access or interface to list of blocks that have changed via C.O.W.?

I'm trying to use snapshots that are active for a day or so to grab differences between successive days for allowing me to display previous versions of files over a month or so...

They aren't a replacement for my daily towero'hanoi-backups, but they are a great deal easier to use.

The chink in my plan is the time it is taking to do a diff -- not really a fatal chink but annoying. It takes anywhere from 80-100 minutes for rsync to sum up the changes and copy them to a third volume each day.

I figured well sheesh, the lvm should already know what sectors are different. If I could combine that info against a inode+blockmap, I might get that diff computation down to under 10 minutes maybe under 5.

The contents of the files that are computed as differences by rsync usually take a minute or less to actually copy into final volume -- anywhere between
800Mb - 2.5GB out of a 1TB volume. So I I could shortcut that diff process,
might make it practical to do more than one snapshot a day;...

Is there some interface for getting at that information or would it have to be written from scratch?

Thnks...

Linda