[Openstreetmap-dev] Cache the whole world.

[Immanuel Scholz]

Hi,

After reading Lars's measurements, I came up to the (probably very old)
idea of having the tile server cache the background images fetched from
NASA. ;)

Lars: Can you rerun your script, this time not accessing the tileserver
from OSM, but directly the URL from the NASA server where the tile server
get its pictures from? And better: can you do this from a shell from the
development server (which should be the same performance than the tile
server ,since they are placed in the same network, right?)

Steve: Can you provide Lars with the URL to the NASA server used? If the
tile server is not in the same network, can you give Lars a temporary
shell account and an wget-install on the tile server?

Man, I sound like a boss... nice feeling, hehe ;-) Please don't feel
bugged around by me, that was not my intention! Some explaining for my
bossing around:

I assume that it is true, that the squid server is in the network just
next to the tile server? (Or on the same hardware). This means, the tile
server need the time fetching the uncached tiles from the NASA server,
right? Since there were no difference between delivering data without
streets and deliver data with streets, the time the tile server need to
layer the streets seems to be negliable to me.

However, it could be possible that the tile server need much of the time
rendering but fails in some last step to actually do it right and deliver
the unlayered tile. This is the reason I ask Lars and Steve for the
additional tests.



Since the background of the world isn't changing, the tiles could be
cached for an infinite time or in other words, the tile server could hold
the data instead of asking the NASA server?

How big is the world at zoom <= 7? How much disk space can we afford on
the tile server to hold the background images? (Better use caches instead
of raw-copy all the data. I suspect nobody ever zooms at the middle of
atlantic, so this data would be wasted space ;)


To my current understanding, squid caches the background images overlayed
with the rendered streets. Would it be more benefitting to cache only the
background images?


Aren't the 48 hours because some people complaining that they are note
able to correctly invalidate the cache and don't see their map changes
drawn in OSM?

If I remember correctly, the reason why OSM does not display the
background independend from the map layer was, because IExplorer <= 6.0
has problems displaying PNG's with transparency. But I also remember, that
there were a (hackish) fix for this problem. Also, we could try an other
format which support transparency.


So is it possible to layer both pictures above each other in the browser
and have the background tiles cached forever, while the OSM data has its
cache expiration some hours only?


Ciao, Imi.

PS: I am new to this html and cache stuff. If there is already progress in
this done by others and I am just a nag cooking old problems, tell me! ;)