Installation guide

Using NFS over TCP

100005 2 tcp 839 mountd

100005 3 udp 836 mountd

100005 3 tcp 839 mountd

If one of the NFS services does not start up correctly, rpcbind will be unable to map RPC requests

from clients for that service to the correct port. In many cases, if NFS is not present in rpcinfo

output, restarting NFS causes the service to correctly register with rpcbind and begin working. For

instructions on starting NFS, refer to Section 10.5, “Starting and Stopping NFS”.

For more information and a list of options on rpcinfo, refer to its man page.

10.9. Using NFS over TCP

The default transport protocol for NFS is TCP; however, the Red Hat Enterprise Linux kernel includes

support for NFS over UDP. To use NFS over UDP, include the mount option -o udp when mounting

the NFS-exported file system on the client system. Note that NFSv4 on UDP is not standards-

compliant, since UDP does not feature congestion control; as such, NFSv4 on UDP is not supported.

There are three ways to configure an NFS file system export:

• On demand via the command line (client side)

• Automatically via the /etc/fstab file (client side)

• Automatically via autofs configuration files, such as /etc/auto.master and /etc/auto.misc

(server side with NIS)

For example, on demand via the command line (client side):

mount -o udp shadowman.example.com:/misc/export /misc/local

When the NFS mount is specified in /etc/fstab (client side):

server:/usr/local/pub /pub nfs rsize=8192,wsize=8192,timeo=14,intr,udp

When the NFS mount is specified in an autofs configuration file for a NIS server, available for NIS

enabled workstations:

myproject -rw,soft,intr,rsize=8192,wsize=8192,udp penguin.example.net:/proj52

Since the default is TCP, if the -o udp option is not specified, the NFS-exported file system is

accessed via TCP.

The advantages of using TCP include the following:

• UDP only acknowledges packet completion, while TCP acknowledges every packet. This results in

a performance gain on heavily-loaded networks that use TCP when mounting shares.

• TCP has better congestion control than UDP. On a very congested network, UDP packets are

the first packets that are dropped. This means that if NFS is writing data (in 8K chunks) all of that

8K must be retransmitted over UDP. Because of TCP's reliability, only parts of that 8K data are

transmitted at a time.

• TCP also has better error detection. When a TCP connection breaks (due to the server being

unavailable) the client stops sending data and restarts the connection process once the server