Could you give p7zip hardlink support? A hardlink entry in a .7z
file would mean something like "create a hardlink if supported and
not disabled by a user option, otherwise make a file copy".
The preferred format should be to store a file's names before its
contents. Otherwise attempting to extract a single file can fail:
If B is archived after A as a hardlink to A, '7z x foo.7z B' has
bypassed A by the time sees B and realizes it needed A's contents.
If each hardlink entry would contain the name of the file where
the actual contents is stored: I think p7zip should *not* blindly
link B to A when it sees an entry "B is a hardlink to A". It
should report an error if it does not know A on the disk matches
the A in the archive or was just extracted from the archive.
Otherwise '7z x foo.7z B' could get an old copy of B (linked to
old A on the disk) instead of the one in the archive.