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Libvirt includes logging facilities starting from version 0.6.0, this complements the error handling mechanism and APIs to allow tracing through the execution of the library as well as in the libvirtd daemon.
The logging functionalities in libvirt are based on 3 key concepts, similar to the one present in other generic logging facilities like log4j:
log messages: they are information generated at runtime by the libvirt code. Each message includes a priority level (DEBUG = 1, INFO = 2, WARNING = 3, ERROR = 4), a category, function name and line number, indicating where it originated from, and finally a formatted message. In addition the library adds a timestamp at the beginning of the message
log filters: a set of patterns and priorities to accept or reject a log message. If the message category matches a filter, the message priority is compared to the filter priority, if lower the message is discarded, if higher the message is output. If no filter matches, then a general priority level is applied to all remaining messages. This allows, for example, capturing all debug messages for the QEMU driver, but otherwise only allowing errors to show up from other parts.
log outputs: once a message has gone through filtering a set of output defines where to send the message, they can also filter based on the priority, for example it may be useful to output all messages to a debugging file but only allow errors to be logged through syslog.
The library configuration of logging is through 3 environment variables allowing to control the logging behaviour:
LIBVIRT_DEBUG: it can take the four following values:
1 or "debug": asking the library to log every message emitted, though the filters can be used to avoid filling up the output
2 or "info": log all non-debugging information
3 or "warn": log warnings and errors, that's the default value
4 or "error": log only error messages
LIBVIRT_LOG_FILTERS: defines logging filters
LIBVIRT_LOG_OUTPUTS: defines logging outputs
Note that, for example, setting LIBVIRT_DEBUG= is the same as unset. If you specify an invalid value, it will be ignored with a warning. If you have an error in a filter or output string, some of the settings may be applied up to the point at which libvirt encountered the error.
Similarly the daemon logging behaviour can be tuned using 3 config variables, stored in the configuration file:
log_level: accepts the following values:
4: only errors
3: warnings and errors
2: information, warnings and errors
1: debug and everything
log_filters: defines logging filters
log_outputs: defines logging outputs
When starting the libvirt daemon, any logging environment variable settings will override settings in the config file. Command line options take precedence over all. If no outputs are defined for libvirtd, it will try to use
0.10.0 or later: systemd journal, if /run/systemd/journal/socket exists
0.9.0 or later: file /var/log/libvirt/libvirtd.log if running as a daemon
before 0.9.0: syslog if running as a daemon
all versions: to stderr stream if running in the foreground
Libvirtd does not reload its logging configuration when issued a SIGHUP. If you want to reload the configuration, you must do a service libvirtd restart or manually stop and restart the daemon yourself.
Starting from 0.9.0, the daemon can save all the content of the debug buffer to the defined error channels (or /var/log/libvirt/libvirtd.log by default) in case of crash, this can also be activated explicitly for debugging purposes by sending the daemon a USR2 signal:
killall -USR2 libvirtd
The syntax for filters and outputs is the same for both types of variables.
The format for a filter is:
x:name
where name is a string which is matched against the category given in the VIR_LOG_INIT() at the top of each libvirt source file, e.g., remote, qemu, or util.json (the name in the filter can be a substring of the full category name, in order to match multiple similar categories), and x is the minimal level where matching messages should be logged:
1: DEBUG
2: INFO
3: WARNING
4: ERROR
Multiple filters can be defined in a single string, they just need to be separated by spaces, e.g: "3:remote 4:event" to only get warning or errors from the remote layer and only errors from the event layer.
If you specify a log priority in a filter that is below the default log priority level, messages that match that filter will still be logged, while others will not. In order to see those messages, you must also have an output defined that includes the priority level of your filter.
The format for an output can be one of the following forms:
x:stderr output goes to stderr
x:syslog:name use syslog for the output and use the given name as the ident
x:file:file_path output to a file, with the given filepath
x:journald output goes to systemd journal
In all cases the x prefix is the minimal level, acting as a filter:
1: DEBUG
2: INFO
3: WARNING
4: ERROR
Multiple output can be defined, they just need to be separated by spaces, e.g.: "3:syslog:libvirtd 1:file:/tmp/libvirt.log" will log all warnings and errors to syslog under the libvirtd ident but also log all debug and information included in the file /tmp/libvirt.log
When logging to the systemd journal, the following fields are defined, in addition to any automatically recorded standard fields:
The log message string
The log priority value
The source type, one of "file", "error", "audit", "trace", "library"
The name of the file emitting the log record
The line number of the file emitting the log record
The name of the function emitting the log record
The libvirt error domain (values from virErrorDomain enum), if LIBVIRT_SOURCE="error"
The libvirt error code (values from virErrorCode enum), if LIBVIRT_SOURCE="error"
Certain areas of the code will emit log records tagged with well known unique id values, which are guaranteed never to change in the future. This allows applications to identify critical log events without doing string matching on the MESSAGE field.
Generated by the QEMU driver when it identifies a QEMU system emulator binary, but is unable to extract information about its capabilities. This is usually an indicator of a broken QEMU build or installation. When this is emitted, the LIBVIRT_QEMU_BINARY message field will provide the full path of the QEMU binary that failed.
The journalctl command can be used to search the journal matching on specific message ID values
$ journalctl MESSAGE_ID=8ae2f3fb-2dbe-498e-8fbd-012d40afa361 --output=json
{ ...snip...
"LIBVIRT_SOURCE" : "file",
"PRIORITY" : "3",
"CODE_FILE" : "qemu/qemu_capabilities.c",
"CODE_LINE" : "2770",
"CODE_FUNC" : "virQEMUCapsLogProbeFailure",
"MESSAGE_ID" : "8ae2f3fb-2dbe-498e-8fbd-012d40afa361",
"LIBVIRT_QEMU_BINARY" : "/bin/qemu-system-xtensa",
"MESSAGE" : "Failed to probe capabilities for /bin/qemu-system-xtensa:" \
"internal error: Child process (LC_ALL=C LD_LIBRARY_PATH=/home/berrange" \
"/src/virt/libvirt/src/.libs PATH=/usr/lib64/ccache:/usr/local/sbin:" \
"/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin HOME=/root " \
"USER=root LOGNAME=root /bin/qemu-system-xtensa -help) unexpected " \
"exit status 127: /bin/qemu-system-xtensa: error while loading shared " \
"libraries: libglapi.so.0: cannot open shared object file: No such " \
"file or directory\n" }
Examples with useful log settings along with more information on how to properly configure logging for various situations can be found in the logging knowledge base article.