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* docs/logging.html.in: document the fact that starting from 0.9.0 the server logs goes to libvirtd.log instead of syslog by default, describe the debug buffer, restructure the page and add a couple more examples
184 lines
8.6 KiB
XML
184 lines
8.6 KiB
XML
<?xml version="1.0"?>
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<html>
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<body>
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<h1 >Logging in the library and the daemon</h1>
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<p>Libvirt includes logging facilities starting from version 0.6.0,
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this complements the <a href="errors.html">error handling</a>
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mechanism and APIs to allow tracing through the execution of the
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library as well as in the libvirtd daemon.</p>
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<ul>
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<li>
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<a href="#log_library">Logging in the library</a>
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</li>
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<li>
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<a href="#log_config">Configuring logging in the library</a>
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</li>
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<li>
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<a href="#log_daemon">Logging in the daemon</a>
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</li>
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<li>
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<a href="#log_syntax">Syntax for filters and output values</a>
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</li>
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<li>
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<a href="#log_examples">Examples</a>
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</li>
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</ul>
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<h3>
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<a name="log_library">Logging in the library</a>
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</h3>
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<p>The logging functionalities in libvirt are based on 3 key concepts,
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similar to the one present in other generic logging facilities like
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log4j:</p>
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<ul>
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<li><b>log messages</b>: they are information generated at runtime by
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the libvirt code. Each message includes a priority level (DEBUG = 1,
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INFO = 2, WARNING = 3, ERROR = 4), a category, function name and
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line number, indicating where it originated from, and finally
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a formatted message. In addition the library adds a timestamp
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at the beginning of the message</li>
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<li><b>log filters</b>: a set of patterns and priorities to accept
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or reject a log message. If the message category matches a filter,
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the message priority is compared to the filter priority, if lower
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the message is discarded, if higher the message is output. If
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no filter matches, then a general priority level is applied to
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all remaining messages. This allows, for example, capturing all
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debug messages for the QEmu driver, but otherwise only allowing
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errors to show up from other parts.</li>
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<li><b>log outputs</b>: once a message has gone through filtering a set of
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output defines where to send the message, they can also filter
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based on the priority, for example it may be useful to output
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all messages to a debugging file but only allow errors to be
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logged through syslog.</li>
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</ul>
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<p>Note that the logging module saves all logs to a <b>debug buffer</b>
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filled in a round-robin fashion as to keep a full log of the
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recent logs including all debug. The debug buffer can be resized
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or deactivated in the daemon using the log_buffer_size variable,
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default is 64 kB. This can be used when debugging the library
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(see the virLogBuffer variable content).</p>
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<h3>
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<a name="log_config">Configuring logging in the library</a>
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</h3>
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<p>The library configuration of logging is through 3 environment variables
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allowing to control the logging behaviour:</p>
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<ul>
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<li>LIBVIRT_DEBUG: it can take the four following values:
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<ul>
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<li>1 or "debug": asking the library to log every message emitted,
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though the filters can be used to avoid filling up the output</li>
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<li>2 or "info": log all non-debugging informations</li>
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<li>3 or "warn": log warnings and errors, that's the default value</li>
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<li>4 or "error": log only error messages</li>
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</ul></li>
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<li>LIBVIRT_LOG_FILTERS: defines logging filters</li>
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<li>LIBVIRT_LOG_OUTPUTS: defines logging outputs</li>
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</ul>
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<p>Note that, for example, setting LIBVIRT_DEBUG= is the same as unset. If
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you specify an invalid value, it will be ignored with a warning. If you
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have an error in a filter or output string, some of the settings may be
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applied up to the point at which libvirt encountered the error.</p>
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<h3>
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<a name="log_daemon">Logging in the daemon</a>
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</h3>
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<p>Similarly the daemon logging behaviour can be tuned using 3 config
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variables, stored in the configuration file:
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<ul>
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<li>log_level: accepts the following values:
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<ul>
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<li>4: only errors</li>
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<li>3: warnings and errors</li>
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<li>2: informations, warnings and errors</li>
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<li>1: debug and everything</li>
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</ul></li>
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<li>log_filters: defines logging filters</li>
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<li>log_outputs: defines logging outputs</li>
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</ul>
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<p>When starting the libvirt daemon, any logging environment variable
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settings will override settings in the config file. Command line options
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take precedence over all. If no outputs are defined for libvirtd, it
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defaults to logging to /var/log/libvirt/libvirtd.log (before 0.9.0
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it was using syslog) when it is running as a daemon, or to
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stderr when it is running in the foreground.</p>
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<p>Libvirtd does not reload its logging configuration when issued a SIGHUP.
