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Virtualization Guide / Troubleshooting / Gathering system information and logs
Applies to SUSE Linux Enterprise Server 15 SP3

38 Gathering system information and logs

  

When a virtualization host encounters a problem, it is often necessary to collect a detailed system report, which can be done with the help of the supportconfig tool. See https://documentation.suse.com/sles/15-SP3/html/SLES-all/cha-adm-support.html for more information about supportconfig.

In some cases, the information gathered by supportconfig is insufficient, and logs generated from a custom logging or debugging configuration might be required to determine the cause of a problem.

38.1 libvirt log controls

  

libvirt provides logging facilities for both the library and the daemon. The behavior of the logging facility is controlled by adjusting the log level, filter, and output settings.

Log level

libvirt log messages are classified into four priority levels: DEBUG, INFO, WARNING, and ERROR. The DEBUG level is very verbose and capable of generating gigabytes of information in a short time. The volume of log messages progressively decreases with the INFO, WARNING, and ERROR log levels. ERROR is the default log level.

Log filters

Log filters provide a way to log only messages matching a specific component and log level. Log filters allow collecting the verbose DEBUG log messages of specific components, but only ERROR level log messages from the rest of the system. By default, no log filters are defined.

Log outputs

Log outputs allow specifying where the filtered log messages are sent. Messages can be sent to a file, the standard error stream of the process, or journald. By default, filtered log messages are sent to journald.

See https://libvirt.org/logging.html for more details on libvirt's log controls.

A default libvirt installation has the log level set to ERROR, no log filters defined, and log outputs set to journald. Log messages from the libvirt daemon can be viewed with the journalctl command: 

# jounalctl --unit libvirtd

The default log facility settings are fine for normal operations and provide useful messages for applications and users of libvirt, but internal issues often require DEBUG level messages. As an example, consider a potential bug in the interaction between libvirt and the QEMU monitor. In this case, we only need to see the debug messages of the communication between libvirt and QEMU. The following example creates a log filter to select debug messages from the QEMU driver and send them to a file named /tmp/libvirtd.log 

   log_filters="1:qemu.qemu_monitor_json"
   log_outputs="1:file:/tmp/libvirtd.log"

Log controls for the libvirt daemon can be found in /etc/libvirt/libvirtd.conf. The daemon must be restarted after making any changes to the configuration file. 

# systemctl restart libvirtd.service