This is interesting. Do you know for sure that YouTube actually runs on literally the same disks as the Google search index? Because I used to work for Bing and they definitely did not let any other services share hardware with the search hotpath. Serving search results was a case where every millisecond counted (even delays below the threshold of perception caused a stat-sig drop in revenue), and so even suggesting that the index should share disks with e.g. OneDrive would've gotten you laughed out of the room.