mirror of https://gitee.com/openkylin/libvirt.git
244 lines
11 KiB
XML
244 lines
11 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html>
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<html xmlns="http://www.w3.org/1999/xhtml">
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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 id="toc"/>
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<h2>
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<a id="log_library">Logging in the library</a>
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</h2>
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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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<h2>
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<a id="log_config">Configuring logging in the library</a>
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</h2>
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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 information</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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<h2>
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<a id="log_daemon">Logging in the daemon</a>
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</h2>
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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:</p>
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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: information, 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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will try to use</p>
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<ul>
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<li>0.10.0 or later: systemd journal, if <code>/run/systemd/journal/socket</code> exists</li>
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<li>0.9.0 or later: file <code>/var/log/libvirt/libvirtd.log</code> if running as a daemon</li>
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<li>before 0.9.0: syslog if running as a daemon</li>
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<li>all versions: to stderr stream if running in the foreground</li>
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</ul>
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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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<h2>
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<a id="log_syntax">Syntax for filters and output values</a>
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</h2>
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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>
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x:name</pre>
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<p>where <code>name</code> is a string which is matched against
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the category given in the VIR_LOG_INIT() at the top of each
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libvirt source file, e.g., <code>remote</code>, <code>qemu</code>,
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or <code>util.json</code> (the name in the filter can be a
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substring of the full category name, in order to match multiple
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similar categories), and <code>x</code> is the minimal
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level where matching messages 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 the following 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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<li><code>x:journald</code> output goes to systemd journal</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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<h2><a id="journald">Systemd journal fields</a></h2>
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<p>
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When logging to the systemd journal, the following fields
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are defined, in addition to any automatically recorded
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<a href="http://www.freedesktop.org/software/systemd/man/systemd.journal-fields.html">standard fields</a>:
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</p>
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<dl>
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<dt><code>MESSAGE</code></dt>
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<dd>The log message string</dd>
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<dt><code>PRIORITY</code></dt>
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<dd>The log priority value</dd>
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<dt><code>LIBVIRT_SOURCE</code></dt>
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<dd>The source type, one of "file", "error", "audit", "trace", "library"</dd>
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<dt><code>CODE_FILE</code></dt>
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<dd>The name of the file emitting the log record</dd>
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<dt><code>CODE_LINE</code></dt>
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<dd>The line number of the file emitting the log record</dd>
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<dt><code>CODE_FUNC</code></dt>
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<dd>The name of the function emitting the log record</dd>
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<dt><code>LIBVIRT_DOMAIN</code></dt>
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<dd>The libvirt error domain (values from virErrorDomain enum), if LIBVIRT_SOURCE="error"</dd>
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<dt><code>LIBVIRT_CODE</code></dt>
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<dd>The libvirt error code (values from virErrorCode enum), if LIBVIRT_SOURCE="error"</dd>
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</dl>
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<h3><a id="journaldids">Well known message ID values</a></h3>
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<p>
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Certain areas of the code will emit log records tagged with well known
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unique id values, which are guaranteed never to change in the future.
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This allows applications to identify critical log events without doing
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string matching on the <code>MESSAGE</code> field.
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</p>
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<dl>
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<dt><code>MESSAGE_ID=8ae2f3fb-2dbe-498e-8fbd-012d40afa361</code></dt>
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<dd>Generated by the QEMU driver when it identifies a QEMU system
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emulator binary, but is unable to extract information about its
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capabilities. This is usually an indicator of a broken QEMU
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build or installation. When this is emitted, the <code>LIBVIRT_QEMU_BINARY</code>
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message field will provide the full path of the QEMU binary that failed.
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</dd>
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</dl>
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<p>
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The <code>journalctl</code> command can be used to search the journal
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matching on specific message ID values
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</p>
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<pre>
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$ journalctl MESSAGE_ID=8ae2f3fb-2dbe-498e-8fbd-012d40afa361 --output=json
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{ ...snip...
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"LIBVIRT_SOURCE" : "file",
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"PRIORITY" : "3",
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"CODE_FILE" : "qemu/qemu_capabilities.c",
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"CODE_LINE" : "2770",
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"CODE_FUNC" : "virQEMUCapsLogProbeFailure",
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"MESSAGE_ID" : "8ae2f3fb-2dbe-498e-8fbd-012d40afa361",
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"LIBVIRT_QEMU_BINARY" : "/bin/qemu-system-xtensa",
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"MESSAGE" : "Failed to probe capabilities for /bin/qemu-system-xtensa:" \
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"internal error: Child process (LC_ALL=C LD_LIBRARY_PATH=/home/berrange" \
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"/src/virt/libvirt/src/.libs PATH=/usr/lib64/ccache:/usr/local/sbin:" \
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"/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin HOME=/root " \
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"USER=root LOGNAME=root /bin/qemu-system-xtensa -help) unexpected " \
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"exit status 127: /bin/qemu-system-xtensa: error while loading shared " \
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"libraries: libglapi.so.0: cannot open shared object file: No such " \
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"file or directory\n" }
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</pre>
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<h2>
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<a id="log_examples">Examples</a>
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</h2>
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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_outputs="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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</body>
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</html>
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