Libvirt: Use the virtlogd deamon

If the serial console feature is enabled on a compute node with [serial_console].enabled = True it deactivates the logging of the boot messages. From a REST API perspective, this means that the two APIs os-getConsoleOutput and os-getSerialConsole are mutually exclusive. Both APIs can be valuable for cloud operators in the case when something goes wrong during the launch of an instance. This blueprint wants to lift the XOR relationship between those two REST APIs.

Problem description

The problem can be seen in the method _create_serial_console_devices in the libvirt driver. The simplified logic is:

def _create_serial_console_devices(self, guest, instance, flavor,
    if CONF.serial_console.enabled:
        console = vconfig.LibvirtConfigGuestSerial()
        console.type = "tcp"
        consolelog = vconfig.LibvirtConfigGuestSerial()
        consolelog.type = "file"

This if-else establishes the XOR relationship between having a log of the guest’s boot messages or getting a handle to the guest’s serial console. From a driver point of view, this means getting valid return values for the method get_serial_console or get_console_output which are used to satisfy the two REST APIs os-getConsoleOutput and os-getSerialConsole.

Use Cases

From an end user point of view, this means that, with the current state, it is possible to get the console output of an instance on host A (serial console is not enabled) but after a rebuild on host B (serial console is enabled) it is not possible to get the console output. As an end user is not aware of the host’s configuration, this could be a confusing experience. Written that down I’m wondering why the serial console was designed with a compute node scope and not with an instance scope, but that’s another discussion I don’t want to do here.

After the implementation, deployers will have both means by hand if there is something wrong during the launch of an instance. The persisted log in case the instance crashed AND the serial console in case the instance launched but has issues, for example a failed establishing of networking so that SSH access is not possible. Also, they will be impacted with a new dependency on the hosts (see Dependencies).

Developers won’t be impacted.

Proposed change

I’d like to switch from the log file to the virtlogd deamon. This logging deamon was announced on the libvirt ML [1] and is available with libvirt version 1.3.3 and Qemu 2.6.0. This logging deamon handles the output from the guest’s console and writes it into the file /var/log/libvirt/qemu/guestname-serial0.log on the host but truncates/rotates that log so that it doesn’t exhaust the hosts disk space (this would solve an old bug [3]).

Nova would generate:

<serial type="tcp">
  <source mode="connect" host="" service="2445"/>
  <log file="/var/log/libvirt/qemu/guestname-serial0.log" append="on"/>
  <protocol type="raw"/>
  <target port="1"/>

For providing the console log data, nova would need to read the console log file from disk directly. As the log file gets rotated automatically we have to ensure that all necessary rotated log files get read to satisfy the upper limit of the get_console_output driver API contract.


  1. How is the migration/rebuild handled? The 4 cases which are possible (based on the node’s patch level):

    1. N -> N: Neither source nor target node is patched. That’s what we have today. Nothing to do.

    2. N -> N+1: The target node is patched, which means it can make use of the output from virtlogd. Can we “import” the existing log of the source node into the virtlogd logs of the target node?

      A: The guest will keep its configuration from the source host and don’t make use of the virtlogd service until it gets rebuilt.

    3. N+1 -> N: The source node is patched and the instance gets migrated to a target node which cannot utilize the virtlogd output. If the serial console is enable on the target node, do we throw away the log because we cannot update it on the target node

      A: In the case of migration to an old host, we try to copy the existing log file across, and configure the guest with the type=tcp backend. This provides ongoing support for interactive console. The log file will remain unchanged if possible. A failed copy operation should not prevent the migration of the guest.

    4. N+1 -> N+1: Source and target node are patched. Will libvirt migrate the existing log from the source node too, which would solve another open bug [4].

  2. Q: Could a stalling of the guest happen if nova-compute is reading the log file and virtlogd tries to write to the file but is blocked?

    A: No, virtlogd will ensure things are fully parallelized

  3. Q: The virtlogd deamon has a 1:1 relationship to a compute node. It would be interesting how well it performs when, for example, hundreds of instances are running on one compute node.

    A: We could add a I/O rate limit to virtlogd so it refuses to read data too quickly from a single guest. This prevents a single guest DOS’ing the host.

  4. Q: Are there architecture dependencies? Right now, a nova-compute node on a s390 architecture depends on the serial console feature because it cannot provide the other console types (VNC, SPICE, RDP). Which means it would benefit from having both.

    A: No architecture dependencies.

  5. Q: How are restarts of the virtlogd deamon handled? Do we lose information in the timeframe between stop and start?

    A: The virtlogd daemon will be able to re-exec() itself while keeping file handles open. This will ensure no data loss during restart of virtlogd.

  6. Q: Do we need a version check of libvirt to detect if the virtlodg is available on the host? Or is it sufficient to check if the folder /var/log/virtlogd/ is present?

    A: We will do a version number check on libvirt to figure out if it is capable to use it.


  1. In case where the serial console is enabled, we could establish a connection to the guest with it and execute tail /var/log/dmesg.log and return that output in the driver’s get_console_output method which is used to satisfy the os-getConsoleOutput REST API.

    Counter-arguments: We would need to save the authentication data to the guest, which would not be technically challenging but the customers could be unhappy that Nova can access their guests at any time. A second argument is, that the serial console access is blocking, which means if user A uses the serial console of an instance, user B is not able to do the same.

  2. We could remove the if-else and create both devices.

    Counter-arguments: This was tried in [2] and stopped because this could introduce a backwards incompatibility which could prevent the rebuild of an instance. The root cause for this was, that there is an upper bound of 4 serial devices on a guest, and this upper bound could be exceeded if an instance which already has 4 serial devices gets rebuilt on a compute node which would have patch [2].

Data model impact


REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact


Other deployer impact

  • The virtlogd service has to run for this functionality and should be monitored.

  • This would also solve a long-running bug which can cause a host disc space exhaustion (see [3]).

Developer impact




Primary assignee:

Markus Zoeller (

Work Items

  • (optional) get a gate job running which has the serial console activated

  • add version check if libvirt supports the virtlogd functionality

  • add “happy path” which creates a guest device which uses virtlogd

  • ensure “rebuild” uses the new functionality when migrated from an old host

  • add reconfiguration of the guest when migrating from N+1 -> N hosts to keep backwards compatibility


  • Libvirt 1.3.3 which brings the libvirt virtlod logging deamon as described in [1].

  • Qemu 2.6.0


The tempest tests which are annotated with CONF.compute_feature_enabled.console_output will have to work with a setup which

  • has the dependency to the virtlogd deamon resolved.

  • AND has the serial console feature enabled (AFAIK there is not job right now which has this enabled)

  • A new functional test for the live-migration case has to be added

Documentation Impact



[1] libvirt ML, “[libvirt] RFC: Building a virtlogd daemon”:

[2] Gerrit; “libvirt: use log file and serial console at the same time”:

[3] Launchpad; “ console.log grows indefinitely “:

[4] Launchpad; “live block migration results in loss of console log”:

[5] A set of patches on the libvirt/qemu ML:

  • [PATCH 0/5] Initial patches to introduce a virtlogd daemon

  • [PATCH 1/5] util: add API for writing to rotating files

  • [PATCH 2/5] Import stripped down virtlockd code as basis of virtlogd

  • [PATCH 3/5] logging: introduce log handling protocol

  • [PATCH 4/5] logging: add client for virtlogd daemon

  • [PATCH 5/5] qemu: add support for sending QEMU stdout/stderr to virtlogd

[6] libvirt ML, “[libvirt] [PATCH v2 00/13] Introduce a virtlogd daemon”:



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