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# Direct Chunk Write

When a user application is working with a chunked dataset and is writing a single chunk of data with `H5Dwrite`, the data goes through several steps inside the HDF5 library. The library first examines the hyperslab selection. Then it converts the data from the datatype in memory to the datatype in the file if they are different. Finally, the library processes the data in the filter pipeline.

This can create an I/O bottleneck in a very high-throughput environment.

The high-level C function `H5DOwrite_chunk` provides a mechanism enabling the application to write a data chunk directly to the file bypassing the library’s hyperslab selection, data conversion, and filter pipeline processes. If an application can pre-process the data properly, the application can use `H5DOwrite_chunk` to write the data much faster.

The following documents describe the use of this feature:

- “Using the Direct Chunk Write Function,” a user-guide (PDF only)
- “RFC: Direct Chunk Write,” the design document for this feature (PDF only)
- The `H5DOwrite_chunk` function entry in the “HDF5 Optimized Functions” section of the HDF5 High-level APIs Reference Manual

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