[ceph-users] easy way to find out the number of allocated objects for a RBD image

Hi, Other than get all objects of the pool and filter by image ID, is there any easier way to get the number of allocated objects for a RBD image? What I really want to know is the actual usage of an image. An allocated object could be used partially, but that's fine, no need to be 100% accurate. To get the object count and times object size, that should be sufficient. "rbd export" exports actual used data, but to get the actual usage by exporting the image seems too much. This brings up another question, is there any way to know the export size before running it? Thanks! Tony