[OSM-dev] render_expired for zoom 19 and metatiles

[Stephan Knauss]

During my Christmas vacation I set up a rendering up to zoom 19 on 
thaimap.osm-tools.org

I recompiled mod_tile with MAX_ZOOM 19 (render_config.h)
I set maxz in tirex backend config
I even adjusted the maximum zoom in the munin scripts.

After that all looked quite nice.

Now I discovered my tile expiration not working as expected.
I feed osm2pgsql expire list into render_expired.
Inside render_expired the max zoom is also hard-coded to 18 in a lot of 
places (at least the 18 appears more than once)

Question:
Would changing all these magic numbers into 19 do the trick? I would 
skip the twopow array. Or will I be breaking something not that obvious?

Additional question:
The more I look into the metatile format the more it is confusing me.
Why is the -e parameter of osm2pgsql my maxzoom-3?
This comment adds confusion:
     // excess_zoomlevels is how many zoom levels at the large end
     // we can ignore because their tiles will share one meta tile.
     // with the default METATILE==8 this is 3.

Will metatiles really share tiles of *different* zoom levels?
The different zooms have different styles in mapnik...

http://wiki.openstreetmap.org/wiki/Tirex/Internals#Metatile_File_Structure
Does not list the zoom per entry. So all tiles inside a metatile would 
be from the same zoom level.

Looking in my filesystem I also have directories for all zoom levels 
down to 19, all containing meta files.

So it is more like a metatile contains 64 tiles of ONE zoom level.

But if it's the later: How do these tiles get expired if all tools stop 
3 levels ahead?

I asked some questions regarding metatile removal before. In this thread 
it was mentioned two times that the filesystem structure needs to be 
adjusted for higher zoom levels. Why?

Stephan