The problem of creating tiles seems to be really simple: You have the OSM data
in one end, which can be downloaded from [here](http://download.geofabrik.de/osm/),
and the `.png` tiles in the other. In the middle there should be
something that reads that data, applies some templates or description, and
generates the tiles. But life is never so simple.

`mapnik` cannot read any of OSM's exported files (XML or `.pbf`), but only from
a SQLite or PostgreSQL/GIS database; and we can only conver to the latter either
with `imposm` or `osm2pgsql`, so that road we go. It's mostly a matter of
following TileMill's page about
[using OSM's data](http://mapbox.com/tilemill/docs/guides/osm-bright-ubuntu-quickstart/)
(and some template called [`osm-bright`](https://github.com/mapbox/osm-bright/tree/master/osm-bright)).
For Debian `sid` you follow its instructions for Ubuntu Oneiric Ocelot.

Importing the data should be as simple as:

    mdione@mustang:~/src/projects/osm$ sudo su -c "osm2pgsql --database osm --input-reader pbf --verbose --create $(pwd)/france.osm.pbf" postgres
    [...]
    Unable to open /home/mdione/src/projects/osm/data/france.osm.pbf

No error message. `strace` gives us a clue:

    open("/home/mdione/src/projects/osm/data/france.osm.pbf", O_RDONLY) = -1 EOVERFLOW (Value too large for defined data type)

A little bit cryptic, but basically it says the file is too large. So I
downloaded only the region where I live (for some countries there are individual
files) and after some cache tweaking:

    mdione@mustang:~/src/projects/osm/data$ sudo su -c "osm2pgsql --database osm --input-reader pbf --verbose --cache $((1024+512)) --create $(pwd)/provence-alpes-cote-d-azur.osm.pbf" postgres

This time I got it right. Some numbers so you have an idea how much time and
space this takes:

    Input file size: 184054kB
    Processing: Node(19153k 832.8k/s) Way(3019k 23.59k/s) Relation(9110 112.47/s)  parse time: 233s
    node cache: stored: 19153339(100.00%), storage efficiency: 16.41% (dense blocks: 113981, sparse nodes: 0), hit rate: 0.00%
    Osm2pgsql took 2125s overall
    Final DB size: ~1GiB

That's some 35 minutes.

Once finished you fire TileMill, create a new project and add a PostGIS layer.
It took me some time to figure out what to put in the different fields, even when
[there is a tutorial](http://mapbox.com/tilemill/docs/guides/postgis-work/) for
that, but I used these, mostly taken from inspecting the database's schemas:

* ID and Class: osm-roads
* Connection: dbname=osm host=localhost user=postgres
* Table or subquery: planet_osm_roads
* Unique key field: osm_id
* Geometry field: way
* The rest: default

And then add the following to the `style.mss` which you can edit in the right
box of TileMill:

```css
#osm-roads {
  ::outline {
    line-color: #7f0000;
    line-width: 2;
    line-join: round;
  }
}
```

The result is kind of dissapointing: first, you have to specify more in the
"Table or subquery" field, because the data actually is not only some kind of
roads but also borders. I used
`(select * from planet_osm_roads where highway!='') as foo` for all roads and
`(select * from planet_osm_roads where boundary='administrative') as foo` for
borders as a first attempt to be more selective. Second, as I said, it's only
the main roads (down to
secondary in [OSM's terms](http://wiki.openstreetmap.org/wiki/Map_Features#Highway),
list which by the way seems to grow everytime I see it), but nothing smaller.
But as a start I think is enough.