http://gen.lib.rus.ec/dbdumps/

It's something like 75TB last time I checked, over 82 million articles in total. Each torrent contains 100,000 articles, named by DOI.

If sci-hub ever gets taken offline, there are enough full backups out there that it can be re-instated fairly quickly (at least the archive of existing articles; the credentials necessary to obtain new ones are not included in these torrents or database dumps).