appending to a memory-mapped file

I'm constantly appending to a file of stock quotes (ints, longs, doubles, etc.). I have this file mapped into memory with mmap.

What's the most efficient way to make newly appended data available as part of the memory mapping?

I understand that I can open the file again (new file descriptor) and then mmap it to get the new data but that seems to be inefficient. Another approach that has been suggested to me is to pre-allocate the file in 1mb chunks, write to a specific position until reaching the end then ftruncate the file to +1mb.

Are there other approaches?

Doest Boost help with this?