It's in the MM2.x application code. The old gdbm always sync the data with disk no matter how small change it is. The gdbm 1.8.3 seems to give user some choice and provides a sync command(basically fsync), you can choose to sync at your time.

The MM2.x does sync for each tile downloading. The structure of gdmb requires you to update data, hash table and header, not very good. But nevertheless, even you don't issue sync command, the gdbm will do a lot of flushing disk also since it has very little cache allocated.