nova/doc/api_samples/os-assisted-volume-snapshots/snapshot-create-assisted-req.json

{
    "snapshot": {
        "volume_id": "521752a6-acf6-4b2d-bc7a-119f9148cd8c",
        "create_info": {
            "snapshot_id": "421752a6-acf6-4b2d-bc7a-119f9148cd8c",
            "type": "qcow2",
            "new_file": "new_file_name",
            "id": "421752a6-acf6-4b2d-bc7a-119f9148cd8c"
        }
    }
}