"Btrfs (B-tree file system, variously pronounced "Butter F S", "Butterfuss", "Better F S",[1] or "B-tree F S"[2]) is a GPL-licensed copy-on-write file system for Linux. Development began at Oracle Corporation in 2007. It is still in heavy development and marked as unstable. Especially when the filesystem becomes full, no-space conditions arise which might make it challenging to delete files" http://en.wikipedia.org/wiki/Btrfs

You are very brave. I don't dare to store my backups on a filesystem still marked unstable.