Investigate stronger overlayfs integration, deprecate rofiles-fuse

[cgwalters]

In Linux 5.11, there's support for unprivileged overlayfs mounts: https://kernelnewbies.org/Linux_5.11?highlight=%28overlayfs%29#Unprivileged_Overlayfs_mounts

ostree was intentionally very conservative with kernel features (we just require link() with some more recent support for optionally using reflinks). But I think we should experiment with overlayfs+ostree and better tie them together.

We've long dipped our toes in this water with ostree admin unlock.

If successful I think we should consider rofiles-fuse deprecated after the basics of that are done (and port e.g. rpm-ostree away from it).

This is also related to #2259 - what would ostree look like if we supported a hard dependency on overlayfs?

[wmanley]

FWIW this wouldn't work for us. On our embedded device we're stuck on an ancient kernel that doesn't even have overlayfs - or at least doesn't have one that works very well. A dependency on overlayfs for the image creation tools wouldn't be a problem, but for deploying it would be.

[cgwalters]

Thanks for the feedback. To be clear this is just an "investigate" issue - I'm definitely not proposing any kind of hard switch to an overlayfs dependency in the next few years at least. In particular even just on the build side, it's going to take a while for 5.11 to propagate.