network-nfs.html

FreeBSD安装说明概述 FreeBSD 提供了一个以文字为主

<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">portmap</kbd>
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">nfsd -u -t -n 4</kbd>
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">mountd -r</kbd>
</pre>

<p>On the <acronym class="ACRONYM">NFS</acronym> client:</p>

<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">nfsiod -n 4</kbd>
</pre>

<p>Now everything should be ready to actually mount a remote file system. In these
examples the server's name will be <var class="LITERAL">server</var> and the client's
name will be <var class="LITERAL">client</var>. If you only want to temporarily mount a
remote filesystem or would rather test the configuration, just execute a command like
this as <tt class="USERNAME">root</tt> on the client:</p>

<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">mount server:/home /mnt</kbd>
</pre>

<p>This will mount the <tt class="FILENAME">/home</tt> directory on the server at <tt
class="FILENAME">/mnt</tt> on the client. If everything is set up correctly you should be
able to enter <tt class="FILENAME">/mnt</tt> on the client and see all the files that are
on the server.</p>

<p>If you want to automatically mount a remote filesystem each time the computer boots,
add the filesystem to the <tt class="FILENAME">/etc/fstab</tt> file. Here is an
example:</p>

<pre class="PROGRAMLISTING">
server:/home   /mnt    nfs rw  0   0
</pre>

<p>The <span class="CITEREFENTRY"><span class="REFENTRYTITLE">fstab</span>(5)</span>
manual page lists all the available options.</p>
</div>

<div class="SECT2">
<h2 class="SECT2"><a id="AEN26295" name="AEN26295">19.6.3. Practical Uses</a></h2>

<p><acronym class="ACRONYM">NFS</acronym> has many practical uses. Some of the more
common ones are listed below:</p>

<ul>
<li>
<p>Set several machines to share a CDROM or other media among them. This is cheaper and
often a more convenient method to install software on multiple machines.</p>
</li>

<li>
<p>On large networks, it might be more convenient to configure a central <acronym
class="ACRONYM">NFS</acronym> server in which to store all the user home directories.
These home directories can then be exported to the network so that users would always
have the same home directory, regardless of which workstation they log in to.</p>
</li>

<li>
<p>Several machines could have a common <tt class="FILENAME">/usr/ports/distfiles</tt>
directory. That way, when you need to install a port on several machines, you can quickly
access the source without downloading it on each machine.</p>
</li>
</ul>
</div>

<div class="SECT2">
<h2 class="SECT2"><a id="NETWORK-AMD" name="NETWORK-AMD">19.6.4. Automatic Mounts with <b
class="APPLICATION">amd</b></a></h2>

<i class="AUTHORGROUP"><span class="CONTRIB">Contributed by</span> Wylie Stilwell.</i> <i
class="AUTHORGROUP"><span class="CONTRIB">Rewritten by</span> Chern Lee.</i> 

<p><span class="CITEREFENTRY"><span class="REFENTRYTITLE">amd</span>(8)</span> (the
automatic mounter daemon) automatically mounts a remote filesystem whenever a file or
directory within that filesystem is accessed. Filesystems that are inactive for a period
of time will also be automatically unmounted by <b class="APPLICATION">amd</b>. Using <b
class="APPLICATION">amd</b> provides a simple alternative to permanent mounts, as
permanent mounts are usually listed in <tt class="FILENAME">/etc/fstab</tt>.</p>

<p><b class="APPLICATION">amd</b> operates by attaching itself as an NFS server to the
<tt class="FILENAME">/host</tt> and <tt class="FILENAME">/net</tt> directories. When a
file is accessed within one of these directories, <b class="APPLICATION">amd</b> looks up
the corresponding remote mount and automatically mounts it. <tt
class="FILENAME">/net</tt> is used to mount an exported filesystem from an IP address,
while <tt class="FILENAME">/host</tt> is used to mount an export from a remote
hostname.</p>

