+=head1 MODES
+
+collectd can operate in three different operating modes. The modes are
+described below.
+
+The simplest mode is the so called B<local mode>. Data is collected locally and
+written in RRD files that reside in I<DataDir>. This is the default mode when
+collectd is linked against C<librrd>.
+
+The other two modes, B<client mode> and B<server mode>, are used to send data
+over a network and receive it again.
+
+In B<client mode> the daemon collectd the data locally and send it's result to
+one or more network addresses. No RRD files are written in this case. This is
+the only mode available if collectd is not linked against C<librrd>.
+
+If started in B<server mode> the daemon will listen on one or more interfaces
+and write the data it receives to RRD files. No data is collected locally.
+
+Please refer to L<collectd.conf(5)> for the configuration options and default
+values.
+
+=head1 SPECIAL PLUGINS
+
+=head2 cpufreq
+
+This module reads F</sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq> (for
+the first CPU installed) to get the current CPU frequency. If this file does
+not exist make sure B<cpufreqd> (L<http://cpufreqd.sourceforge.net/>) or a
+similar tool is installed.
+
+=head2 mysql
+
+Requires B<mysqlclient> to be installed. It connects to the database when
+started and keeps the connection up as long as possible. When the connection is
+interrupted for whatever reason it will try to re-connect. The syslog will
+contain loud complaints in case anything goes wrong.
+
+This plugin issues C<SHOW STATUS> and evaluates C<Bytes_{received,sent}>,
+C<Com_*> and C<Handler_*> which correspond to F<traffic-mysql.rrd>,
+F<mysql_commands-*.rrd> and F<mysql_handler-*.rrd>. Also, the values of
+C<Qcache_*> are put in F<mysql_qcache.rrd> and values of C<Threads_*> are put
+in F<mysql_threads.rrd>. Please refer to the B<MySQL reference manual>,
+I<5.2.4. Server Status Variables> for an explanation of these values.
+
+=head2 sensors
+
+The B<sensors> module uses lm_sensors to retrieve sensor-values. This means
+that all the needed modules have to be loaded and lm_sensors has to be
+configured (most likely by editing F</etc/sensors.conf>. Read
+L<sensors.conf(5)> for details.
+
+The B<lm_sensors> homepage can be found at
+L<http://secure.netroedge.com/~lm78/>.
+
+=head2 hddtemp
+
+To get values from B<hddtemp> collectd connects to B<localhost> (127.0.0.1),
+port B<7634/tcp>. The B<Host> and B<Port> options can be used to change these
+default values. See L<collectd.conf(5)> for details. C<hddtemp> has to be
+running to work correctly. If C<hddtemp> is not running timeouts may appear
+which may interfere with other statistics..
+
+The B<hddtemp> homepage can be found at
+L<http://www.guzu.net/linux/hddtemp.php>.
+
+=head2 vserver
+
+B<VServer> support is only available for Linux. It cannot yet be found in a
+vanilla kernel, though. To make use of this plugin you need a kernel that has
+B<VServer> support built in, i.e. you need to apply the patches and compile
+your own kernel, which will then provide the /proc/virtual filesystem that is
+required by this plugin.
+
+The B<VServer> homepage can be found at L<http://linux-vserver.org/>.
+