prune empty dirs doesn't seem to be doing so.... && optimizations?

[Linda Walsh]

I'm using: (according to ps)
/usr/bin/rsync --archive --human-readable --acls --xattrs --hard-links 
--no-inc-recursive --prune-empty-dirs --whole-file --one-file-system 
--compare-dest=/home/. --exclude=**/.recycle/**  
--exclude=CPAN-ishtar-build-cache/** 
/home/.snapdir/@GMT-2012.10.11-02.23.25/. /home.diff/

But the difference FS systems get a full compliment of every dir in the 
source.

Perhaps it wasn't well tested when using a 3rd dir to compare against?

Also, anyway to optimize the same-system case (using shared memory). 

Also an option to use inode numbers would greatly speed things -- since 
these
are snapshots of each other both file systems will have the same inode 
numbers
for files that equal.

As it stands, running the above rsync takes about 90 minutes/invokation.

The resulting 'differences' can be copied by "cp" in usually under 1 minute.
(a few Gig or less).