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If you want to reload the configuration, you must do a <code>service
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libvirtd restart</code> or manually stop and restart the daemon
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yourself.</p>
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<p>Starting from 0.9.0, the daemon can save all the content of the debug
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buffer to the defined error channels (or /var/log/libvirt/libvirtd.log
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by default) in case of crash, this can also be activated explicitly
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for debugging purposes by sending the daemon a USR2 signal:</p>
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<pre>killall -USR2 libvirtd</pre>
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<h3>
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<a name="log_syntax">Syntax for filters and output values</a>
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</h3>
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<p>The syntax for filters and outputs is the same for both types of
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variables.</p>
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<p>The format for a filter is:</p>
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<pre>x:name</pre>
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<p>where <code>name</code> is a match string e.g. <code>remote</code> or
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<code>qemu</code> and the x is the minimal level where matching messages
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should be logged:</p>
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<ul>
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<li>1: DEBUG</li>
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<li>2: INFO</li>
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<li>3: WARNING</li>
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<li>4: ERROR</li>
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</ul>
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<p>Multiple filters can be defined in a single string, they just need to be
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separated by spaces, e.g: <code>"3:remote 4:event"</code> to only get
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warning or errors from the remote layer and only errors from the event
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layer.<p>
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<p>If you specify a log priority in a filter that is below the default log
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priority level, messages that match that filter will still be logged,
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while others will not. In order to see those messages, you must also have
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an output defined that includes the priority level of your filter.</p>
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<p>The format for an output can be one of those 3 forms:</p>
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<ul>
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<li><code>x:stderr</code> output goes to stderr</li>
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<li><code>x:syslog:name</code> use syslog for the output and use the
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given <code>name</code> as the ident</li>
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<li><code>x:file:file_path</code> output to a file, with the given
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filepath</li>
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</ul>
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<p>In all cases the x prefix is the minimal level, acting as a filter:</p>
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<ul>
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<li>1: DEBUG</li>
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<li>2: INFO</li>
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<li>3: WARNING</li>
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<li>4: ERROR</li>
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</ul>
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<p>Multiple output can be defined, they just need to be separated by
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spaces, e.g.: <code>"3:syslog:libvirtd 1:file:/tmp/libvirt.log"</code>
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will log all warnings and errors to syslog under the libvirtd ident
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but also log all debug and information included in the
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file <code>/tmp/libvirt.log</code></p>
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<h3>
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<a name="log_examples">Examples</a>
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</h3>
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<p>For example setting up the following:</p>
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<pre>export LIBVIRT_DEBUG=1
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export LIBVIRT_LOG_OUTPUTS="1:file:virsh.log"</pre>
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<p>and then running virsh will accumulate the logs in the
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<code>virsh.log</code> file in a way similar to:</p>
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<pre>14:29:04.771: debug : virInitialize:278 : register drivers
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14:29:04.771: debug : virRegisterDriver:618 : registering Test as driver 0</pre>
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<p>the messages are timestamped, there is also the level recorded,
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if debug the name of the function is also printed and then the formatted
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message. This should be sufficient to at least get a precise idea of
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what is happening and where things are going wrong, allowing to then
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put the correct breakpoints when running under a debugger.</p>
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<p>To activate full debug of the libvirt entry points, utility
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functions and the QEmu/KVM driver, set:</p>
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<pre>log_filters=1:libvirt 1:util 1:qemu
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log_output=1:file:/var/log/libvirt/libvirtd.log</pre>
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<p>in libvirtd.conf and restart the daemon will allow to
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gather a copious amount of debugging traces for the operations done
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in those areas.</p>
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<p>On the other hand to deactivate the logbuffer in the daemon
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for stable high load servers, set</p>
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<pre>log_buffer_size=0</pre>
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<p>in the libvirtd.conf.</p>
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</body>
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</html>
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