The mounting of NFS volumes closely resembles regular file systems. Invoke mount using the following syntax:
mount -t nfs
nfs_volume local_dir options
nfs_volume is given as
Since this notation is unique to NFS filesystems, you can leave out
-t nfs option.
There are a number of additional options that you can specify to
mount upon mounting an NFS volume. These may be
given either following the
-o switch on the
command line or in the options field of the
/etc/fstab entry for the volume. In both cases,
multiple options are separated by commas and must not
contain any whitespace characters. Options specified on the command
line always override those given in the
Here is a sample entry from
# volume mount point type options news:/var/spool/news /var/spool/news nfs timeo=14,intr
This volume can then be mounted using this command:
In the absence of an
fstab entry, NFS
mount invocations look a lot uglier. For instance,
suppose you mount your users’ home directories from a machine named
moonshot, which uses a default
block size of 4 K for read/write operations. You might increase the
block size to 8 K to obtain better performance by issuing the command:
mount moonshot:/home /home -o rsize=8192,wsize=8192
These specify the datagram size used by the NFS clients on read and write requests, respectively. The default depends on the version of kernel, but is normally 1,024 bytes.
This sets the time (in tenths of a second) the NFS client will wait for a request to complete. The default value is 7 (0.7 seconds). What happens after a timeout depends on whether you use the hard or soft option.
Explicitly mark this volume as hard-mounted. This is on by default. This option causes the server to report a message to the console when a major timeout occurs and continues trying indefinitely.
Soft-mount (as opposed to hard-mount) the driver. This option causes an I/O error to be reported to the process attempting a file operation when a major timeout occurs.
Allow signals to interrupt an NFS call. Useful for aborting when the server doesn’t respond.
Except for rsize and wsize, all of these options apply to the client’s behavior if the server should become temporarily inaccessible. They work together in the following way: Whenever the client sends a request to the NFS server, it expects the operation to have finished after a given interval (specified in the timeout option). If no confirmation is received within this time, a so-called minor timeout occurs, and the operation is retried with the timeout interval doubled. After reaching a maximum timeout of 60 seconds, a major timeout occurs.
By default, a major timeout causes the client to print a message
to the console and start all over again, this time with an initial
timeout interval twice that of the previous cascade. Potentially, this
may go on forever. Volumes that stubbornly retry an operation until
the server becomes available again are called
hard-mounted. The opposite variety, called
soft-mounted, generate an I/O error for the
calling process whenever a major timeout occurs. Because of the
write-behind introduced by the buffer cache, this error condition is
not propagated to the process itself before it calls the
write function the next time, so a program can
never be sure that a write operation to a soft-mounted volume has
succeeded at all.
Whether you hard- or soft-mount a volume depends partly on taste but also on the type of information you want to access from a volume. For example, if you mount your X programs by NFS, you certainly would not want your X session to go berserk just because someone brought the network to a grinding halt by firing up seven copies of Doom at the same time or by pulling the Ethernet plug for a moment. By hard-mounting the directory containing these programs, you make sure that your computer waits until it is able to re-establish contact with your NFS server. On the other hand, non-critical data such as NFS-mounted news partitions or FTP archives may also be soft-mounted, so if the remote machine is temporarily unreachable or down, it doesn’t hang your session. If your network connection to the server is flaky or goes through a loaded router, you may either increase the initial timeout using the timeo option or hard-mount the volumes. NFS volumes are hard-mounted by default.
Hard mounts present a problem because, by default, the file operations are not interruptible. Thus, if a process attempts, for example, a write to a remote server and that server is unreachable, the user’s application hangs and the user can’t do anything to abort the operation. If you use the intr option in conjuction with a hard mount, any signals received by the process interrupt the NFS call so that users can still abort hanging file accesses and resume work (although without saving the file).
Usually, the rpc.mountd daemon in some way or other keeps track of which directories have been mounted by what hosts. This information can be displayed using the showmount program, which is also included in the NFS server package:
showmount -e moonshotExport list for localhost: /home <anon clnt> #
showmount -d moonshotDirectories on localhost: /home #
showmount -a moonshotAll mount points on localhost: localhost:/home
 One doesn’t say filesystem because these are not proper filesystems.