插入以后，如果是插入到新的數據塊中，數據塊就會被自動格式化等待數據訪問。而在自動段空間管理中，數據插入到新的數據塊以后，數據塊并沒有被格式化，而是在第一次在第一次訪問這個數據塊的時候才格式化這個塊。所以我們又需要一條水位線，用來標示已經被格式化的塊。這條水位線就叫做低 HWM。

未格式化，意思就是這個塊，已經是屬于這個段了，但是還保留著原來的樣子沒動
格式化就是把塊中的數據清除掉，并把塊頭改為這個對象的
MSSM 表空間中的段，只有一個高水位，高水位下的塊都是格式化了的
但是 ASSM 表空間中的段，有兩個高水位：低高水位和高高水位
低高水位下的塊全部是格式化了的
但是低高水位和高高水位之間的塊，則可能是格式化了的，也可能是沒有

全表掃描時，通常都是讀至低高水位線，然后根據位圖去讀低高與高高之間格式化過的塊，避開未格式化的塊

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@rh6 ~]$ 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@rh6 ~]$ 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、驗證 hwm 的變化

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@rh6 ~]$ 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@rh6 ~]$ 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: rh6
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, p_w_picpath: oracle@rh6 (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@rh6 (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 中發生了變化
#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