mpd startup notes
What I had to do to make mpd work
mpd uses shared memory to load the music database.  For large MP3
collections, the sysctl "kern.shminfo.shmall" might need to be
increased.

I added this to my /etc/sysctl.conf:
 kern.shminfo.shmall=9216

and then I ran:
 $ sudo sysctl -w kern.shminfo.shmall=9216

You should not have to make changes like this unless mpd complains about
an insufficient amount of shared memory.