Rsync is not the best fit tool for Btrfs. Better use btrfs send.
You find good Information about Btrfs in the wiki
Btrfs snapshots take up significantly less disk space than rsync, even when transferred to external storage with send!
I have 1600 (!) snapshots of 5 pcs on 1 external disk with 2TB (<50%full) 
(Try to do this with rsync 
without using backuppc)
And every snapshot is mountable 
