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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
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Merge branch 'main' into vrt_manual
ninsbl Oct 25, 2024
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consistent recommendation
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11 changes: 11 additions & 0 deletions 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, there are known performance issues with GRASS VRTs build
over external, registered data (see below).

<p>
Reading the whole VRT is slower than reading the equivalent single
Expand Down Expand Up @@ -48,6 +50,15 @@ <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 build
with <em>r.buildvrt</em> over GDAL-linked (<em>r.external</em>) raster maps, especially
with slower file systems with latency like NFS. Performance degradation can also be
noticable on local file systems, but is usually less severe.
<p>
For GDAL-linked raster maps it is thus recommended to build GDAL VRTs, e.g with
<em>gdalbuildvrt</em>.

<h2>SEE ALSO</h2>

<em>
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