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

Chris Jones rollercow at sucs.org
Wed Jun 24 16:28:14 BST 2009


Joe Richards wrote:
> 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.
>   
Here blank tiles are 103bytes...

Yes, a fair way to reduce space and inode usage...

However file systems will have some limit to the number of names which 
can be linked to an inode so you need to be a bit smart about it.

> I did an md5sum on the, 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:
>
> <snip>
>
> 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?
Pass, all truly blue tiles here are 103 bytes and have the same md5sum.

--
Chris Jones, SUCS Admin
http://sucs.org





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