[Gdal-dev] RasterIO tile size issue

[Sheykhet, Rostic]

Hi Frank, All:

	I have found an issue with GDALRasterIO() when dealing with very
big(long) images and/or very small tile cache.  The problem seems to
occur when RasterIO() attempts to write an area of a certain width (say
AW) to an image with a smaller tile width (say BW), and the tile cache
is not large enough to keep ceil(AW/BW) blocks cached.
	Assuming that the tile cache is completely turned off, here is
the scenario that I am observing.  Passed-in buffer size = 128x128,
image tile size=64x64.  RasterIO() starts writing the first line of the
passed in buffer, acquires the first 64x64 block B[0,0] of the image and
writes the first 64 samples to it.  Then it needs to continue writing
the left-over 64 bytes of same line and therefore acquires the next
block B[1,0] (to the right) and flushes B[0,0] block to disk.  After
writing the last 64 samples of this scanline, it moves on to writing the
second scan line, at which point the block B[0,0] is required again.  At
this point, the code at line 286 of rasterio.cpp is executed:
                int bJustInitialize = 
                    eRWFlag == GF_Write
                    && nYOff <= nLBlockY * nBlockYSize
                    && nYOff + nYSize >= (nLBlockY+1) * nBlockYSize
                    && nXOff <= nLBlockX * nBlockXSize
                    && nXOff + nXSize >= (nLBlockX+1) * nBlockXSize;

This code checks whether or not the block is completely contained within

the area covered by the passed in buffer.  If so, the code simply
allocates
a new tile without reading the data that has already been written to
disk.

I think that a check should be put in for figuring out whether this
RasterIO call has already written to this block, and the block should be
read from disk, if that's the case.

Thanks

Rostic