[gdal-dev] GDAL ConfigOptions

[Even Rouault]

Le 25/10/2021 à 14:25, Azat.Nurgaliev at dlr.de a écrit :
> Hello everyone,
>
> I am working on optimizing a GDAL application that uses RasterIO and GeoTiff.
> I am using the C++ library.
>
> I would like to experiment with different ConfigOptions, but they have no effect on the application.
> I specify them in an environment variable and directly in the code.
>
> I am especially interested in GTIFF_DIRECT_IO.
>
> I use blktrace I/O trace to check.
> It traces all I/O operations, and through it I see that the application sends "RA" operations, which means readahead.
> The readahed operations only happen when buffered I/O is enabled, and readahead operations should no longer happen when direct I/O is enabled.

That's not what GTIFF_DIRECT_IO does. It "just" bypasses libtiff and the 
GDAL block cache (when possible). But it uses the regular GDAL I/O, 
which means buffered I/O (fread() calls). Using unbuffered reads would 
probably have only a marginal effect (but maybe not ? this is left to 
the reader as an exercise :-))

Even


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