[OSM-dev] mapnik blank (sea/land) tiles and hardlinking

[Joe Richards]

I am generating a bunch of tiles for a rendering of the data imported from LINZ/NZOGPS (pre-merge).

There are obviously quite a few sea & land tiles, and my virtual machine is likely to run out of space.  I got thinking about this, and realised the land (grey) tiles could be hardlinked to a single grey image, and the sea (blue) tiles could be hardlinked to a single blue image.

The blank tiles are all 256 bytes, but I assume they will use up a disk block (I think 4k).  There are more than half a million of these, and at 4k, this makes about 2GB (and counting), ie 76% of the overall tileset is rubbish.

I did an md5sum on them, and for some reason the grey (land) ones all have the same checksum:
e.g. 91591e38499af234ff1e55b634329fed  13/7906/5276.png

The blue ones, however, don't seem to have this property for whatever reason:
md5sum 13/7906/5279.png
80d8ddebacbc0a7a16ef799f4b9763d7  13/7906/5279.png

md5sum 13/7906/5280.png
edede53ee6ba4e06efa8ae2bdb3965a4  13/7906/5280.png

md5sum 13/7906/5281.png
3fe002719775913621f3ef23f0d2589b  13/7906/5281.png

Anyway, has anyone got any recipes for hardlinking these?  I'm not sure why the blue tiles have different content (even if they are all rendered blue), any ideas?