Le 19/04/2024 à 17:28, Sean Gillies a écrit :
Even,
Does the shared attribute of a VRT sourceFilename element not affect
caching at all?
If shared is set to 0, then one GDALDataset per VRTSource will be
opened. This has little benefit.
The scope of sharing was initially greater since it extented to
potentially several GDALDataset instance of VRT, but this wasn't safe in
multithreaded scenarios where you would read one VRT in a thread,
another one in another thread, but both would point to let's same the
same TIFF GDALDataset. Hence the scope of sharing was reduced to a
single GDALDataset VRT instance to be safe by default.
Is the cache avoided so that potentially stale data isn't propagated,
or for other reasons?
Are you reacting to "if it is triggered. VRTSource::IRasterIO() calls
IRasterIO() on the source band, which doesn't necessarily always trigger
block-based reading" ? I just meant that calling IRasterIO() on a
GDALDataset/GDALRasterBand doesn't necessarily cause the block cache to
trigger. This is dependent of the driver and the parameters of the
request. And this isn't specific to use it from a VRT.
On Fri, Apr 19, 2024 at 9:09 AM Even Rouault
<even.roua...@spatialys.com> wrote:
Sean,
Within a given GDALDataset opened on a VRT, if the VRT references
several times the same source, only one GDALDataset will be opened
for it, so you may benefit from the block cache mechanism (if it
is triggered. VRTSource::IRasterIO() calls IRasterIO() on the
source band, which doesn't necessarily always trigger block-based
reading). But if you open another VRT (or the same one), it will
not share the same GDALDataset for sources that may be common with
the first one, so no re-use of existing block cache. For network
sources, the I/O cache at the /vsicurl/ level works however on
filenames, not VSIFILE* instances, so you will save network reads
Even
Le 19/04/2024 à 16:48, Sean Gillies via gdal-dev a écrit :
Happy Friday, folks!
Are the source rasters of a VRT added to the block cache such
that different VRTs using the same source can avoid reads from
disk or the network? Or is it intended that the VSI cache covers
this need instead?
Thanks,
--
Sean Gillies
--
http://www.spatialys.com
My software is free, but my time generally not.
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