If you only mmap the file, how do you handle the case of inserting or deleting bytes in the middle of a large file? Answering that might also help imagining wh

If you only mmap the file, how do you handle the case of inserting or deleting bytes in the middle of a large file? Answering that might also help imagining why and how B+Trees are used (the "+" for iteration actually being at least extremely useful, if not important).