Mounting an NFS Volume

NFS volumes are mounted very much the way usual file systems are mounted. You invoke mount using the following syntax:

     # mount -t nfs nfs volume local dir options

nfs_volume is given as remote_host:remote_dir. Since this notation is unique to NFS file systems, you can leave out the -t nfs option.

There are a number of additional options that you may specify to mount upon mounting an NFS volume. These may either be given 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 from each other by commas. Options specified on the command line always override those given in the fstab file.

A sample entry in /etc/fstab might be

     # volume              mount point       type  options
     news:/usr/spool/news  /usr/spool/news   nfs   timeo=14,intr

This volume may then be mounted using

     # mount news:/usr/spool/news

In the absence of a 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 4K for read/write operations. You might decrease block size to 2K to suit ' datagram size limit by issuing

     # mount moonshot:/home /home -o rsize=2048,wsize=2048

The list of all valid options is described in its entirety in the nfs(5) manual page that comes with Rick Sladkey's NFS-aware mount tool which can be found in Rik Faith's util-linux package). The following is an incomplete list of those you would probably want to use:

rsize=n and wsize=n

These specify the datagram size used by the NFS clients on read and write requests, respectively. They cur- rently default to 1024 bytes, due to the limit on UDP datagram size described above.

timeo=n

This sets the time (in tenths of a second) the NFS client will wait for a request to complete. The default values is 0.7 sec- onds.

hard

Explicitly mark this volume as hard-mounted. This is on by default.

soft

Soft-mount the driver (as opposed to hard-mount).

intr

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 inaccessible temporarily. They play 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 will cause 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, soft-mounted volumes gerenates 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(2) 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 is not simply a question of taste, but also has to do with what sort of information you want to access from this 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 xv at the same time, or by pulling the Ethernet plug for a moment. By hard-mounting these, you make sure that your computer will wait 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 as well be soft-mounted, so it doesn't hang your session in case the remote machine should be temporarily unreachable, or down. 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, but allow for signals interrupting the NFS call so that you may still abort any hanging file access.

Usually, the mountd daemon will in some way or other keep 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. The mountd, however, does not do this yet.