详解dbfileparallelwrite,writecompletewaits

SQL SELECT * FROM v$version; 
BANNER
 ---------------------------------------------------------------- 
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Prod PL/SQL Release 10.2.0.3.0 
- Production SQL select name,parameter1,paramet
 SQL> SELECT * FROM v$version;
 
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 
- ProdPL/SQL Release 10.2.0.3.0 - Production
 
SQL>SELECT name,parameter1,parameter2,parameter3,wait_class from v$event_name  
2   where name in ('free buffer waits','db file parallel write','write complete waits');
NAME                      
PARAMETER1 PARAMETER2 PARAMETER3 WAIT_CLASS
------------------------- ---------- ---------- ---------- ------------------------------
free buffer waits         
file#      
block#     
set-id#    
Configurationwrite complete waits      
file#      
block#                
Configurationdb file parallel write    
requests   interrupt  timeout    System I/O
 
在Oracle 10g中，db file parallel write 被归属为系统I/0等待类，如果你检查到了系统出现了大量的
db file parallel write事件，表明你的I/O有问题。这时你可以检查
db file parallel write ，write complete waits，free buffer waits的平均等待时间
SQL> select event,time_waited,average_wait from v$system_event where event in  
2  ('db file parallel write','write complete waits','free buffer waits');
EVENT                              
TIME_WAITED AVERAGE_WAIT
---------------------------------------------------------------- ----------- ------------
db file parallel write         284          .67 

我这里由于是测试机器，没有出现 write complete waits,free buffer waits 等待事件

AVERAGE_WAIT的值应该小于1(百分之一秒)，如果AVERAGE_WAIT大于10（十分之一秒）,表明非常缓慢的I/O，出现了db file parallel write 时常也会伴随write complete waits等待，如果是非常繁忙的OLTP系统，还可能出现free buffer waits，因为缓慢的I/0势必影响前台进程。db file parallel write只属于DBWR进程独有，不要被名字迷惑了，不要误解为该事件与并行DML有关。

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