一般说来,异步I/O是和同步I/O相比较来说的,如果是同步I/O,当一个I/O操作执行时,应用程序必须等待,直到此I/O执行完. 相反,异步I/O操作在后台运行,I/O操作和应用程序可以同时运行,提高了系统性能; 使用异步I/O会提高I/O流量,如果应用是对裸设备进行操作,这种优势更加明显, 因此象数据库,文件服务器等应用往往会利用异步I/O,使得多个I/O操作同时执行.

Oracle在默认情况是不使用异步IO的。可查看参数filesystemio_options（默认值为none），有四种选项

■ 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.

官方解释如下:

Asynchronous I/O Support

Note:

On Linux, Automatic Storage Management uses asynchronous I/O by default. Asynchronous I/O is not supported for database files stored on NFS file systems.

Oracle Database supports kernel asynchronous I/O. This feature is disabled by default.

By default, the DISK_ASYNCH_IO initialization parameter in the parameter file is set to TRUE to enable asynchronous I/O on raw devices. To enable asynchronous I/O on file system files:

Ensure that all Oracle Database files are located on file systems that support asynchronous I/O.
Set the FILESYSTEMIO_OPTIONS initialization parameter in the parameter file to one of the following values:
Linux Distribution Recommended Value
SUSE Linux Enterprise Server 9 SETALL
Other distributions ASYNCH

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ORACLE 异步IO

Asynchronous I/O Support

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Linux Distribution	Recommended Value
SUSE Linux Enterprise Server 9	SETALL
Other distributions	ASYNCH