def summarize_timesteps(hdf5_file):
    """Summarizes total read/write bytes at each timestamp.

    Summarizes read/write bytes for each time step using the HDF5 interface
    instead of converting to a DataFrame or TimeSeries first.  Returns a dict
    of form::

        {
            "1546761600": {
                "read_bytes": 6135848142.0,
                "write_bytes": 6135848142.0
            },
            "1546761630": {
                "read_bytes": 5261439143.0,
                "write_bytes": 6135848142.0
            },
            "1546761660": {
                "read_bytes": 4321548241.0
                "write_bytes": 6135848142.0,
            },
            ...
        }

    """
    datasets = {
        '/datatargets/writebytes': 'write_bytes',
        '/datatargets/readbytes': 'read_bytes'
    }
    results = {}
    for dataset_name in datasets.keys():
        timestamps = hdf5_file.get_timestamps(dataset_name)[...]
        sum_bytes = hdf5_file[dataset_name][:, :].sum(axis=1)
        for index, timestamp in enumerate(timestamps):
            output_key = datasets.get(dataset_name)
            output_val = sum_bytes[index]
            timestamp_str = str(timestamp)
            if timestamp_str not in results:
                results[timestamp_str] = {}
            results[timestamp_str][output_key] = output_val

    return results