[OSM-talk] Multiple Layers for OSM

[Jochen Topf]

On Tue, Sep 25, 2012 at 12:38:20PM +0200, Simon Poole wrote:
> IMHO the easiest, but very hackish, way to achieve this would be to
> split the (64bit) object ID space up  in to sub-spaces (a la CIDR).
> 
> All tools will have to support 64bit IDs rsn and we are currently
> throwing away half of the ID space for no real good reason except
> convenience. The main concern is that 64bits may not be enough ID space
> long term: for example 63 bits for OSM proper would allow ~18'000 nodes
> per square meter of the earth's surface (~62'000 without oceans), which
> may not be enough for detailed multi-storey indoor mapping :-).  Such a
> scheme would not preclude switching to larger IDs down the road (the ID
> size is essentially just a practical limitation of the databases and
> tools we are currently using) the individual layers would simply get
> non-continuous ID spaces allocated if necessary .

Currently the IDs are rather dense in the ID space. (There are holes from
deleted objects, but not that many.) This is a very good thing, because it
allows efficient implementations, for instance all node locations can be
put into an array which needs only 8 bytes * max_node_id.

If you give out part of the ID space to different databases this approach
does not work any more. Thats not the end of the world, but we have to
really start looking into the consequences of these kinds of decisions
because we are not a small database any more...

Jochen
-- 
Jochen Topf  jochen at remote.org  http://www.remote.org/jochen/  +49-721-388298