<p>An access to a file within <tt class="FILENAME">/host/foobar/usr</tt> would tell <b
class="APPLICATION">amd</b> to attempt to mount the <tt class="FILENAME">/usr</tt> export
on the host <tt class="HOSTID">foobar</tt>.</p>

<div class="EXAMPLE"><a id="AEN26348" name="AEN26348"></a>
<p><b>Example 19-1. Mounting an Export with amd</b></p>

<p>You can view the available mounts of a remote host with the <tt
class="COMMAND">showmount</tt> command. For example, to view the mounts of a host named
<tt class="HOSTID">foobar</tt>, you can use:</p>

<pre class="SCREEN">
<samp class="PROMPT">%</samp> <kbd class="USERINPUT">showmount -e foobar</kbd>
Exports list on foobar:
/usr                               10.10.10.0
/a                                 10.10.10.0
<samp class="PROMPT">%</samp> <kbd class="USERINPUT">cd /host/foobar/usr</kbd>
</pre>
</div>

<p>As seen in the example, the <tt class="COMMAND">showmount</tt> shows <tt
class="FILENAME">/usr</tt> as an export. When changing directories to <tt
class="FILENAME">/host/foobar/usr</tt>, <b class="APPLICATION">amd</b> attempts to
resolve the hostname <tt class="HOSTID">foobar</tt> and automatically mount the desired
export.</p>

<p><b class="APPLICATION">amd</b> can be started by the startup scripts by placing the
following lines in <tt class="FILENAME">/etc/rc.conf</tt>:</p>

<pre class="PROGRAMLISTING">
amd_enable="YES"
</pre>

<p>Additionally, custom flags can be passed to <b class="APPLICATION">amd</b> from the
<var class="VARNAME">amd_flags</var> option. By default, <var
class="VARNAME">amd_flags</var> is set to:</p>

<pre class="PROGRAMLISTING">
amd_flags="-a /.amd_mnt -l syslog /host /etc/amd.map /net /etc/amd.map"
</pre>

<p>The <tt class="FILENAME">/etc/amd.map</tt> file defines the default options that
exports are mounted with. The <tt class="FILENAME">/etc/amd.conf</tt> file defines some
of the more advanced features of <b class="APPLICATION">amd</b>.</p>

<p>Consult the <span class="CITEREFENTRY"><span
class="REFENTRYTITLE">amd</span>(8)</span> and <span class="CITEREFENTRY"><span
class="REFENTRYTITLE">amd.conf</span>(5)</span> manual pages for more information.</p>
</div>

<div class="SECT2">
<h2 class="SECT2"><a id="NETWORK-NFS-INTEGRATION" name="NETWORK-NFS-INTEGRATION">19.6.5.
Problems Integrating with Other Systems</a></h2>

<i class="AUTHORGROUP"><span class="CONTRIB">Contributed by</span> John Lind.</i> 

<p>Certain Ethernet adapters for ISA PC systems have limitations which can lead to
serious network problems, particularly with NFS. This difficulty is not specific to
FreeBSD, but FreeBSD systems are affected by it.</p>

<p>The problem nearly always occurs when (FreeBSD) PC systems are networked with
high-performance workstations, such as those made by Silicon Graphics, Inc., and Sun
Microsystems, Inc. The NFS mount will work fine, and some operations may succeed, but
suddenly the server will seem to become unresponsive to the client, even though requests
to and from other systems continue to be processed. This happens to the client system,
whether the client is the FreeBSD system or the workstation. On many systems, there is no
way to shut down the client gracefully once this problem has manifested itself. The only
solution is often to reset the client, because the NFS situation cannot be resolved.</p>

