Cloned from launchpad blueprint https://blueprints.launchpad.net/nova/+spec/libvirt-image-to-local-bdm.

Description:

As part of the havana blueprint improve-block-device-handling we added the possibility to have glance images specified as block devices, and wa also added the field to specify the target of the block device.

The idea behind this is that we would be able to specify weather the image would be created on the hypervisor (dest=local) or will be downloaded onto a volume similar to a Cinder snapshot (dest=volume).

Implementing the 'dest: local' images will require some work:

* Since the way these block devices will be handled is different than what we currently have (for block devices) - we will need to generilize the code that does base image download in creation to be able to accept data passed as image block devices.

* We will likely want to take advantage of using base images and caching them on hypervisor nodes - so this blueprint will cover extending and generalizing the code in nova/virt/libvirt/imagecache.py to be aware of the block devices and handle them according to the system configuration.

* We will need to make sure that drivers can advertise to the manager code that they will accept these new block devices. Most of the groundwork for this is laid out in https://review.openstack.org/#/c/39086/ and it will be mostly a matter of implementing a new DriverBlockDevice subtype.

* We will likely want to add horizon support for this as well.

Specification URL (additional information):

None