On Fri, Sep 12, 2025 at 5:55 PM Nicholas Hubbard <nicholashubbard@xxxxxxxxxx> wrote: > > Hello everybody, > > I have just started working with overlayfs, and ran into a problem. Specifically > I wanted to overlay the root of my filesystem with a command like the following: > > # mount -t overlay overlay -o lowerdir=/,upperdir=./tmp-upper,workdir=./tmp-work ./merged > > However, I noticed that my nested btrfs subvolumes and boot partition were not > included in ./merged. I quickly learned though that you could have multiple > lowerdirs. So next I tried the following command (I have a nested btrfs subvolume > at /home, and my boot partition mounted at /boot): > > # mount -t overlay overlay -o lowerdir=/:/home:/boot,upperdir=./tmp-upper,workdir=./tmp-work ./merged > > I was expecting that now I would have (for example) the following directories: > > ./merged/home/$USER > ./merged/boot/grub > > However I instead had: > > ./merged/$USER > ./merged/grub > > Which shows that all the lowerdirs are placed right at the root of the mergedir. > > So I have two questions. > > 1. Is there a (easy) way say "I want to include all nested mountpoints into the > overlay in their same directories"? No. not in general. but maybe specifically for btrfs subvolumes you can do it because they are all accessible via the btrfs root mount. If all your lower dirs are on the same btrfs filesystem and the subvolumes are also accessible via some path is root mount (e.g. /subvols), then something like may work for you: mount -t overlay overlay -o lowerdir=/,upperdir=./tmp-upper,workdir=./tmp-work ./merged rmdir ./merged/home rmdir ./merged/boot mv ./merged/subvols/home ./merged/home mv ./merged/subvols/boot ./merged/boot Never tried it with btrfs subvols, so it may not work. > 2. If there is not a (easy) way to do this, do you think it would be both feasible > and useful to add such a feature? The general case of "include all nested mountpoints" is just way too far off from the current overlayfs design. Too hard to explain why. I suggest that you look at FUSE alternatives like https://github.com/trapexit/mergerfs Not sure if mergerfs supports this use case, but I think a userspace union fs is in a better position to add support to this if desired. Thanks, Amir.
