Oracle在Linux下使用异步IO（aio）配置
1、首先用root用户安装以下必要的rpm包
# rpm -Uvh libaio-0.3.106-3.2.x86_64.rpm
# rpm -Uvh libaio-devel-0.3.106-3.2.x86_64.rpm
2、在系统级支持异步I/O
与[Note 225751.1]介绍的在RHEL3里面设置异步IO不同，不需要设置aio-max-size，而且'/proc/sys/fs'路径下也没有这个文件。因为从2.6 kernel开始，已经取消了对IO size的限制[Note 549075.1]。另外根据[Note 471846.1]，Oracle建议将aio-max-nr的值设置为1048576或更高。
#echo > /proc/sys/fs/aio-max-nr 1048576
3、在数据库级启用异步I/O
首先修改数据库参数。与[Note 225751.1]在RHEL 3里面设置异步IO不同，Oracle10gR2默认是打开了对异步IO的支持的，不需要重新编译数据库软件。在'$ORACLE_HOME/rdbms/lib'路径下，也没有'skgaioi.o'这个文件。在某些情况下，Oracle无法将IO行为或事件报告给操作系统[Note 365416.1]，因此需要做以下操作。
这里开始换成oracle用户
SQL>alter system set disk_asynch_io=TRUE scope=spfile;
SQL>alter system setfilesystemio_options=asynchscope=spfile;
SQL>shutdown immediate
$ cd $ORACLE_HOME/rdbms/lib
$ ln -s /usr/lib/libaio.so.1 skgaio.o
$ make PL_ORALIBS=-laio -f ins_rdbms.mk async_on
SQL>startup
在Oracle10gR2中AIO默认已经是开启的了。可以通过ldd或者nm来检查oracle是否已经启用了AIO支持，有输出代表已经启用。
[oraprod@db01 ~]$/usr/bin/ldd $ORACLE_HOME/bin/oracle | grep libaio
libaio.so.1 => /usr/lib64/libaio.so.1 (0x00002aaaac4a9000)
[oraprod@db01 ~]$/usr/bin/nm $ORACLE_HOME/bin/oracle | grep io_getevent
w io_getevents@@LIBAIO_0.4
4、检查异步I/O是否在使用
根据[Note 370579.1]，可以通过查看slabinfo统计信息查看操作系统中AIO是否运行，slab是Linux的内存分配器，AIO相关的内存结构已经分配，kiocb值的第二列和第三列非0即是已使用。与kernel 2.4.x不同，没有显示kiobuf，因为从kernel 2.5.43开始，kiobuf已经从内核中被移除。
$cat /proc/slabinfo | grep kio
kioctx 64 110 384 10 1 : tunables 54 27 8 : slabdata 11 11 0
kiocb 13 315 256 15 1 : tunables 120 60 8 : slabdata 21 21 44
我的实验过程：
[Oracle@localhost ~]$ sqlplus /nolog
SQL*Plus: Release 10.2.0.1.0 - Production on Fri Dec 17 10:37:10 2010
Copyright (c) 1982, 2005, Oracle.All rights reserved.
[oracle@localhost ~]$ /usr/bin/nm $ORACLE_HOME/bin/oracle | grep io_getevent
w io_getevents@@LIBAIO_0.4
[oracle@localhost ~]$
[oracle@localhost ~]$
[oracle@localhost ~]$ /usr/bin/ldd $ORACLE_HOME/bin/oracle | grep libaio
libaio.so.1 => /usr/lib/libaio.so.1 (0x00507000)
[oracle@localhost ~]$ cat /proc/slabinfo | grep kio
kioctx1415256151 : tunables120600 : slabdata110
kiocb00128311 : tunables120600 : slabdata000
## 没有启用：
SQL> show parameter disk_asynch_io
NAMETYPE
------------------------------------ ----------------------
VALUE
------------------------------
disk_asynch_ioboolean
TRUE
SQL> sho parameter filesystemio_options
NAMETYPE
------------------------------------ ----------------------
VALUE
------------------------------
filesystemio_optionsstring
none
SQL>
SQL>
SQL> alter system set filesystemio_options = setallscope=both;
alter system set filesystemio_options = setallscope=both
*
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified
SQL>
SQL>alter system set filesystemio_options = setallscope=spfile;
System altered.
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area285212672 bytes
Fixed Size1218992 bytes
Variable Size88082000 bytes
Database Buffers192937984 bytes
Redo Buffers2973696 bytes
Database mounted.
Database opened.
[root@localhost ~]# cat /proc/slabinfo | grep kio
kioctx1315256151 : tunables120600 : slabdata110
kiocb731128311 : tunables120600 : slabdata110
http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/ch23_os.htm#sthref749
9.1.1.1Asynchronous I/O
With synchronous I/O, when an I/O request is submitted to the operating system, the writing process blocks until the write is confirmed as complete. It can then continue processing. With asynchronous I/O, processing continues while the I/O request is submitted and processed. Use asynchronous I/O when possible to avoid bottlenecks.
Some platforms support asynchronous I/O by default, others need special configuration, and some only support asynchronous I/O for certain underlying file system types.
9.1.1.2FILESYSTEMIO_OPTIONS Initialization Parameter
You can use theFILESYSTEMIO_OPTIONSinitialization parameter to enable or disable asynchronous I/O or direct I/O on file system files. This parameter is platform-specific and has a default value that is best for a particular platform. It can be dynamically changed to update the default setting.
FILESYTEMIO_OPTIONScan be set to one of the following values:
ASYNCH:enable asynchronous I/O on file system files, which has no timing requirement for transmission.
DIRECTIO:enable direct I/O on file system files, which bypasses the buffer cache.
SETALL:enable both asynchronous and direct I/O on file system files.
NONE:disable both asynchronous and direct I/O on file system files.
转自http://www.linuxidc.com/Linux/2012-08/67929p2.htm
