Comment 6 for bug 1746073

upvote for this from me - although our use case is slightly different; we need volume backed disks so that we can do backups from Cinder and also dynamic volume resize. So our block would look like this:

"block_device_mapping_v2": [{
    "boot_index": "0",
    "uuid": "ac408821-c95a-448f-9292-73986c790911",
    "source_type": "image",
    "volume_size": "{{ from machine root-disk constraint }}",
    "destination_type": "volume",
    "delete_on_termination": true,
    "tag": "disk1",
    "disk_bus": "scsi"}]

where uuid = source image id

I think you could cover 90% of use cases with:

juju deploy cs:ubuntu --constraints="root-storage=volume root-disk=20G root-disk-autodelete=true/false"

The other properties in block_device_mapping_v2 shouldn't need surfacing back to the juju user.