Memory-Mapped MappedByteBuffer or Direct ByteBuffer for DB Implementation?

Question

This looks like a long question because of all the context. There are 2 questions inside the novel below. Thank you for taking the time to read this and provide assistan

Answer 1

I think you shouldn't worry about mmap'ping files up to 2GB in size.

Looking at the sources of MongoDB as an example of DB making use of memory mapped files you'll find it always maps full data file in MemoryMappedFile::mapWithOptions() (which calls MemoryMappedFile::map()). DB data spans across multiple files each up to 2GB in size. Also it preallocates data files so there's no need to remap as the data grows and this prevents file fragmentation. Generally you can inspire yourself with the source code of this DB.