[OSM-dev] [OSM-talk] Mapnik tileset coherency issues

Christopher Schmidt crschmidt at metacarta.com
Fri Oct 19 13:44:26 BST 2007


On Fri, Oct 19, 2007 at 01:37:16PM +0100, Jon Burgess wrote:
> On 19/10/2007, Christopher Schmidt <crschmidt at metacarta.com> wrote:
> > I believe ext3 gets upset at more than 32000 'links' pointing to a
> > directory. (This is why TileCache has super-deep directory structure.)
> > So although lots of files in a directory is fine, lots of directories in
> > a directory was not... but my memory is poor. This was ext3 ... ah, yes:
> >
> > "Apparently when using ext2 or ext3 there's a limit on the number of
> > subdirectories you can create within a directory. This is a hardcoded
> > number and seems to be set to about 2^15 ~= 32k."
> >
> > http://www.rooftopsolutions.nl/article/135
> >
> 
> I know Squid uses a hashed directory hierachy. I believe it limits a
> single directory to 256 entries.
> 
> If we setup squid in a reverse-proxy type setup then it could take
> care of storing the tiles for us. I wonder if squid has the necessary
> hooks to allow a rendering engine to be hooked directly into the cache
> instead of needing a backend web server & module.

Squid as a TileCache has limitations:
 * Harder to hook in to expire entries
 * Since it's entirely based on URLs, it's trivial to poison the cache
   by visiting (for example) /a/b/c.png?foo=bar
 * No knowledge about groups of tiles -- for example, with metatiling. 

It's possible these aren't that important, but I don't think Squid is
the best solution for a map tile cache. 
 
Regards,
-- 
Christopher Schmidt
MetaCarta




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