link:
http://www.eygle.com/faq/db_file_multiblock_read_count&OracleIO.htm
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OS最大IO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，最大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所支持的最大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最大所能读取得数据量：
$ 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
$
我们可以看到，在以上测试平台中，Oracle最多每次IO能够读取128个Block，由于block_size为8k,也就是每次最多读取了1M数据.
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最多可以读取的Block数量.
