Direct NFS Client
NAS storage devices use the Network File System (NFS) to access data. In Oracle Database 10g, NAS devices were accessed using operating system–specific kernel NFS drivers. This required that you tune many parameters, and the configuration varied across the platforms. The NFS clients tended to be inconsistent across the different operating system platforms. Manageability wasn’t easy because you had to contend with over 20 configuration parameters.
In Oracle Database 11g, the Oracle NFS implements the NFS Version 3 protocol in the Oracle RDBMS kernel. Implementing the Oracle Direct NFS offers the following benefits:
Avoids the bottlenecks and resource constraints by avoiding the kernel NFS layer.
Provides a common NFS interface for Oracle for use on all operating system platforms and supported NFS servers.
Provides load balancing across multiple connections to the NFS servers, thus improving performance.
Performance is predictable because the Oracle NFS implementation enables you to completely control the input/output path to the Network File Servers.
Easier management including simpler configuration and superior diagnosability.
Configuring Direct NFS
You don’t have to configure much to implement Oracle Direct NFS Client. Direct NFS searches for the mount point entries in the following order:
$ORCLE_HOME/dbs/oranfstab
/etc/orafnstab
/etc/mtab
The database uses the first match as the mount point. Oracle always requires that even when you use Direct NFS, the kernel NFS system must perform the mounting. For this reason, Oracle will always crosscheck the information about mount points in the oranfstab file with the operating system NFS mount points. If there’s a mismatch, Direct NFS can’t act as a client to the NFS server and stops serving the NFS server.
Enabling Direct NFS
You must follow these steps to enable Direct NFS:
1.You must mount all NFS mount points with your kernel NFS client. You must make sure you mount any file systems you plan on using through ODM NFS and make the file systems available to Oracle over regular NFS mounts.
2.If you want to specify Oracle-specific options to Direct NFS, you’ll need an oranfstab file. This is an optional step. The oranfstab file must have the following attributes so the database can access all NFS servers through Direct NFS:
server: Provides the NFS server name.
mount: Provides the local mount point for the NFS server.
export: Provides the exported path from the NFS server.
path: Provides the network path to the NFS server. You can specify up to four network paths with an IP address or by name. Using multiple network paths enables the Direct NFS client to use an alternate path if the current path fails. Multiple paths also enable the client to perform load balancing.
Mnt_timeout: Specifies (in seconds) the time Direct NFS Client should wait for a successful mount before timing out. This parameter is optional. The default timeout is 10 minutes (600).
Dontroute: Specifies that outgoing messages should not be routed by the operating system, but instead sent using the IP address to which they are bound. Note that this POSIX option sometimes does not work on Linux systems with multiple paths in the same subnet.
A typical oranfstab file looks similar to the following:
server: TestServer1
path: 130.33.34.11
export: /vol/oradata1 
mount: /mnt/oradata1
In order to remove an NFS path that the database is using currently, you must restart the database.
3. You must replace the standard ODM library, libnfsodm10.so, with the ODM NFS library, as shown here:
$ cd $ORACLE_HOME/lib
$ cp libodmll.so libodmll.so_stub
$ ln -s libnfsodm11.so libodm11.so
You can disable the Direct NFS client by using any of the following three methods.
Delete the oranfstab file.
Replace the ODM NFS library with the stub libodm11.so file.
Modify the oranfstab file by deleting the specific NFS server or the network paths to the NFS server.
If the database can’t open the NFS server using Direct NFS, it will use the operating system kernel client instead.
Monitoring Direct NFS
You can query the following views in order to manage Direct NFS:
V$DNFS_STATS Shows performance statistics for Direct NFS.
V$DNFS_SERVERS Shows servers accessed by Direct NFS.
V$DNFS_FILES Shows files currently using Direct NFS.
V$DNFS_CHANNELS Shows the open network paths being used by Direct NFS.

参考至: 《McGraw.Hill.OCP.Oracle.Database.11g.New.Features.for.Administrators.Exam.Guide.Apr.2008》           
http://docs.oracle.com/cd/E11882_01/install.112/e41961/storage.htm#CWLIN279

http://czmmiao.iteye.com/blog/1965839