Cloud operators requires a transparent way for the end user to upload images (either RAW or QCOW2) where Glance, when importing a new image, would automatically convert the image to the destination format and will
discard of the original image. This way, no matter what format you upload (without the customers intervention) would be stored by glance as raw and together with nova that is set to use RBD as it's backend,
whenever a new VM is spinned the root device would be created as an RBD
COW clone.
This feature was introduced in OSP13 as technology preview, this RFE tracks the graduation of this feature.