https://www.quora.com/Why-does-Wikipedia-use-MySQL-as-a-data-store-rather-than-a-NoSQL-database-since-it-scales-and-will-be-economic/answer/Domas-Mituzas

Billions of views hit the cache layer, not the application - application generates only about 50000 queries a second.

Now, there are two kinds of databases serving the wikis - 'core' - which has all the metadata, and text storage.

There're six shards for core database, split by language (so requests don't really have to hit multiple shards) - and they're all under 500G, and active dataset for each shard fits into 64GB (on some into 32GB) systems and doesn't thrash disks much.