What I dislike most is the GDBM header, it contains all the important information and locates in just one allocation unit. So each tile update it needs to write to the header once, you have 50k tiles, it write 50k times.

If the operating system doesn't have the dynamically changing allocation support for SD card(I don't know if maemo has or not), it pretty much ensure a kill of that block some time down the road.