Oracle数据库异步IO导致查询响应缓慢

最终该问题通过检查等待事件的方式得以解决，下面简单描述一下处理过程：

会话1：

在服务器本地使用sqlplus登陆数据库实例，

1).执行下面的SQL语句确定本会话的SID：

SQL> SELECT DISTINCT SID FROM V$MYSTAT;

2).执行查询2条数据的临时表（固定耗时12秒）。

会话2：

在服务器本地使用sqlplus登陆数据库实例，

执行下面的SQL语句，查询会话1在查询2条数据的临时表时发生的等待事件：

SQL> set linesize 200
SQL> set pagesize 200
SQL> col program format a30
SQL> col machine format a30
SQL> col wait_class format a30
SQL> select username,program,machine,event,wait_class from v$session where wait_class <>'Idle' and sid=572

USERNAME PROGRAM MACHINE EVENT WAIT_CLASS
------------------------------ ------------------------------ ------------------------------ ---------------------------------------------------------------- ------------------------------
SYS sqlplus@rx9900a (TNS V1-V3) rx9900a asynch descriptor resize Other

该等待事件的WAIT_CLASS为Other，比较异常，从EVENT可以大概了解该等待和异步IO有一定的关系，通过在MOS上检索该EVENT找到如下一篇文章：

Bug 9829397 Excessive CPU and many "asynch descriptor resize" waits for SQL using Async IO
This note gives a brief overview of bug 9829397.
The content was last updated on: 28-JUN-2013
Click here for details of each of the sections below.

Affects:

*Product (Component)*	Oracle Server (Rdbms)
*Range of versions believed* to be affected**	Versions >= 11.2 but BELOW 12.1
*Versions confirmed* as being affected**	11.2.0.2 11.2.0.1
Platforms affected	Generic (all / most platforms affected)

It is believed to be a regression in default behaviour thus:
Regression introduced in 11.2.0.2
Fixed:

issue is fixed in

Symptoms:	Related To:
Excessive CPU Usage Performance Of Query/ies Affected Waits for "asynch descriptor resize"	DISK_ASYNCH_IO

Description

Some queries in 11.2 may exhibit higher CPU usage than earlier
releases with many "asynch descriptor resize" waits occurring
compared to the same SQL in earlier releases. 
 
Rediscovery Notes:
 Async IO is in use.
 The total time waiting for "asynch descriptor resize" is
 typically very small but with very high counts. The high
 wait count indicates many resizes of the number of AIO 
 descriptors unnecessarily wasting CPU.

Workaround Disable async IO. 
 eg: Set DISK_ASYNCH_IO = false    <<<< 禁用异步IO
 
References:
 For more information about "asynch descriptor resize" see the following: Note:1273748.1 High Numbers of 'asynch descriptor resize' waits Note:1081977.1 Details of the "asynch descriptor resize" wait event.

Getting a Fix Use one of the "Fixed" versions listed above (for Patch Sets / bundles use the latest version available as
  contents are cumulative - the "Fixed" version listed above is
  the first version where the fix is included) or Click here for suggestions on how to get a fix for this issue

Please note: The above is a summary description only. Actual symptoms can vary. Matching to any symptoms here does not confirm that you are encountering this problem. For questions about this bug please consult Oracle Support.

References

Bug:9829397 (This link will only work for PUBLISHED bugs)
Note:245840.1 Information on the sections in this article

从文章可以看出该问题可能是个bug，通过在数据库实例关闭异步IO即可解决问题，根据文章执行如下的操作：

SQL> show parameter io

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
......
disk_asynch_io boolean TRUE
......

SQL> alter system set disk_asynch_io=false scope=spfile sid='scrk1';

System altered.

重启RAC第一个节点数据库实例之后问题即得到解决。