SquashFu - an alternative backup solution
  Inspired by http://forums.gentoo.org/viewtopic-p-4732709.html

Requirements: aufs, aufs2-util, squashfs-tools, rsync

Goal: To create a backup solution which provides incremental backups and compression,
      and which also provides an easy way to roll back.

Design:
  A directory structure is created as follows:
      backup_root/
       |- seed.sfs
       |- ro/
       |- rw/
       |- bins/
           |-1/   <-- Monday incremental
           |-2/   <-- Tuesday incremental
           |-3/   <-- Wednesday incremental
           |-4/   <-- Thursday incremental
           |-5/   <-- Friday incremental
           |-6/   <-- Saturday incremental
           |-7/   <-- Sunday incremental

  seed.sfs is created from an initial backup and compressed using SquashFS, which i
  is simply a filesystem which focuses on compression, but is read only. It's mounted,
  using a loopback device, on ro/. Using aufs2, a union mount is formed by ro/ and each 
  of the numbered bins, each corresponding to a day.

  Wait, wat? What the heck IS a union mount? In simple terms, a union combines multiple
  directories (which themselves could also be mount points) into a single resultant
  mount point. Using our directory structure above as an example, each day (bin) is 
  overlayed (in order, its important) on the base (seed.sfs) to create what appears 
  to be an up to date backup in rw/. Skip the rest if you've had enough.

  At the time of the backup, the day is determined in order to prepare the union. If
  today is Thursday, `date +%u` returns 4. In order for rsync to properly create our
  incremental, we need to compare current data with the seed plus incrementals leading
  up to Thursday, so we create our Aufs mount with the branches bins/4, bins/3, bins/2,
  bins/1, and ro/ (in that order). When rsync now writes to the resulting union, aufs
  happily receives the data into the first available (writable) branch of the union,
  which is bins/4. The next day, the union mount is reformed adding the next bin...

  If and when you want to roll back, the process is simple. Mount the seed plus
  the necessary bins to increment the seed up to the day of interest.

Warnings:
  It is imperative that you do NOT touch the individual branches. You can and will
  cause irrepairable damage to the union, rendering your incrementals worthless.


Further reading:
   http://en.wikipedia.org/wiki/Aufs
   http://en.wikipedia.org/wiki/UnionFS
   http://aufs.sourceforge.net/
   http://en.wikipedia.org/wiki/SquashFS
   http://en.wikipedia.org/wiki/Rsync