Could anybody point me where to find the internal structures of the COW devices in /dev/mapper?

What I want to do, exactly, is: make a backup of the snapshot in an image file, reading all the used blocks, and leaving the "not-used" ones. Later, I want to get the image file, and restore the cow device. But I can't find an LVM document describing exactly how the COW structure is stored in disk.

Thanks,