Say you want to go to a (large, as in Turkey for me) foreign country where mobile data is expensive, find out that Ovi maps (that can be downloaded like this - use the ones marked to work for 3.04) are of no use and turn to Mappero and Google Maps.

The problem is as follows. Mappero stores tiles by default in the folder /home/user/MyDocs/.maps/ which is a FAT32 partition. On N900, this means that each file takes up 64kB space no matter how small it is (some tiles Mapper downloads are just 177 bytes small, but they still take up 64kB). This information comes from here.

We will solve it by creating a loopback device (a filesystem in a file) that overcomes this limitation. We will be using ext2 filesystem - N900 wouldn't mount ext3 or ext4 for me. As we will be storing the file on N900 itself (and therefore on a FAT32 filesystem), the file can be only 4GB in size, but it should be hopefully enough. If you need more, buy a microsd card, use some saner filesystem then FAT32 on it and store the file there.

This can be (I guess) done on N900 itself, but I used my Ubuntu system (but any linux should do).
First create a file
Then make it 4GB large (it is small enough to fit on FAT32)
Now create an ext2 filesystem on it:
(based on this and mkfs.ext2 man page)
-i switch specifies, that we will have one inode per each 1024 bytes. Since for every file we need one inode, this means we will be able to have around 4 million files in this device. This is needed because lots of files are smaller then 1024 bytes. The value could be set to 2048 or maybe even 4096 (to have 2M or 1M inodes respectively) which would probably free some more space, since inodes take up space too. You can play with that.

-L is a label, you can set it or ignore it

-m 0 specifies that superuser will not have any allocated space. He or she does not need the space anyway in this file, so we might as well use it.

-b specifies that a block size will be 1024. ext2 (or ext 3 or ext4) does not allow smaller block sizes.

Now copy .mappero_image (or whatever you call the file) to the device and mount it with You may want to copy the files in .maps directory onto .mappero_image first - just mount it to some random directory, copy the things over and remount it. If mappero refuses to work, check permissions and that the file poi.db is present.

If everything works as it should, here is how we make it work automatically even after reboots. Since N900 recreates /etc/fstab on each boot, we cannot just put a line there. There are basically two ways - modify the script that autocreates fstab (which is scary, so we will not be doing that) or write a one-purpose upstart job, which is what we will do.
Create a file in /etc/event.d/ and put the following into it:
This just starts script /etc/event.d/mount2, which we must create. The location and name of this script can of course be changed, that is what I used. Put this into the script:
Do not forget to make the file executable!
The script just waits 40 seconds and then mounts the file. I am sure this can be done better, but I do not know hot to do actions in upstart based on what is mounted (ideally, it would work like "start when /home/user/MyDocs is mounted). But this works. If it does not, try to increase the sleep time (i.e. 30 did not work for me).


Now, we can proceed to downloading maps.
To download maps beforehand for later offline use, proceed as follows: (by the author of mappero here)

If number of maps is large (more than 10 000 or so), N900 might start to be very unresponsive or appear to be dead. But it probably is not and just is very busy downloading the maps. You can check over SSH on how it is doing.

I found out that around 250 000 (most of Turkey on zoom 7, or area between 36° and 42° latitude and 30° and 45° longtitude) maps takes up about 450MB, which is about 15% of the file. It took 6 hours to download it (I think the bottleneck was N900 itself) so you might want to plug the device to an adapter because it is very battery intensive. So you could get around 1 500 000 maps onto single file I gues (but YMMV). If that is still not enough, I guess you can use more such files and symlink them in a smart way.

Please note that it seems at the moment that Google Satellite does not work with mappero in zooms below 8 r so. If it works for you, I would be very happy to hear about it. Also, if something does not work or could be done better, say so!

Update: fat filesystem allows lower block size so maybe it is actually better.

You create it with (dd command holds):
Busybox mount is not smart enough to understand enough options to make the file writatable by mappero so download this, unpack it
(.debs are just archives) and put the file mount onto N900. Move it to /usr/bin/ and name it mount2 and make it executable (chmod +x mount2)

Put this instead of the version for ext2 to /etx/event.d/mount2 (I know, my naming sucks):