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.

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.