DBMSes hardly use the filesystem

RDBMSes like Oracle implement their own... well, their own pretty much everything. With modern OSes there is little performance advantage to putting your database on a raw partition, so you create a huge file, but that's about as far as filesystem use goes. The DBMS will typically have its own page cache and even its own internal process scheduling. A Unix filesystem isn't designed for indexing lots of fixed-size 'rows'.