本文解析了Oracle数据库中v$bh视图的结构及用途，介绍了如何利用v$bh和x$bh视图监测数据块的状态，包括热点块分析及数据块在缓存中的分布情况。
v$bh与x$bh
v$bh:非常详细地记录了数据块在数据缓冲区内的使用情况，一条记录对应一个block的详细记录。
v$bh来自于基表x$bh与x$le
查看v$bh结构:
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13:07:20
SYS@ test1 >desc v$bhName Null? Type----------------------------------------------------------------- -------- --------------------------------------------FILE# NUMBERBLOCK# NUMBERCLASS# NUMBERSTATUS VARCHAR2(10)XNC NUMBERFORCED_READS NUMBERFORCED_WRITES NUMBERLOCK_ELEMENT_ADDR RAW(4)LOCK_ELEMENT_NAME NUMBERLOCK_ELEMENT_CLASS NUMBERDIRTY VARCHAR2(1)TEMP VARCHAR2(1)PING VARCHAR2(1)STALE VARCHAR2(1)DIRECT VARCHAR2(1)NEW CHAR(1)OBJD NUMBERTS# NUMBERLOBID NUMBERCACHEHINT NUMBER |
STATUS
free - not currently in use
xcur - exclusive current,表示该数据块处于排外模式,正在被当前Instance占用；
scur - shared current,在RAC环境中表示该数据库正在和其他实例共享数据。
cr - consistent read,一致性读。
read - being read from disk
mrec - in media recovery mode,表示数据块处于介质恢复模式；
irec - in instance recovery mode ,表示数据块处于实例恢复模式；
write - 表示数据库正在往磁盘写入数据；
13:07:19 SYS@ test1 >select * from v$fixed_view_definition t where t.view_name='GV$BH'
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VIEW_NAME VIEW_DEFINITION------------------------------ --------------------------------------------------GV$BH select bh.inst_id, file#, dbablk,
class, decode(st
ate,0,'free',1,'xcur',2,'scur',3,'cr',
4,'read',5,
'mrec',6,'irec',7,'write',8,'pi',
9,'memory',10,'m
write',11,'donated', 12,'protected', 13,'securefi
le', 14,'siop',15,'recckpt', 16, 'flashfree',
17,
'flashcur',
18, 'flashna'),
0, 0,
0, bh.le_addr,
le_id1, le_id2, decode(bitand(flag,1),
0, 'N',
'Y'
), decode(bitand(flag,16),
0, 'N',
'Y'), decode(bi
tand(flag,1536),
0, 'N',
'Y'), decode(bitand(flag,
16384), 0,
'N', 'Y'), decode(bitand(flag,65536),
0
, 'N',
'Y'), 'N', obj, ts#, lobid, bitand(OBJ_FLA
G, 240)/16
from x$bh bh, x$le le
where bh.le_addr
= le.le_addr (+) |
13:08:20 SYS@ test1 >desc x$bh
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Name Null? Type----------------------------------------------------------------- -------- --------------------------------------------ADDR RAW(4)INDX NUMBERINST_ID NUMBERHLADDR RAW(4)BLSIZ NUMBERNXT_HASH RAW(4)PRV_HASH RAW(4)NXT_REPL RAW(4)PRV_REPL RAW(4)FLAG NUMBERFLAG2 NUMBERLOBID NUMBERRFLAG NUMBERSFLAG NUMBERLRU_FLAG NUMBERTS# NUMBERFILE# NUMBERDBARFIL NUMBERDBABLK NUMBERCLASS NUMBERSTATE NUMBERMODE_HELD NUMBERCHANGES NUMBERCSTATE NUMBERLE_ADDR RAW(4)DIRTY_QUEUE NUMBERSET_DS RAW(4)OBJ NUMBERBA RAW(4)CR_SCN_BAS NUMBERCR_SCN_WRP NUMBERCR_XID_USN NUMBERCR_XID_SLT NUMBERCR_XID_SQN NUMBERCR_UBA_FIL NUMBERCR_UBA_BLK NUMBERCR_UBA_SEQ NUMBERCR_UBA_REC NUMBERCR_SFL NUMBERCR_CLS_BAS NUMBERCR_CLS_WRP NUMBERLRBA_SEQ NUMBERLRBA_BNO NUMBERHSCN_BAS NUMBERHSCN_WRP NUMBERHSUB_SCN NUMBERUS_NXT RAW(4)US_PRV RAW(4)WA_NXT RAW(4)WA_PRV RAW(4)OQ_NXT RAW(4)OQ_PRV RAW(4)AQ_NXT RAW(4)AQ_PRV RAW(4)OBJ_FLAG NUMBERTCH NUMBERTIM NUMBERCR_RFCNT NUMBERSHR_RFCNT NUMBER |
