Libvirt runtime image type

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Libvirt qemu/parallels hypervisor is capable to deal with different images type. With this change we are going to add an ability to use a supplied image without converting it according to images_type config parameter.

Problem description

Currently nova libvirt driver sticks to a configured by a nova.conf image type. It is not possible to use supplied image without conversion in case it differs from specified or default one. Libvirt currently supports the following image backends: RBD, LVM, QCOW2, RAW, PLOOP. It is inflexible to limit one node just for one type of image.

Use Cases

Let a user be flexible about image types usage on different compute nodes. This would allow them to use all existing catalog of images across all compute nodes without necessity to reconfigure this by images_type parameter.

Proposed change

LibvirtDriver has a field called image_backend which is initialized just once when compute service starts. Let it be in an instance property rather than a property of the compute service. So, we introduce a new parameter CONF.libvirt.images_type_mapping, which controls image_backend in a more sophisticated way. This new configuration parameter is a list of mappings as follows:

images_type_mapping = <source image type1>:<backend image type1>,

<source image type2>:<backend image type2> … and so on.

If a provided image is not presented in images_type_mapping list, then it is converted to the format defined by images_type parameter.

Correctness of the supplied CONF.libvirt.images_type_mapping should be made by code. In case of invalid input an exception should be thrown by a parsing function.


Correct: images_type = default images_type_mapping = raw:lvm, ploop:ploop

images_type = lvm images_type_mapping = qcow2:qcow2

images_type = qcow2 images_type_mapping = raw:raw, ploop:ploop

images_type = default images_type_mapping = raw:raw, qcow2:qcow2

images_type = default images_type_mapping =

Incorrect: # ambiguous conversion rule for qcow2 format (qcow2->raw or qcow2->qcow2) images_type = default images_type_mapping = qcow2:raw, qcow2:qcow2

# ambiguous conversion rule for raw format (raw->rbd or raw->lvm) images_type = default images_type_mapping = raw:rbd, raw:lvm

# invalid source format (lvm and rbd can’t be specified as a source) images_type = default images_type_mapping = lvm:rbd, rbd:rbd


Let it be as it is.

Data model impact


REST API impact


Security impact


Notifications impact


Other end user impact

A new ListOpt parameter CONF.libvirt.images_type_mapping is added to nova.conf file.

Performance Impact


Other deployer impact

A new parameter CONF.libvirt.images_type_mapping should be specified if a user is interested in altering current behavior, which should be seamless in upgrade.

Developer impact

Other hypervisors detect particular type of image mostly in runtime. Hyperv, for instance, detects vhd or vhdx by header and doesn’t need to specify which one to use by config.



Primary assignee:

Maxim Nestratov

Other contributors:

Dmitry Guryanov

Work Items

  • Introduce ListOpt images_type_mapping config option.

  • Implement parsing images_type_mapping complying with current images_type parameter.

  • Implement image_backend as a property of an instance rather than a service.




Functional test is going to be implemented.

Documentation Impact

It should be reflected in documentation that a new nova.conf parameter images_type_mapping is introduced as described above.