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Direct NFS Tips

Oracle 11g New Features Tips by Donald BurlesonJune 29, 2015

Oracle 11g New Features Tips

Prior to the 11g release of Oracle, the benefits of NFS and the database have been subject to certain drawbacks such as performance degradation and complex configuration requirements of the NFS client. These drawbacks have been minimized in Oracle 11g by the integration of a direct NFS client in the Oracle kernel.  This client allows Oracle to optimize the I/O path between Oracle and the NFS server directly.  Utilizing Oracle's direct NFS client is also automated and simplified in configuration to achieve optimal performance. 

Benefits of Direct NFS

The direct NFS client has two fundamental optimizations that increase performance.  First, the direct NFS client is capable of performing concurrent direct I/O. This removes the need to utilize the operating system's cache and bypassing limitations which are imposed by the operating system configuration.  Using the operating system to configure the NFS client, and data written from the database to the NFS, requires that the data be read from Oracle's memory to the operating system cache.  This is done before it is copied from the system cache to the NFS. By using the direct NFS client, the Oracle data no longer needs to be cached in the operating system.  The process reduces the overall memory consumption and kernel CPU when writing to the network drive. 

In addition to eliminating the dependency on the operating system cache, the second fundamental optimization is that the direct NFS client now supports asynchronous I/O on NFS file systems.  Asynchronous I/O (AIO), also known as non-blocking I/O, allows I/O processing to continue independently from other I/O that has not yet completed.  This setting makes a tremendous difference for read/write performance, whether using an NFS or a local drive. 

Many DBAs have experience enabling AIO on their local file systems and recall the performance improvement when switching from synchronous I/O to asynchronous I/O.  As of Oracle 11.1, the direct NFS client supports up to four parallel network paths.  The direct NFS client automatically performs load balancing across all specified paths.

 Another advantage of the direct NFS client is that Oracle manages the configuration across different platforms.  This greatly simplifies the task of administering NFS storage by allowing Oracle to internally manage the NFS configurations for all platforms.  This eliminates the need to manage different configurations on each platform, or use storage-specific drivers.  This feature also uses simple Ethernet for storage connectivity, eliminating the need for additional architecture components such as redundant host bus adaptors, Fiber channel switches, or bonded network interfaces. 

The direct NFS client supports both single instance databases as well as real application cluster (RAC) environments.  The client recognizes when an instance is part of a RAC environment and automatically optimizes the NFS configuration accordingly. 

The direct NFS client looks for mount settings in the following order:

  • $ORACLE_HOME/dbs/oranfstab for settings on a single database

  • /etc/oranfstab for NFS mounts available to all Oracle databases on the host

  • Lastly, the mount tab file (i.e. /etc/mtab on Linux) to determine available NFS mounts

The oranfstab file should be created to include the server, path, export (path), and mount.  For example, one possible contents of this file could be:

server: MyServer
export: /vol/oradata mount: C:\oracle\11g\oradata\ora11gr1
Server - NFS server name
Path - Up to four network paths to the NFS server
Export - Exported path from the NFS server
Mount - Local mount point for NFS server

Prior to enabling the direct NFS client, the NFS file systems should already be mounted and available over regular NFS mounts.  Once the NFS file systems are mounted, it is necessary to replace the standard Oracle Disk Manager (ODM) library with one that supports the direct NFS client.  This can be accomplished by issuing the following commands on the server:

> cd $ORACLE_HOME/lib
> cp libodm11.so_stub
> ln 's

These commands create a copy of the ODM library. They also create a symbolic link between the ODM library that supports the direct NFS client and old ODM library. 

Monitor Direct NFS

Oracle 11g has introduced several new dynamic views to provide visibility on the configuration and performance of direct NFS in the database. 

  • v$dnfs_servers - View servers accessed using Direct NFS

  • v$dnfs_files - View files currently open using Direct NFS

  • v$dnfs_channels - View open network paths or channels to servers for which Direct NFS is providing files

  • v$dnfs_stats - View performance statistics for Direct NFS

Direct NFS in OLTP and DSS

Case studies from Oracle demonstrate that the direct NFS client provides a significant improvement over the kernel NFS for throughput and CPU utilization in both DSS and OLTP environments.  In DSS environments, the improvement increases dramatically as additional network interfaces are added.  Oracle's case study shows an approximately 40% improvement in DSS environments and an 11% improvement in OLTP environments.  These metrics are only one element of the full benefits of using the direct NFS client rather than the kernel NFS.  Other factors that should be considered when evaluating this feature include the simplicity of setup, maintenance, and the reduced infrastructure requirements.

RAC Using NFS with Direct NFS (DNFS)

Oracle 11g comes with enhanced support for Oracle over NFS using the new Direct NFS feature.  Direct NFS allows for costs savings by sticking with one connection model: the network.  This allows for multipathing and unified storage.  In addition, Direct NFS even works in Windows, even though Windows has no NFS support. Another nice feature of NFS is that files are directly accessible via standard OS commands like ls, mv, cp and such.  

Lastly, Oracle's Direct NFS feature allows for Direct I/O and Asynchronous I/O by default. To use Direct NFS, the client must be installed on RAC nodes.  This can even be performed on nodes where Oracle RAC is already running.  

The Oracle 11g Direct NFS Client is shipped with Oracle 11g.  Once installed, it is very simple to put it in place:

  1. Stop any RAC databases
  2.  Navigate to $ORACLE_HOME/lib
  3. Move the file to a backup
  4. Create a symbolic link called which uses as its
    source:  ln -s
  5. Start the RAC databases

Direct NFS information can then be queried from the following fixed views:

  •  v$dnfs_channels
  • v$dnfs_files
  • v$dnfs_servers
  • v$dnfs_stats

Note: Oracle 11g Direct NFS only works with NFS V3 compatible NAS devices.

 NFS must be set up at the OS level before using the Direct NFS client.  Each OS has different requirements for configuring and using NFS.


This is an excerpt from the new book Oracle 11g New Features: Expert Guide to the Important New Features by John Garmany, Steve Karam, Lutz Hartmann, V. J. Jain, Brian Carr.

You can buy it direct from the publisher for 30% off.


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