It would be useful to allow "lfs setstripe --copy FID" (and "lfs mirror extend ..." and "lfs migrate ...") to inherit the default layout from "FID" instead of having to specify a full pathname. This is useful for operations that are working on the source file by FID and don't know the full pathname of the parent (and don't want the overhead of doing fid2path repeatedly).

In theory it would be possible to also use "MOUNT/.lustre/fid/FID" for this, but at some cost in usability.