x$bh中“touch count”信息对应到x$bh的tch字段，而段的data_object_id信息对应到x$bh的obj，或者是v$bh的objd。
案例分析：
（1）对象有多少个数据块缓存到Data buffer中
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13:29:01
SCOTT@ test1 >truncate table t1;13:29:09
SCOTT@ test1 >begin13:29:12
2 for
i in 1..5000
loop13:29:25
3 insert
into t1 values (i);13:29:35
4 end loop;13:29:38
5 end;13:29:39
6 /PL/SQL procedure successfully completed.Elapsed: 00:00:00.6713:29:41
SCOTT@ test1 >commit; |
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创建一存储过程：show space读取数据块信息13:29:43
SCOTT@ test1 >create or
replace procedure show_space13:34:44
213:34:44
3 ( p_segname in
varchar2,13:34:44
413:34:44
5 p_owner in
varchar2 default user,13:34:44
613:34:44
7 p_type in
varchar2 default 'TABLE',13:34:44
813:34:44
9 p_partition in
varchar2 default NULL )13:34:44
1013:34:44
11 as13:34:44
1213:34:44
13 l_total_blocks number;13:34:44
1413:34:44
15 l_total_bytes number;13:34:44
1613:34:44
17 l_unused_blocks number;13:34:44
1813:34:44
19 l_unused_bytes number;13:34:44
2013:34:44
21 l_LastUsedExtFileId number;13:34:44
2213:34:44
23 l_LastUsedExtBlockId number;13:34:44
2413:34:44
25 l_last_used_block number;13:34:44
2613:34:44
27 procedure p( p_label
in varchar2, p_num
in number )13:34:44
2813:34:44
29 is13:34:44
3013:34:44
31 begin13:34:44
3213:34:44
33 dbms_output.put_line( rpad(p_label,40,'.') ||13:34:44
3413:34:45
35
p_num );13:34:45
3613:34:45
37 end;13:34:45
3813:34:45
39 begin13:34:45
4013:34:45
4113:34:45
4213:34:45
43 dbms_space.unused_space13:34:45
4413:34:45
45 ( segment_owner => p_owner,13:34:45
4613:34:45
47 segment_name => p_segname,13:34:45
4813:34:45
49 segment_type => p_type,13:34:45
5013:34:45
51 partition_name => p_partition,13:34:45
5213:34:45
53 total_blocks => l_total_blocks,13:34:45
5413:34:45
55 total_bytes => l_total_bytes,13:34:45
5613:34:45
57 unused_blocks => l_unused_blocks,13:34:45
5813:34:45
59 unused_bytes => l_unused_bytes,13:34:45
6013:34:45
61 last_used_extent_file_id => l_LastUsedExtFileId,13:34:45
6213:34:45
63 last_used_extent_block_id => l_LastUsedExtBlockId,13:34:45
6413:34:45
65 last_used_block => l_last_used_block );13:34:45
6613:34:45
6713:34:45
6813:34:45
69 p( 'Total Blocks', l_total_blocks );13:34:45
7013:34:45
71 p( 'Total Bytes', l_total_bytes );13:34:45
7213:34:45
73 p( 'Unused Blocks', l_unused_blocks );13:34:45
7413:34:45
75 p( 'Unused Bytes', l_unused_bytes );13:34:45
7613:34:45
77 p( 'Last Used Ext FileId', l_LastUsedExtFileId );13:34:45
7813:34:45
79 p( 'Last Used Ext BlockId', l_LastUsedExtBlockId );13:34:45
8013:34:45
81 p( 'Last Used Block', l_last_used_block );13:34:45
8213:34:45
83 end;13:34:47
84 / |
13:34:49 SCOTT@ test1 >set serverout on
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13:35:01
