The chunk size calculation policy for thin pool volumes.
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Accepted values:
  generic
    If thin_pool_chunk_size is defined, use it. Otherwise, calculate
    the chunk size based on estimation and device hints exposed in
    sysfs - the minimum_io_size. The chunk size is always at least
    64KiB.
  performance
    If thin_pool_chunk_size is defined, use it. Otherwise, calculate
    the chunk size for performance based on device hints exposed in
    sysfs - the optimal_io_size. The chunk size is always at least
    512KiB.
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