db file parallel write,write complete waits

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# Configuration

write complete waits file# block# Configuration

db 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。