I didn't realize that. I'm sure there are third party repositories that add kernels that support AUFS. I compile my own kernel and AUFS on Ubuntu right now anyways so that I can use a 4+ kernel and AUFS4. It's not that challenging, but I understand this is a big step for most users. mhddfs doesn't work well for me because of the disconnects that occur from large transfers or rsyncs.Nope - it it included in many/most debian/ubuntu based distributions, but is neither available or easily added to Fedora, RHEL, CentOS, etc. where you must use a 3rd-party kernel (or compile your own) to get AUFS support. Not available in Gentoo either without some extra work which is what I'm running on my snapraid box (I use mhddfs for pooling).
And @OBasel is right about it having a low chance of being upstreamed. Especially since OverlayFS was merged into the kernel in 3.18 - if I was building a new snapraid system now I would be looking at OverlayFS.
Thanks for the comment, but Overlayfs isn't a solution for a media server. It is one filesystem laid on top of another. And the "lower" filesystem is always read-only. As result, it doesn't meet the needs most people would be using SnapRAID for, a fileserver. The primary use of it are livecd/liveusb distros. If you know if a different way to use Overlayfs that overcomes these shortcomings, I'd love to have a third option for pooling.