Debug Logs

Turning on debug logs

If you report a bug against libvirt, in most cases you will be asked to attach debug logs. These are bare text files which tracks transition between different states of libvirtd, what it has tried to achieve, etc. Because of client -- server schema used in libvirt, the logs can be either client or server too. Usually, it's server side that matters as nearly all interesting work takes place there. Moreover, libvirt catches stderr of all running domains. These can be useful as well.

How to turn on debug logs for libvirt

Persistent setting

The daemon configuration files location is dependent on connection URI. For qemu:///system:

  • open /etc/libvirt/libvirtd.conf in your favourite editor

  • find & replace, or set these variables:

log_level = 1
log_filters="1:qemu 3:remote 4:event 3:util.json 3:rpc"
log_filters="3:remote 4:event 3:util.json 3:rpc 1:*"
  • save and exit

  • restart libvirtd service

systemctl restart libvirtd.service

In the config variables above, we have set logging level to 1 (debug level), set some filters (to filter out noise), e.g. from rpc only warnings (=level 3) and above will be reported. The logs are saved into /var/log/libvirt/libvirtd.log. Since libvirt 4.4.0 log filters support shell globbing, therefore the usage of log_level is considered deprecated in favour of pure usage of log_filters.

In case you want to get the client logs, you need to set this environment variable:

export LIBVIRT_LOG_OUTPUTS="1:file:/tmp/libvirt_client.log"

However, when you are using the session mode qemu:///session or you run the libvirtd as unprivileged user you will find configuration file under $XDG_CONFIG_HOME/libvirt/libvirtd.conf.

Runtime setting

Debugging anomalies can be very painful, especially when trying to reproduce it after the daemon restarts, since the new session can make the anomaly "disappear". Therefore, it's possible to enable the debug logs during runtime using libvirt administration API. To use it conveniently, there's a virt-admin client provided by the libvirt-admin package. Use the package manager provided by your distribution to install this package. Once you have it installed, run the following as root to see the set of log filters currently being active:

# virt-admin daemon-log-filters
 Logging filters: 3:remote 4:util.json 4:rpc

In order to change this set, run the same command as root, this time with your own set of filters:

# virt-admin daemon-log-filters "1:util 1:libvirt 1:storage 1:network 1:nodedev 1:qemu"
# virt-admin daemon-log-filters "3:remote 4:util.json 4:rpc 1:*"

Analogically, the same procedure can be performed with log outputs:

# virt-admin daemon-log-outputs
 Logging outputs: 3:syslog:libvirtd
# virt-admin daemon-log-outputs "1:file:/var/log/libvirt/libvirtd.log"

NOTE: It's always good practice to return the settings to the original state once you're finished debugging, just remember to save the original sets of filters and outputs and restore them at the end the same way as described above.

Removing filters and outputs

It's also possible to remove all the filters and produce an enormous log file, but it is not recommended since some of libvirt's modules can produce a large amount of noise. However, should you really want to do this, you can specify an empty set of filters:

# virt-admin daemon-log-filters ""
 Logging filters:

The situation is a bit different with outputs, since libvirt always has to log somewhere and resetting the outputs to an empty set will restore the default setting which depends on the host configuration, journald in our case:

# virt-admin daemon-log-outputs
 Logging outputs: 1:file:/var/log/libvirt/libvirtd.log
# virt-admin daemon-log-outputs ""
 Logging outputs: 2:journald

What to attach?

Now you should go and reproduce the bug. Once you're finished, attach:

  • /var/log/libvirt/libvirtd.log or whatever path you set for the daemon logs.

  • If the problem is related to a domain named $dom attach:

    • /var/log/libvirt/qemu/$dom.log (Or substitute qemu with whatever hypervisor you are using.)

    • The XML configuration of the vm/domain obtained by virsh dumpxml $dom

  • If the problem involves a crash of libvirtd or any other component, also attach the backtrace from the core dump if possible (e.g. using coredumpctl).

  • If you are asked for client logs, /tmp/libvirt_client.log.

  • Ideally don't tear down the environment in case additional information is required.

Example filter settings

Some filter setting suggestions for debugging more specific things. Unless it's explicitly stated, these work on libvirt 4.4.0 and later. Please note that some of the filters below may not log enough information for filing a proper libvirt bug. Usually it's better to log more than less.

Targeted logging for debugging QEMU VMs

Specifying only some sections allows for a targeted filter configuration which works on all versions and is sufficient for most cases.

1:libvirt 1:qemu 1:conf 1:security 3:event 3:json 3:file 3:object 1:util

Less verbose logging for QEMU VMs

Some subsystems are very noisy and usually not the culprit of the problems. They can be silenced individually for a less verbose log while still logging everything else. Usual suspects are the JSON code, RPC, authentication and such. A permissive filter is good for development use cases.

3:remote 4:event 3:util.json 3:util.object 3:util.dbus 3:util.netlink 3:node_device 3:rpc 3:access 1:*

Minimalistic QEMU QMP monitor logging

This filter logs only QMP traffic and skips most of libvirt's messages.

2:qemu.qemu_monitor 3:*