Partitioning may be the answer but this is a huge problem for corporations like Facebook (and the one I am working for). If things have been done with one giant

Partitioning may be the answer but this is a huge problem for corporations like Facebook (and the one I am working for). If things have been done with one giant repo from day one, splitting it is going to be a major engineering/political/social problem when there's thousands of engineers working on the code base and you can't just shut down business for the duration of the migration.