Tue 04 September 2007
There's been some good talk on the mapserver list (thanks to Gregor's diligent testing) about performance related to serving up raster imagery.
First off, comparisons of image formats. Then a look at some TIFF optimization techniques like overviews (similar to "pyramids" in ESRI land) and internal tiling to boost rendering speed.
Most of the conclusions are not all that staggering:
TIFF is fastest but takes up more space compared to ECW and JPEG2000.
Overviews speed up TIFFs tremendously when zoomed out (ie when mapserver would otherwise have to perform some heavy downsampling)
Internal tiles in GeoTIFF format give a boost when zoomed in (only the necessary tiles are read from disk)
The TIFF comparison was run on two setups; a monsterous 8-core, RAID-5 equipped beast and a low-memory virtual machine on low-end PC hardware. The TIFF optimizations are very noticeable on the lesser machine but almost completely negligible on the high-end machine.
Both tiling and overviews are useful, but only on machines with resource
shortages, such as slow disks or a lack of spare RAM for caching.
Nothing earth-shattering (these techniques are often mentioned as best practices) but is very nice to see some hard numbers to back it up. Plus the verbose test logs provide a good example for a newbie trying to implement them. Good stuff Gregor!