Did you ever down-truncate it? Did your backup program take forever to crunch through non-existent 4TB (and take as much space to restore?)

Atomic access wise, that’s fine as long as you don’t map it through a network file system if any sort; also, $DIETY help you recover a consistent state if the power goes out or you kernel panic

Mmap is awesome, I use it all the time. But writing into mmaps robustly is hard. Best example to learn from I am familiar with is symas LMDB source code.