db file parallel write和db file parallel read

<pre name="code" class="sql">db file parallel write 
This event occurs in the DBWR. It indicates that the DBWR is performing a parallel write to files and blocks. The parameter requests indicates the real number of I/Os that are 

being performed. When the last I/O has gone to disk, the wait ends. 

Wait Time: Wait until all of the I/Os are completed 

Parameters: 

files
This indicates the number of files to which the session is writing

blocks
This indicates the total number of blocks to be written

requests
This indicates the total number of I/O requests, which will be the same as blocks

个等待是磁盘IO引起的问题，跟其他都没关系，这是后台进程DBWR的等待。从BUFFER CACHE写数据到硬盘里，写的速度太慢，所以等


待


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

db file parallel read

This happens during recovery. It can also happen during buffer prefetching, as an optimization (rather than performing multiple single-block reads). Database blocks that need to be changed as part of recovery are read in parallel from the database.