跟着吕大师（VAGE）揭密隐含参数：_db_writer_coalesce_area_size

最近在看吕大师的大作《Oracle核心揭密》，马上要上市了，期待中。。。
这部大作可以与Jonathan Lewis大师的《Oracle Core_ Essential Internals for DBA》相提并论，看了几天收益颇多，哈哈美国有Lewis，中国有VAGE。
gyj@OCM> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production

gyj@OCM> show parameter mem

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
hi_shared_memory_address integer 0
memory_max_target big integer 0
memory_target big integer 0
shared_memory_address integer 0
gyj@OCM> show parameter sga

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 372M
sga_target big integer 0
gyj@OCM> show parameter db_cache

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_cache_advice string ON
db_cache_size big integer 100M

gyj@OCM> show parameter mttr

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
fast_start_mttr_target integer 0

_db_writer_coalesce_area_size Size of memory allocated to dbwriter for coalescin 1048576

gyj@OCM>create table gyj100 (id int,name varchar2(2000));

gyj@OCM> declare
2 c number :=0 ;
3 begin
4 for i in 1 .. 5000000 loop
5 insert into gyj100 values(i,'gyj'||i);
6 c := c+1;
7 if mod(c,5000)=0 then
8 commit;
9 end if;
10 end loop;
11 end;
12 /

PL/SQL procedure successfully completed.

gyj@OCM> select bytes/1024/1024 sz from user_segments where segment_name='GYJ100';

SZ
----------
120

gyj@OCM> select sid from v$mystat where rownum=1;

SID
----------
125

gyj@OCM> update gyj100 set id=id+0 where rownum<=5000000;

5000000 rows updated.

Elapsed: 00:00:47.50



再开一个窗口观察等待事件：
gyj@OCM> select * from V$SESSION_wait where sid=125;

SID SEQ# EVENT P1TEXT P1 P1RAW P2TEXT P2 P2RAW P3TEXT P3 P3RAW WAIT_CLASS_ID WAIT_CLASS# WAIT_CLASS WAIT_TIME SECONDS_IN_WAIT STATE WAIT_TIME_MICRO TIME_REMAINING_MICRO TIME_SINCE_LAST_WAIT_MICRO
---------- ---------- ---------------------------------------------------------------- ---------------------------------------------------------------- ---------- ---------------- ---------------------------------------------------------------- ---------- ---------------- ---------------------------------------------------------------- ---------- ---------------- ------------- ----------- ---------------------------------------------------------------- ---------- --------------- ------------------- --------------- -------------------- --------------------------
125 45323 log buffer space 0 00 0 00 0 00 3290255840 2 Configuration -1 0 WAITED SHORT TIME 4608 7446

gyj@OCM> select event,TOTAL_WAITS , TIME_WAITED_MICRO from V$SYSTEM_EVENT where EVENT='db file parallel write';

no rows selected

gyj@OCM> select event,TOTAL_WAITS , TIME_WAITED_MICRO from V$SYSTEM_EVENT where EVENT like 'db file%';

EVENT TOTAL_WAITS TIME_WAITED_MICRO
---------------------------------------------------------------- ----------- -----------------
db file sequential read 91162 37712572
db file scattered read 1830 8543678
db file single write 372 1934286
db file async I/O submit 553 84199251
db file parallel read 67 1863758

始终没看到db file parallel write等待。。。。

在吕大师的指点下：







select event,TOTAL_WAITS , TIME_WAITED_MICRO from V$SYSTEM_EVENT where EVENT='db file async I/O submit';

MOS上有篇文章
'db file async I/O submit' when FILESYSTEMIO_OPTIONS=NONE [ID 1274737.1]
'db file async I/O submit' should be treated as 'db file parallel write' in previous releases.

**********************************************************************
在吕大的指点下，开始测试：
**********************************************************************
第一把：
参数：_db_writer_coalesce_area_size=1048576

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 1944 88188121
gyj@OCM> set timing on;
gyj@OCM> update gyj100 set id=id+0 where rownum<=5000000;

5000000 rows updated.

Elapsed: 00:00:46.10
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 2222 128007271

sys@OCM> select (2222-1944)/46||'次/秒' from dual;

(2222-1944)/46
--------------
6.04347826次/秒

第二把测试，修改参数：
sys@OCM> alter system set "_db_writer_coalesce_area_size"=8048576 scope=spfile;

System altered.

sys@OCM> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
sys@OCM> startup
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