SCOTT@ test1 >exec show_space(p_segname=>'T1');Total Blocks............................16Total Bytes.............................131072Unused Blocks...........................0Unused Bytes............................0Last Used Ext FileId....................4Last Used Ext BlockId...................136Last Used Block.........................8PL/SQL procedure successfully completed. |
数据文件id为4，t1表总共使用了16个块
分析数据块的rowid：
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13:35:40
SCOTT@ test1 >select f,b from
(13:41:15
2 select dbms_rowid.rowid_relative_fno(rowid) f,13:41:44
3 dbms_rowid.rowid_block_number(rowid) b13:42:04
4 from
t1) group by f,b; F B---------- ---------- 4
143 11
691 4
142 11
693 11
694
4 141 11
692 11
6958
rows selected. |
Users 表空间有两个datafile，id为4，11
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13:42:52
SYS@ test1 >select file#,name from
v$datafile; FILE# NAME---------- -------------------------------------------------- 1
/u01/app/oracle/oradata/test1/system01.dbf 2
/u01/app/oracle/oradata/test1/sysaux01.dbf 3
/u01/app/oracle/oradata/test1/undotbs01.dbf 4
/u01/app/oracle/oradata/test1/users01.dbf 10
/u01/app/oracle/oradata/test1/index01.dbf 11
/dsk1/oradata/test1/users02.dbf6
rows selected. |
T1表总共为16个块，数据占用了8个块
13:47:43 SYS@ test1 >select file#,dbablk,tch from x$bh where obj=
2 (select data_object_id from dba_objects
3 where owner='SCOTT' AND object_name='T1')
4* order by dbablk
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FILE# DBABLK TCH---------- ---------- ---------- 4
136 2 4
137 2 4
138 2 4
139 2 4
140 2 4
141 2 4
142 2 4
143 2 11
688 1 11
689 1 11
690 4 11
690 1 11
691 2 11
692 2 11
693 2 11
694 2 11
695 217
rows selected. |
13:48:59 SYS@ test1 >select file#,block#,status from v$bh where objd=
13:49:18 2 (select data_object_id from dba_objects
13:49:33 3 where owner='SCOTT' and object_name='T1')
13:49:51 4 order by block#;
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FILE# BLOCK# STATUS---------- ---------- ---------- 4
136 xcur 4
137 xcur 4
138 xcur 4
139 xcur 4
140 xcur 4
141 xcur 4
142 xcur 4
143 xcur 11
688 xcur 11
689 xcur 11
690 xcur 11
690 cr 11
691 xcur 11
692 xcur 11
693 xcur 11
694 xcur 11
695 xcur17
rows selected. |
查看x$bh,返回了block的touch count信息，表示了块的热点程度，现在最热的块是690，此块应该是段头块。
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13:56:05
SYS@ test1 >select header_file,header_block from
dba_segments13:56:25
2 where
owner='SCOTT' and
segment_name='T1';HEADER_FILE HEADER_BLOCK----------- ------------ 11
690 |
手工清空buffer_cache：
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14:03:30
SYS@ test1 >alter system flush buffer_cache;System altered.14:06:02
SYS@ test1 >select file#,dbablk,tch from
x$bh where
obj=(select data_object_id from
dba_objects where
owner='SCOTT' AND object_name='T1') order by dbablk;no rows selected14:06:16
SYS@ test1 >select file#,block#,status from
v$bh where
objd=(select data_object_id from
dba_objects where
owner='SCOTT' and