<p>Though the ``correct'' solution is to get a higher performance and capacity Ethernet
adapter for the FreeBSD system, there is a simple workaround that will allow satisfactory
operation. If the FreeBSD system is the <span class="emphasis"><i
class="EMPHASIS">server</i></span>, include the option <var class="OPTION">-w=1024</var>
on the mount from the client. If the FreeBSD system is the <span class="emphasis"><i
class="EMPHASIS">client</i></span>, then mount the NFS filesystem with the option <var
class="OPTION">-r=1024</var>. These options may be specified using the fourth field of
the <tt class="FILENAME">fstab</tt> entry on the client for automatic mounts, or by using
the <var class="OPTION">-o</var> parameter of the mount command for manual mounts.</p>

<p>It should be noted that there is a different problem, sometimes mistaken for this one,
when the NFS servers and clients are on different networks. If that is the case, make
<span class="emphasis"><i class="EMPHASIS">certain</i></span> that your routers are
routing the necessary UDP information, or you will not get anywhere, no matter what else
you are doing.</p>

<p>In the following examples, <tt class="HOSTID">fastws</tt> is the host (interface) name
of a high-performance workstation, and <tt class="HOSTID">freebox</tt> is the host
(interface) name of a FreeBSD system with a lower-performance Ethernet adapter. Also, <tt
class="FILENAME">/sharedfs</tt> will be the exported NFS filesystem (see <span
class="CITEREFENTRY"><span class="REFENTRYTITLE">exports</span>(5)</span>), and <tt
class="FILENAME">/project</tt> will be the mount point on the client for the exported
filesystem. In all cases, note that additional options, such as <var
class="OPTION">hard</var> or <var class="OPTION">soft</var> and <var
class="OPTION">bg</var> may be desirable in your application.</p>

<p>Examples for the FreeBSD system (<tt class="HOSTID">freebox</tt>) as the client in <tt
class="FILENAME">/etc/fstab</tt> on freebox:</p>

<pre class="PROGRAMLISTING">
fastws:/sharedfs /project nfs rw,-r=1024 0 0
</pre>

<p>As a manual mount command on <tt class="HOSTID">freebox</tt>:</p>

<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd
class="USERINPUT">mount -t nfs -o -r=1024 fastws:/sharedfs /project</kbd>
</pre>

<p>Examples for the FreeBSD system as the server in <tt class="FILENAME">/etc/fstab</tt>
on <tt class="HOSTID">fastws</tt>:</p>

<pre class="PROGRAMLISTING">
freebox:/sharedfs /project nfs rw,-w=1024 0 0
</pre>

<p>As a manual mount command on <tt class="HOSTID">fastws</tt>:</p>

<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd
class="USERINPUT">mount -t nfs -o -w=1024 freebox:/sharedfs /project</kbd>
</pre>

<p>Nearly any 16-bit Ethernet adapter will allow operation without the above restrictions
on the read or write size.</p>

<p>For anyone who cares, here is what happens when the failure occurs, which also
explains why it is unrecoverable. NFS typically works with a ``block'' size of 8&nbsp;k
(though it may do fragments of smaller sizes). Since the maximum Ethernet packet is
around 1500&nbsp;bytes, the NFS ``block'' gets split into multiple Ethernet packets, even
though it is still a single unit to the upper-level code, and must be received,
assembled, and <span class="emphasis"><i class="EMPHASIS">acknowledged</i></span> as a
unit. The high-performance workstations can pump out the packets which comprise the NFS
unit one right after the other, just as close together as the standard allows. On the
smaller, lower capacity cards, the later packets overrun the earlier packets of the same
unit before they can be transferred to the host and the unit as a whole cannot be
reconstructed or acknowledged. As a result, the workstation will time out and try again,
but it will try again with the entire 8&nbsp;K unit, and the process will be repeated, ad
infinitum.</p>

<p>By keeping the unit size below the Ethernet packet size limitation, we ensure that any
complete Ethernet packet received can be acknowledged individually, avoiding the deadlock
situation.</p>

<p>Overruns may still occur when a high-performance workstations is slamming data out to
a PC system, but with the better cards, such overruns are not guaranteed on NFS
``units''. When an overrun occurs, the units affected will be retransmitted, and there
will be a fair chance that they will be received, assembled, and acknowledged.</p>
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