if he's anything like me, it's virtual machines. i have about 50 virtualbox disk images, about 100 linux containers and since i've enabled deduplication i've become extrely lazy and use duplicity instead of hardlinks for my files. e.g. i have same photos in ~/Pictures/by_occassion/whatshisname_wedding as i have in ~/Pictures/by_date/2013-06. i know it's bad practice but if the FS can deal with it, why not?