跟着吕大师（VAGE）揭密隐含参数：_db_writer_coalesce_area_size(三)
ORACLE instance started.
Total System Global Area 388354048 bytes
Fixed Size 2228584 bytes
Variable Size 276827800 bytes
Database Buffers 104857600 bytes
Redo Buffers 4440064 bytes
Database mounted.
Database opened.
sys@OCM> select event,TOTAL_WAITS , TIME_WAITED_MICRO from V$SYSTEM_EVENT where EVENT='db file async I/O submit';
no rows selected
gyj@OCM> update gyj100 set id=id+0 where rownum<=5000000;
5000000 rows updated.
Elapsed: 00:00:16.34
sys@OCM> select event,TOTAL_WAITS , TIME_WAITED_MICRO from V$SYSTEM_EVENT where EVENT='db file async I/O submit';
EVENT TOTAL_WAITS TIME_WAITED_MICRO
---------------------------------------------------------------- ----------- -----------------
db file async I/O submit 31 15256544
sys@OCM> select 31/16||'次/秒' from dual;
31/16
----------
1.9375次/秒
再测一把，把参数改回去：_db_writer_coalesce_area_size=1048576
sys@OCM> alter system set "_db_writer_coalesce_area_size"=1048576 scope=spfile;
System altered.
sys@OCM> startup force;
ORACLE instance started.
Total System Global Area 388354048 bytes
Fixed Size 2228584 bytes
Variable Size 276827800 bytes
Database Buffers 104857600 bytes
Redo Buffers 4440064 bytes
Database mounted.
Database opened.
sys@OCM> select event,TOTAL_WAITS , TIME_WAITED_MICRO from V$SYSTEM_EVENT where EVENT='db file async I/O submit';
no rows selected
gyj@OCM> update gyj100 set id=id+0 where rownum<=5000000;
5000000 rows updated.
Elapsed: 00:00:48.49
sys@OCM> sys@OCM> select event,TOTAL_WAITS , TIME_WAITED_MICRO from V$SYSTEM_EVENT where EVENT='db file async I/O submit';
EVENT TOTAL_WAITS TIME_WAITED_MICRO
---------------------------------------------------------------- ----------- -----------------
db file async I/O submit 282 40353435
sys@OCM> select 282/48||'次/秒' from dual;
282/48
----------
5.875次/秒
果然把参数db_writer_coalesce_area_size改大起作用了：
参数：_db_writer_coalesce_area_size=1048576， 产生db file async I/O submit等待6次/秒
参数：_db_writer_coalesce_area_size=8048576，产生db file async I/O submit等待2次/秒