[xxs]

There is zero issue to have a flat file database that's all multi. MySQL InnoDB, the transactional one engine, can be a single file database. [Small exception - part of the schema is stored on two separates files for easier access, there won't be a big deal to store them in the big file. Transaction log is also not in the same file for obvious reasons]

If you want a full serialization of the memory to the 'flat' file - then no. It just makes no sense. Yet, a single file databases that's multi-user, mulch-threaded and mult-transaction is all viable.