[gdal-dev] Fast Pixel Access

[Brian Case]

-Jukka

tileindex, mapserver, and the gdal wms driver



On Mon, 2014-02-03 at 17:20 +0000, Jukka Rahkonen wrote:
> Luke Roth <roth.luke <at> gmail.com> writes:
> 
> > 
> > Another thing that might speed up access is setting the config
> option GDAL_DISABLE_READDIR_ON_OPEN = TRUE, either as an environment
> variable or on the command line.  That should help with GDAL reading the
> directory each time it opens a dataset.  I have an application which reads
> one value from each of a large number of datasets and setting this option
> made it run about 3 times faster.
> 
> 
> Hi,
> 
> You are right. This config option makes GDAL to skip the reading of the
> remote directory and saves a lot of bandwidth:
> 
> VRT case: 
> Bytes Received:  4 244 509 (of which the vrt file: 4 192 577)
> Sequence (clock) duration:	00:00:09.9996000
> Was:
> Bytes Received:  6 459 443
> Sequence (clock) duration:	00:00:37.8130000
> 
> BigTIFF case: 
> Bytes Received:  2 158 917
> Sequence (clock) duration:	00:00:04.4368000
> Was:
> Bytes Received:  4 374 137
> Sequence (clock) duration:	00:00:30.9192000
> 
> 
> Conclusion:
> Both options are unsuitable for serious use while amusing to play with.
> Reading the BigTIFF tile offset index (or whatever it is) seems to mean
> about 2 MB of compultory payload traffic. Reading the VRT file means in this
> example 4 MB of payload. If this sort of net access to a large directory of
> raster files should be important for someone there should be a way to find
> the right raster file and righ data range in that file with minimum amount
> of bytes. Perhaps some kind of rtree indexed vrt file? First aid might be to
> keep the vrt file on the client side.
> 
> -Jukka Rahkonen- 
> 
> 
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