[gdal-dev] Block cache and VRT sources

[Even Rouault]

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.rouault at 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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