In my writing about dar, I recently made that point that dar is a filesystem differ and patcher. We can exploit this property to do something really cool: build an Asynchronous rsync. What does that mean? rsync is a tool that has been in may *nix admins’ toolboxes for years. Typically used over ssh, rsync will compare the state of a local directory tree (or file) to the state of a remote tree, and efficiently make the remote match the local (or vice-versa).