We had a discussion on IRC about how creating a file for each user won't be a good idea in the future as it would create too many files as the number of users increases.

The basic idea is: "So to recap, keep as many users as possible in 1 GB files with each user's listens in one file always, keep filename, offset and size for each user in index, and use uuids for filenames and directory structure"

See IRC conversation here: https://chatlogs.metabrainz.org/brainzbot/metabrainz/2017-12-29/?msg=4085037&page=1