@classmethod
def update(cls, resource, name, size, snapshot_profile, background, cmdline
    =None, kernel=None):
    """ Update this disk. """
    if isinstance(size, tuple):
        prefix, size = size
        if prefix == '+':
            disk_info = cls.info(resource)
            current_size = disk_info['size']
            size = current_size + size
    disk_params = cls.disk_param(name, size, snapshot_profile, cmdline, kernel)
    result = cls.call('hosting.disk.update', cls.usable_id(resource),
        disk_params)
    if background:
        return result
    cls.echo('Updating your disk.')
    cls.display_progress(result)
