I worked at a company that built a custom database on top of mmap, back in the early 2000's. It was single threaded, intended for very limited OLTP use cases,

I worked at a company that built a custom database on top of mmap, back in the early 2000's. It was single threaded, intended for very limited OLTP use cases, not a generic DBMS, but worked wonderfully for those use cases. A couple gigabytes was more than enough address space. During this time, a machine with 4 gigabytes RAM was considered "high end"! It only ran on a couple of posix platforms. At startup, it would read everything into memory so everything was hot, ready to go. The "database" was intended to run on a dedicated node with no other applications running. It also kept a write ahead log for transaction recovery.