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[linux-lvm] 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?



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