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NFS, the network filesystem, is
probably the most prominent network services using RPC. It allows to access
files on remote hosts in exactly the same way as a user would access any local
files. This is made possible by a mixture of kernel functionality on the client
side (that uses the remote file system) and an NFS server on the server side
(that provides the file data). This file access is completely transparent to
the client, and works across a variety of server and host architectures.
NFS offers a number of advantages:
Data accessed by all users can be kept on a central
host, with clients mounting this directory at boot time. For example, you can
keep all user accounts on one host, and have all hosts on your network Ø
mount /home from that host. If
installed alongside with NIS, users can then log into any system, and still
work on one set of files.
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Data consuming large amounts of disk
space may be kept on a single host. For example, all files and programs
relating to LaTeX and METAFONT could be kept and maintained in one place.
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Administrative data may be kept on a
single host. No need to use rcp anymore to install the same stupid file on 20
different machines.
Preparing NFS
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Before you can use NFS, be it as
server or client, you must make sure your kernel has NFS support compiled in.
Newer kernels have a simple interface on the proc filesystem for this, the
/proc/filesystems file, which you can display using cat:
$ cat
/proc/filesystems
minix
ext2
msdos
nodev proc
nodev nfs
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If nfs is missing from this list,
then you have to compile your own kernel with NFS enabled. Configuring the
kernel network options is explained in section ``Kernel Configuration'' .
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For older kernels prior to -1.1, the
easiest way to find out whether your kernel has NFS support enabled is to
actually try to mount an NFS file system. For this, you could create a
directory below /tmp, and try to mount a local directory on it:
# mkdir /tmp/test
# mount
localhost:/etc /tmp/test
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If this mount attempt fails with an
error message saying ``fs type nfs no supported by kernel'', you must make a
new kernel with NFS enabled. Any other error messages are completely harmless,
as you haven't configured the NFS daemons on your host yet.
Mounting an NFS
Volume
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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.
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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.
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A sample entry in /etc/fstab might be
# volume mount point type
options
news:/usr/spool/news
/usr/spool/news nfs timeo=14,intr
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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
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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.
