pdumpfs: a daily backup system similar to Plan9's dumpfs

What's pdumpfs?

pdumpfs is a simple daily backup system similar to Plan9's dumpfs
which preserves every daily snapshot.  pdumpfs is written in Ruby.
You can access the past snapshots at any time for retrieving a certain
day's file.  Let's backup your home directory with pdumpfs!

pdumpfs constructs the snapshot YYYY/MM/DD in the destination
directory.  All source files are copied to the snapshot directory for
the first time.  On and after the second time, pdumpfs copies only
updated or newly created files and stores unchanged files as hard
links to the files of the previous day's snapshot for saving a disk
space.