object_name='T1') order by block#;no rows selected |
统计一个object的非free状态的v$bh的记录数，基本就反映了一个对象再data buffer中被cache的块数。
14:10:29 SYS@ test1 >select count(*) from v$bh where objd=
14:10:45 2 (select data_object_id from dba_objects
14:10:59 3 where owner='SCOTT' and object_name='T1')
14:11:23 4 and status !='free';
COUNT(*)
----------
0
对t1表进行事务操作：
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14:12:05
SCOTT@ test1 >insert
into t1 values (5001);1
row created.Elapsed: 00:00:00.0014:13:38
SCOTT@ test1 >commit;Commit complete.14:13:12
SYS@ test1 >select count(*) from
v$bh where
objd= 2
(select data_object_id from
dba_objects 3
where owner='SCOTT'
and object_name='T1') 4*
and status !='free' COUNT(*)----------
4 |
14:13:43 SYS@ test1 >select file#,block#,status from v$bh where objd=(select data_object_id from dba_objects where owner='SCOTT' and object_name='T1') order by block#;
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FILE# BLOCK# STATUS---------- ---------- ---------- 4
143 xcur 11
688 xcur 11
689 xcur 11
690 xcur |
（2）热点块问题
x$bh的touch count，这个数字将作为LRU算法的一个重要参考信息，如果一个块被多次访问，每次访问都会导致该块的记录加一。
1）查看t1的object_id
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14:26:37
SYS@ test1 >select data_object_id from dba_objects14:26:56
2 where owner='SCOTT'
and object_name='T1';DATA_OBJECT_ID-------------- 16399 |
2）查看block 143的tch
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14:29:05
SYS@ test1 >select tch from x$bh14:29:20
2 where obj=16399
and dbablk=143
and file#=4 and tch>0; TCH---------- 1 |
3）访问block 143
采用了dbms_rowid.rowid_create函数来创建rowid，表示新创建的rowid类型为扩展rowid，类型为1；data_object_id为16399；数据文件id为4；块的id为143；行数为一行（0）。
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14:30:47
SYS@ test1 >select count(*) from scott.t114:31:02
2 where rowid=dbms_rowid.rowid_create(1,16399,4,143,0); COUNT(*)---------- 1 |
多次访问后，block 143的tch在增加
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14:32:19
SYS@ test1 >select tch from
x$bh where
obj=16399 and
dbablk=143
and file#=4
and tch>0; TCH---------- 2 14:32:35
SYS@ test1 >select count(*) from
scott.t1 where
rowid=dbms_rowid.rowid_create(1,16399,4,143,0); COUNT(*)---------- 114:33:23
SYS@ test1 >select tch from
x$bh where
obj=16399 and
dbablk=143
and file#=4
and tch>0; TCH----------
3 |
block 143的初始tch=1，经过连续两次查询后，tch=3.根据这一变化，可以基本判断x$bh表中的tch大的块，一般都是热点块。
查询top 10热点块所在的对象：
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14:43:43
SYS@ test1 >select /*+ rule */
owner,object_name from dba_objects 2
where data_object_id in 3
(select obj from 4
(select obj from x$bh order by tch desc ) 5* where rownum <11)OWNER OBJECT_NAME------------------------------ ------------------------------SYS I_OBJ#SYS I_OBJ2SYS I_ICOL1SYS I_CDEF2SYS I_OBJ#_INTCOL#SYS I_HH_OBJ#_INTCOL#SYS I_TAB_STATS$_OBJ#SYS I_IND_STATS$_OBJ#8
rows selected. |
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