[gdal-dev] does gdal support multiple simultaneous writers to raster

[Jan Hartmann]

I would be interested in an implementation. I'm preparing a proposal to 
georeference the complete cadastral map of the Netherlands in 1832 at a 
10 cm/pixel scale with Cloud facilities. Gdalwarp is the central piece 
of software, and distributed processing capabilities would be very 
important. Could you please think about the possibilities, and what kind 
of funding would be required, so I can take that in during the next few 
months?

Jan

On 01/12/2013 04:35 PM, Even Rouault wrote:
>> ex. convert
>> multiple datasets to different output datasets in a parallel way.
> As Frank underlined, there's currently an issue with the global block cache
> regarding write support.
>
> Imagine that you have 2 threads A and B.
> Thread A deal with dataset A, and thread B deal with dataset .
> Thread A is in the middle of writing some tile/line of dataset A.
> Thread B is trying to fill a new entry in the block cache (with new read data,
> or new data to write). But the block cache is full. So the last recently used
> entry must be discarded. If that entry is a dirty block of dataset A, then it
> must be flushed to disk, in the context of thread B, but at that time thread A
> is also writing data... Which might be an issue since drivers are re-entrant
> (can be invoked by multiple threads, if each thread deal with different
> datasets) not thread-safe.
>
> This specific case here could be fixed in different ways :
> A) Making drivers thread-safe (or accessing them through a thread-safe layer),
> that is to say add a dataset level mutex
> B) or having a per-dataset block cache instead of a global block cache
> C) deal differently with dirty blocks. Only flush them if the operation that
> need to discard the dirty block is initiated by an operation on the same
> dataset as the dirty block.
>
>
>> Would
>> those parallel operations not be affected by GDAL caching for bot read and
>> write.Since the cache is set to a limit. Is Accessing the current used
>> cache value concurrent safeto increase it/decrease it ?
> Hum, I see that GDALGetCacheMax() and GDALSetCacheMax() are not thread safe
> currently. We would need to protect them by the raster block mutex, with a
> leading call to CPLMutexHolderD( &hRBMutex );
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