By now, the /usr-merge is an old transition. Effectively, it turns top-level directories such as /bin into symbolic links pointing below /usr. That way the entire operating system can be contained below the /usr hierarchy enabling e.g. image based update mechanisms. It was first supported in Debian 9, which is no longer in active use at this point (except for users of Freexian’s ELTS offer). When it became mandatory in Debian 12, it wasn’t really done though, because Debian’s package manager was not prepared to handle file system objects being referred to via two different paths. With nobody interested in handling the resulting issues, Freexian stepped in and funded a project lead by Helmut Grohne to resolve the remaining issues.

While the initial idea was to enhance the package manager, Debian’s members disagreed. They preferred an approach where files were simply tracked with their physical location while handling the resulting misbehavior of the package manager using package-specific workarounds. This has been recorded in the DEP17 document. During the Debian 13 release cycle, the plan has been implemented. A tool for detecting possible problems was developed specifically for this transition. Since all files are now tracked with their physical location and necessary workarounds have been added, problematic behavior is no longer triggered. An upgrade from Debian 12 to Debian 13 is unlikely to run into aliasing problems as a result.

This whole project probably consumed more than 1500 hours of work from Debian contributors, of which 700 were sponsored by Freexian through the work of Helmut Grohne. What remains is eventually removing the workarounds.