|Welcome to the Monastery
Wow, that's a lot of bad advice for one message!
I'm amazed to see anyone offering SysV IPC as a solution these days - it was a bad choice years ago when I last used it. So many limitations, so many traps, so much pain, so DAMN SLOW!
I have no idea what mmap would gain you here aside from shared storage. This is clearly a database problem, so you might as well use one. Most likely BDB and MySQL are both using mmap for you.
And for god's sake, don't write your own DB in C!