Let's consider the following scenario. I have a development container in /var/lib/machines/gentoo-amd64 where I build most of the software. Then I use the binpkgs there to speed up upgrades of my main system. As a result, most (but not all) files from rootfs are duplicates of files in the development container. I would find it really helpful to have an auto-deduplicate function in Portage. Basically, I'd set something like: AUTO_DEDUPLICATE="/var/lib/machines/gentoo-amd64" and while merging, Portage would compare every installed file against "${AUTO_DEDUPLICATE}/${path}" and if it were identical, used the equivalent of `cp --reflink=auto` from that path instead of installing the file from image. This should provide for roughly permanent space savings without the need to repeatedly run duperemove, and the performance impact of that.
Hmm, or instead of copying from the dedupe source, it could try calling the kernel sysctl for deduping — that would probably be both easier and safer.
