[OSM-dev] Daily Diff parsing - tile expiring
brett at bretth.com
Thu Aug 21 00:52:12 BST 2008
This sounds much simpler :-)
Frederik Ramm wrote:
>> I think that adding the Lat/Lon tag to node references in the ways is not an
>> option since it would require a huge amount of queries on the database when
>> creating the diffs. The positive side of this would that it would only have
>> to be done once.
> If you are happy with supporting only zoom level 16 granularity, you can
> store the tile x and tile y coordinates for each node, that's only 4
> bytes per node.
> If you create a file that contains four bytes for every node, indexed
> simply by node id (node #1 x at file position 0, node #1 y at 2, node #2
> x at 4 etc.), then your file will have about 1.2 Gig for the current
> planet. When processing a diff you would simply mmap that file,
> effectively loading into memory and granting very fast access to node
> This wastes some memory because it reserves space for deleted nodes as
> well, but when I last looked the number of "deleted" node ids was about
> 20%, and this is an acceptable overhead - otherwise you would have to
> use something like a hashtable to index your node list which would use
> more than that.
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