Allocating space for dataset elements

Space can be allocated for dataset elements when a dataset is created, or when the dimensions of an existing dataset with unlimited maximum dimensions are extended. When using a non-MPI VFD, the space allocation can be deferred until elements are actually written to the dataset’s storage, but because there is no central coordinating agent for allocating space in HDF5 files, all space for storing data elements in dataset must be allocated collectively in parallel applications, so that the space allocation information on all processes is kept synchronized.  Additionally, metadata in the file is also created or modified during these dataset operations, which also requires them to be collective operations.


Allocating space involves different algorithms for each type of dataset storage:


Additionally, when space is allocated for dataset elements, the application may request writing a fill value to each element, which involves different algorithms for each type of dataset storage:

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