Great article. I'm very much looking forward to your next article, with practical advice about the various union filesystem options and their performance.
I suspect there is a relatively new use case for union filesystems, due to the increasing popularity of solid state drives. For example, I help run a server with many hundred gigabytes of small files on on XFS, on RAID-1, on rotating rust. There is enough reading and writing (and therefore seeking) going on to push the performance limits of the storage subsystem. So I bought one of those whiz-bang Intel SSD drives, with the intention of sending all writes to the flash drive, and using the rotating rust purely for reads (at least during normal operation). After some investigation of various unionfs options, I tried mhddfs since it sounded like the simplest thing to deal with. It didn't perform so hot, with load shooting up to about 700 before I killed it (maybe it was trying to replicate the admittedly large directory structure somewhere? Who knows?) So now I'm back to the primitive "manage everything with a rats nest of symbolic links" strategy.