> My question is: how is this better than mmapping the file

for a non-toy database you typically need to know that the data
you are writing has gone to disk.  mmap doesn't give you that.
mmap may be good enough for read, but there aren't too
many read-only database managers.

mmap also doesn't cope to well with anything other than a small
database - you spend too much time re-mapping a window onto one
of many data files.

jan-simon.