Writing by Chunk

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In this example each process writes a "chunk" of data to a dataset. The C and Fortran 90 examples result in the same data layout in the file.

Figure a C Example	Figure b Fortran Example

For this example, four processes are used, and a 4 x 2 chunk is written to the dataset by each process.

To do this, you would:

• Use the block parameter to specify a chunk of size 4 x 2 (or 2 x 4 for Fortran).
• Use a different offset (start) for each process, based on the chunk size:

  C	Process 0	Process 1	Process 2	Process 3
  	offset[0] = 0	offset[0] = 0	offset[0] = 4	offset[0] = 4
  	offset[1] = 0	offset[1] = 2	offset[1] = 0	offset[1] = 2
  Fortran	Process 0	Process 1	Process 2	Process 3
  	offset(1) = 0	offset(1) = 2	offset(1) = 0	offset(1) = 2
  	offset(2) = 0	offset(2) = 0	offset(2) = 4	offset(2) = 4

For example, the offset and block parameters for Process 2 would look like:

Figure a C Example	Figure b Fortran Example

Below are example programs for writing hyperslabs by pattern in Parallel HDF5:

hyperslab_by_chunk.c

hyperslab_by_chunk.F90

The following is the output from h5dump for the HDF5 file created in this example:

HDF5 "SDS_chnk.h5" {
GROUP "/" {
   DATASET "IntArray" {
      DATATYPE  H5T_STD_I32BE  
      DATASPACE  SIMPLE { ( 8, 4 ) / ( 8, 4 ) } 
      DATA {
         1, 1, 2, 2,
         1, 1, 2, 2,
         1, 1, 2, 2,
         1, 1, 2, 2,
         3, 3, 4, 4,
         3, 3, 4, 4,
         3, 3, 4, 4,
         3, 3, 4, 4
      } 
   } 
} 
} 

The h5dump utility is written in C so the output is in C order.

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