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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+==========================================
+Libvirt runtime image type
+==========================================
+
+Include the URL of your launchpad blueprint:
+
+https://blueprints.launchpad.net/nova/+spec/runtime-image-type
+
+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.
+
+Examples:
+---------
+
+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
+
+Alternatives
+------------
+
+Let it be as it is.
+
+Data model impact
+-----------------
+
+None.
+
+REST API impact
+---------------
+
+None.
+
+Security impact
+---------------
+
+None.
+
+Notifications impact
+--------------------
+
+None.
+
+Other end user impact
+---------------------
+
+A new ListOpt parameter CONF.libvirt.images_type_mapping is added to nova.conf
+file.
+
+Performance Impact
+------------------
+
+None.
+
+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.
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  Maxim Nestratov mnestratov@virtuozzo.com
+
+Other contributors:
+  Dmitry Guryanov dguryanov@virtuozzo.com
+
+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.
+
+Dependencies
+============
+
+None.
+
+Testing
+=======
+
+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.
+
+References
+==========
+
+None.
+