`git annex migrate` leaves old, unlinked backend versions lying around. It
would be great if these were purged automatically somehow. 

> Yes, this is an issue mentioned in the
> [[tips/migrating_data_to_a_new_backend]].
> 
> Since multiple files can point to the same content, it could be that
> only one file has been migrated, and the content is still used. So
> the content either has to be retained, or an operation as expensive
> as `git annex unused` used to find if something else still uses it. 
> 
> Rather than adding such an
> expensive operation to each call to migrate, I focused on hard-linking
> the values for the old and new keys, so that the old keys don't actually
> use any additional resources (beyond an extra inode).
> 
> This way a lot of migrations can be done, and only when you're done you
> can do the more expensive cleanup pass if you want to. --[[Joey]]
> [[done]]