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dbfilemultiblockreadcountandOracleIO大小

发布时间:2019-11-05 06:29:49

初始化参数db_file_multiblock_read_count 影响Oracle在执行全表扫描时一次读取的block的数量.

db_file_multiblock_read_count的设置要受OSIO能力影响,也就是说,如果你系统的硬件IO能力有限,即使设置再大的db_file_multiblock_read_count也是没有用的。

理论上,db_file_multiblock_read_count和系统IO能力应该有如下关系:

Max(db_file_multiblock_read_count) = MaxOsIOsize/db_block_size

当然这个Max(db_file_multiblock_read_count)还要受Oracle的限制,

目前Oracle所支持的db_file_multiblock_read_count 值为128.

我们可以通过db_file_multiblock_read_count来测试Oracle在不同系统下,单次IO所能读取得数据量:

我们可以看到,在以上测试平台中,Oracle多每次IO能够读取128个Block,由于block_size为8k,也就是每次多读取了1M数据.

$ sqlplus "/ as sysdba"

SQL*Plus: Release 10.1.0.2.0 - Production on Wed Aug 11 23:43:52 2004

Copyright (c) 1982, 2004, Oracle. All rights reserved.

Connected to:

Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - 64bit Production

With the Partitioning, OLAP and Data Mining options

SYS AS SYSDBA on 11-AUG-04 >show parameter read_count

NAME TYPE VALUE

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

db_file_multiblock_read_count integer 16

SYS AS SYSDBA on 11-AUG-04 >create tablespace dfmbrc

2 datafile '/opt/oracle/oradata/eygle/dfmbrc.dbf'

3 size 20M extent management local uniform size 2M;

Tablespace created.

SYS AS SYSDBA on 11-AUG-04 >create table t tablespace dfmbrc as select * from dba_objects;

Table created.

SYS AS SYSDBA on 11-AUG-04 >insert into t select * from t;

9149 rows created.

SYS AS SYSDBA on 11-AUG-04 >/

18298 rows created.

SYS AS SYSDBA on 11-AUG-04 >/

36596 rows created.

SYS AS SYSDBA on 11-AUG-04 >commit;

Commit complete.

SYS AS SYSDBA on 11-AUG-04 >alter session set db_file_multiblock_read_count=1000;

Session altered.

SYS AS SYSDBA on 12-AUG-04 >show parameter read_count

NAME TYPE VALUE

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

db_file_multiblock_read_count integer 128

SYS AS SYSDBA on 11-AUG-04 >alter session set events '10046 trace name context forever,level 12';

Session altered.

SYS AS SYSDBA on 11-AUG-04 >alter system flush buffer_cache;

System altered.

SYS AS SYSDBA on 11-AUG-04 >select count(*) from t;

COUNT(*)

----------

73192

SYS AS SYSDBA on 12-AUG-04 >@gettrace

TRACE_FILE_NAME

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

/opt/oracle/soft/eygle_ora_24432.trc

$ cat /opt/oracle/soft/eygle_ora_24432.trc|grep sca

WAIT #26: nam='db file scattered read' ela= 18267 p1=10 p2=10 p3=128

WAIT #26: nam='db file scattered read' ela= 8836 p1=10 p2=138 p3=127

WAIT #26: nam='db file scattered read' ela= 8923 p1=10 p2=265 p3=128

WAIT #26: nam='db file scattered read' ela= 8853 p1=10 p2=393 p3=128

WAIT #26: nam='db file scattered read' ela= 8985 p1=10 p2=521 p3=128

WAIT #26: nam='db file scattered read' ela= 8997 p1=10 p2=649 p3=128

WAIT #26: nam='db file scattered read' ela= 9096 p1=10 p2=777 p3=128

WAIT #26: nam='db file scattered read' ela= 583 p1=10 p2=905 p3=12

$

系统平台为:

$ uname -a

SunOS billing 5.8 Generic_108528-23 sun4u sparc SUNW,Ultra-4

当然具体的,Oracle一次IO能读取多少block还和很多因素有关,比如存储是否连续,磁盘是否经过条带等方式划分,并且Oracle的

单次IO读取不能跨越Extent边界等.某些平台还和操作系统的参数设置有关.

大家可以测试一下不同的平台,Oracle的单次IO多可以读取的Block数量.

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