[gdal-dev] performance with VRT dataset containing a large number of raster datasets

[Dmitriy Baryshnikov]

Hi Paolo,
the te option set the output raster size, not the subraster. You have to 
use gdal_translate to get subraster and then use gdalwarp.
Also *multi option is preferable to multi threaded execution.
*

Best regards,
     Dmitry

24.12.2013 21:48, Paolo Corti ?????:
>
> Hi all
>
> I am experiencing problems using a VRT dataset built from a large 
> directory of .tif files.
>
> While if a build the VRT from just one of these datasets, gdalwarp 
> will work very fast with it, if I use the VRT will all of these 
> images, it becomes undefinite times slower.
>
> Debugging the gdalwarp command, it seems that gdalwarp accesses every 
> single raster composing the vrt datasource, even if that single raster 
> is not within the extent defined by the -te gdalwarp option.
>
> Here is the output for a vrt composed by just one single .tif file:
>
> $ time gdalwarp -te -88.88 20.28 -87.88 21.28 single.vrt single.tiff
> ...
> Src=1344,4463,1201x1201 Dst=0,0,1200x1200
> 0...10...20...30...40...50...60...70...80...90...100 - done.
> GDAL: GDALClose(single.vrt, this=0x8ca6ef8)
> GDAL: GDALClose(/home/capooti/training/vrt/srtm_19_08.tif, this=0x8ca9aa8)
> GDAL: GDALClose(/home/capooti/training/vrt/srtm_19_08.tif, this=0x8ca7208)
> GDAL: GDALClose(single.tiff, this=0x8ca7e98)
> GDAL: In GDALDestroy - unloading GDAL shared library.
>
> real 0m0.137s
> user 0m0.076s
> sys 0m0.052s
>
> and here is the output for a vrt composed of all of the .tif files:
>
> capooti at ubuntu:~/training/vrt$ time gdalwarp -te -88.88 20.28 -87.88 
> 21.28 all.vrt all.tiff
> ...
> Src=7344,16464,1201x1201 Dst=0,0,1200x1200
> 0...10...20...30...40...50...60...70...80...90...100 - done.
> GDAL: GDALClose(all.vrt, this=0x9d332b0)
> GDAL: GDALClose(/home/capooti/training/vrt/srtm_18_09.tif, this=0x9d46618)
> GDAL: GDALClose(/home/capooti/training/vrt/srtm_19_08.tif, this=0x9d467c8)
> ...
> GDAL: GDALClose(/home/capooti/training/vrt/srtm_35_09.tif, this=0x9d4c1b8)
> GDAL: GDALClose(/home/capooti/training/vrt/srtm_35_10.tif, this=0x9d4c3a8)
> GDAL: GDALClose(/home/capooti/training/vrt/srtm_19_08.tif, this=0x9d30800)
> GDAL: GDALClose(/home/capooti/training/vrt/srtm_35_11.tif, this=0x9d4c630)
> GDAL: GDALClose(all.tiff, this=0x9d33990)
> GDAL: In GDALDestroy - unloading GDAL shared library.
>
> real 61m12.130s
> user 0m6.076s
> sys 0m18.052s
>
> Any hint here to prevent gdalwarp to access the unneeded source raster 
> files - the defined extent in -te should just hit a couple of ones?
> thanks in advance
> p
>
> -- 
> Paolo Corti
> Geospatial software developer
> web: http://www.paolocorti.net
> twitter: @capooti
> skype: capooti
>
>
>
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