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r.buildvrt: document performance issues with external data #4441

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Oct 31, 2024
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r.buildvrt: document performance issues with external data #4441

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document performance issue
ninsbl Oct 3, 2024
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address code review
ninsbl Oct 25, 2024
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Merge branch 'main' into vrt_manual
ninsbl Oct 25, 2024
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ninsbl Oct 25, 2024
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consistent recommendation
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15 changes: 14 additions & 1 deletion raster/r.buildvrt/r.buildvrt.html
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Expand Up @@ -12,6 +12,8 @@ <h2>NOTES</h2>
the original raster maps which is only valid if the original raster
maps remain in the originally indicated mapset. A VRT can also be built
from raster maps registered with <em><a href="r.external.html">r.external</a></em>.
However, GRASS VRTs built from external registered data (see below)
are known to have performance issues.

<p>
Reading the whole VRT is slower than reading the equivalent single
Expand Down Expand Up @@ -48,18 +50,29 @@ <h3>VRT from a DEM in the North Carolina sample dataset</h3>
r.buildvrt file=tilelist.csv output=elev_state_50m_vrt
</pre></div>

<h2>KNOWN ISSUES</h2>

Users may experience significant performance degradation with virtual rasters built
with <em>r.buildvrt</em> over GDAL-linked (<em>r.external</em>) raster maps,
especially on slower file systems with latency like NFS. Performance degradation
may also occur on local file systems, but is usually less severe. For such use cases
consider using the GRASS GIS addon
<a href="https://grass.osgeo.org/grass-stable/manuals/addons/r.buildvrt.gdal.html">r.buildvrt.gdal</a>
or building GDAL VRTs, e.g. with <em>gdalbuildvrt</em>.

<h2>SEE ALSO</h2>

<em>
<a href="r.tile.html">r.tile</a>,
<a href="r.patch.html">r.patch</a>,
<a href="r.external.html">r.external</a>
<a href="https://grass.osgeo.org/grass-stable/manuals/addons/r.buildvrt.gdal.html">r.buildvrt.gdal</a>
</em>

<p>
The equivalent GDAL utility
<em>
<a href="http://gdal.org/gdalbuildvrt.html">gdalbuildvrt</a>
<a href="https://gdal.org/gdalbuildvrt.html">gdalbuildvrt</a>
</em>

<h2>AUTHOR</h2>
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