Retrieve changed block with snapshots

Hi all,
I'm using LVM2 as VM backend and I am very happy with the result.

One thing that I feel useful but that I can't figure how to get working is to use snapshots to obtain the list of changed block after the snapshot was taken.

To be more clear:
- immagine to have a LV called "LV1"
- take a snapshot of "LV1" and name it "SNAP1"
- now "SNAP1" has a point-in-time image of "LV1", and by reading from it I can go "back in time" to when I issued the snapshot
- but how I can get the list of only the changed block between "LV1" and "SNAP1"?

Its my understanding that some Ruby/Python scripts can read the COW table and retrieve the list of the changed block, however I am not so inclined to use them on production server. So, we have a method _inside_ the current LVM tools that enable us to retrieve the changed block list?

Regards.