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NFS Servers
Available for Linux
There have been several important changes in
Linux's NFS support between 1998 and 2002. This section summarizes two of these
changes, in case you have older documentation or are using an older
distribution. You can often use whatever NFS server ships with your
distribution, but sometimes you may have to upgrade (or occasionally downgrade)
your server to achieve compatibility with other systems. You can find
information on the latest NFS developments for Linux at
User-Mode
and Kernel-Mode Servers
An NFS server basically directs data between
a disk file and a network interface. The original Linux NFS servers operated in
user mode, which means that they had no
privileged access to or help from the kernel. This design, however, means that
data must pass from the disk through the kernel to a user-mode program and back
through the kernel and to the network (or in the opposite direction). The
transfer of data from the kernel to the user-mode program and back again
impedes performance, so the kernel and the NFS server were redesigned so that
the kernel could handle more of the necessary transfers itself. This resulted
in a speed increase. To take advantage of this improvement, you must activate
the NFS Server Support option in the Network File Systems menu of the kernel's
File Systems menu (see
without this support, your kernel won't do its part of the NFS server job. You
must also use an NFS server that's designed to use the kernel's NFS server
support. This server is often called knfsd , as opposed to the
regular nfsd , but this distinction is often not made explicit.
Figure 8.1. The Linux
kernel includes support for both client and server NFS functions.
align=left border=0> The Linux kernel configuration tools also
provide an option called NFS File System Support. This option provides the
kernel's NFS client tools, which work in
conjunction with the normal mount command to mount a remote NFS
server's exports on your local directory tree. The kernel's NFS client and
server support options are independent of each other; you can include
neither, just one, or both, as you see fit.
NFS Versions 2
and 3
Like many other protocols and programs, NFS
has undergone periodic revisions. In 2002, version 3 of NFS, or NFSv3 for short, is the latest version in common use.
(In fact, NFSv4 also exists, but Linux support for NFSv4 is embryonic at best. Check
http://www.nfsv4.org for more
information.) Many NFS clients and servers, however, don't yet support NFSv3;
they support NFSv2. Most of the Linux 2.2.x kernels, for instance, support NFSv2 but not NFSv3.
Only with kernel 2.2.18 and later does NFSv3 support appear as a standard
kernel option. (You can obtain NFSv3 patches for some earlier versions of the
kernel.) NFSv3 introduces several improvements, including better file locking,
improved performance via an asynchronous mode (Linux's NFSv2 implements an
asynchronous mode, but in a non-standard way), extensions taken from BSD's Not Quite NFS (NQNFS), and optional use of TCP
connections (NFSv2 uses UDP exclusivelybut TCP connections are not fully
implemented in Linux's NFSv3, as of early 2002). As a general rule, NFSv2 is
adequate for many small networks with casual users, but NFSv3 is very desirable
for higher server loadsat least, when fully implemented. Early experimental
versions of Linux's NFSv3 performed poorly because they didn't implement the
faster asynchronous mode operation, but the 2.4.x
kernel's NFSv3 support implements this feature for server operations. (Client
operations are still slow as of kernel 2.4.17.) If you want to use an NFSv3 server that uses
the kernel's NFS acceleration, you must select the Provide NFSv3 Server Support
option in the kernel configuration tools (this is a suboption of the NFS Server
Support option described earlier and shown in
the Provide NFSv3 Client Support option is required to use NFSv3 features as a
client. The NFS protocols allow for fall-back operation, so if one system
provides NFSv3 support but the other system only supports NFSv2, the two
computers will still be able to communicate using NFSv2 protocols. Thus,
chances are you'll want to select NFSv3 support.In addition to kernel options, you need
versions of NFS support utilities that support NFSv3 if you want to use it. Specifically,
you need nfs-utils version 0.1.6 or later for NFSv3 server support and mount version
2.10m or later for NFSv3 client support. Most distributions include these tools
in packages of these names, so you can check your installation CD-ROM or use rpm or dpkg to check
for the appropriate tools. For instance, you might type the following to check
for the appropriate version of mount on an RPM-based system: $ rpm -q mount mount-2.11b-5mdk
In this example, the output reveals that mount version
2.11b is installed, which is more than adequate for NFSv3 client support.
