During the early part of the Maverick cycle we once again revisited out Union Mount solution. At that time VFS union-mounts was the hit of the day, set to finally to produce something which might get into the kernel. Since then the complexity of changing every filesystem to support whiteouts, its invasiveness, and its affects on Posix semantics have lead to it falling by the wayside. In its place has sprung overlayfs.
overlayfs is a small patch set which is a hybrid of the VFS union-mount approach and that of aufs/unionfs in that it also provides a filesystem. This greatly reduces the complexity of the patch set, reducing its invasiveness and thus increasing its chances of ever being merged. So much so simpler is it that your author is actually able to understand and debug it. Win.
We have been tickling overlayfs for most of the Natty cycle, but with Natty in the can I have had had some time to catch up with its development and help out a little, both with testing and bug fixing. Culminating today in my being able to inject a kernel containing overlayfs support into an Ubuntu LiveCD and boot it, then update it to the latest Natty, all without error.
overlayfs may shortly be in a mergable state, nirvana for all union mount lovers. Only time and testing will tell.