[gdal-dev] Performance issues with gdal vrt

[Craig Leat]

Hi list

Craig Leat:
> I need to investigate further, but I am suspecting that this issue may
> be specific to the MrSid format.

I have tested further and can report as follows:
1. I have 24 MrSid files with overview levels 2 4 8 16 32 64 128
2. I can point QGIS to the directory and load all these sid tiles very
quickly (IOW nearly instantaneously). I mean the tile names appear in
the legend and the full extent of the tiles appears on the map canvas.
3. I have built a vrt file containing these 24 sid tiles and loading
the vrt into QGIS takes 23 minutes.
4. I converted the sid files to GeoTiff and created external overviews
with the same levels.
5. I can point QGIS to the directory of tiffs and I'm able to load the
collection very quickly.
6. I have built a vrt for the tiffs and I can load this vrt very quickly.

I clearly have a problem working with MrSid tiles and vrt mosaics. I
am happy to open a ticket, but I don't think I can provide any more
useful information beyond the following:
1. MrSid SDK v7.0.0 built with gcc 4.1
2. gdal 1.6.1 built with gcc 4.3.3 (note the compiler version mismatch)
3. The sid and tiff tiles reside on the same physical device
4. I tested with other sid files from different suppliers and the
behaviour is similar.

I'm going to avoid the vrt format for the time being, but if a
developer is interested in investigating further you are welcome to
contact me.

Regards

Craig