[thristian]

The POSIX standard requires certain behaviours from the filesystem, that POSIX-compliant software can rely on.

Unfortunately, those behaviours are mutually exclusive with transparent layering.

It's certainly possible to build a file-system whose behaviours are compatible with that kind of transparent layering - Plan9 was built on exactly that model, for example - but then it wouldn't be a POSIX-compliant filesystem anymore.

The promise of Docker was that you'd be able to deploy your existing applications in a more reliable, repeatable way, but that breaks down when you have to tinker with your application's file-handling code, or jump through extra hoops to flatten the layers of your container's filesystem image.