The Overlay File-System has been in development for several years and is used by some notable Linux distributions, but has yet to be merged into the mainline Linux kernel after having to be pulled a few times in the past. The new plan is to merge OverlayFS for the Linux 3.10 kernel
With the patches now up to their sixteenth revision, Miklos Szeredi has called upon Linus Torvalds and Al Viro (the VFS maintainer) to consider pulling OverlayFS for inclusion into the Linux 3.10 release.
Szeredi writes in his latest plea with the Linux kernel developers, "It's included in Ubuntu and openSUSE, used by OpenWrt and various other projects. I regularly get emails asking when it will be included in mainline."
Introducing the new Linux file-system is made-up of thirteen patches and the v16 revision to them can be found on the kernel mailing list
As far as the Overlay File-System approach to a union file-system, the kernel documentation describes itself:
This document describes a prototype for a new approach to providing overlay-filesystem functionality in Linux (sometimes referred to as union-filesystems). An overlay-filesystem tries to present a filesystem which is the result over overlaying one filesystem on top of the other.
The result will inevitably fail to look exactly like a normal filesystem for various technical reasons. The expectation is that many use cases will be able to ignore these differences.
This approach is 'hybrid' because the objects that appear in the filesystem do not all appear to belong to that filesystem. In many cases an object accessed in the union will be indistinguishable from accessing the corresponding object from the original filesystem. This is most obvious from the 'st_dev' field returned by stat(2).
While directories will report an st_dev from the overlay-filesystem, all non-directory objects will report an st_dev from the lower or upper filesystem that is providing the object. Similarly st_ino will only be unique when combined with st_dev, and both of these can change over the lifetime of a non-directory object. Many applications and tools ignore these values and will not be affected.
Linus Torvalds looks like he may finally pull the file-system for Linux 3.10. He already responded to the thread
Yes, I think we should just do it. It's in use, it's pretty small, and the other alternatives are worse. Let's just plan on getting this thing done with.
Al, I realize you may not love this, but can you please give this alook? People clearly want to use it. In particular the new interfaces, like the inode ops open function with dentry passed in or whatever? The changes outside of overlayfs looked fine to me.
Al's response at the moment is, "I'll post a review tonight or tomorrow. FWIW, I was not too happy with it the last time I looked, but I'll obviously need to reread the whole thing."