title: TensorFlow Dataset.shuffle - large dataset
body: <p>I'm using TensorFlow 1.2 with a dataset in a 20G TFRecord file. There is about half a million samples in that TFRecord file.</p>
  
  <p>Looks like if I choose a value smaller than the amount of records in the dataset for <code>buffer_size</code>, only the first N records in the TFRecord will be used. <a href="https://www.tensorflow.org/api_docs/python/tf/contrib/data/Dataset#shuffle" rel="noreferrer">https://www.tensorflow.org/api_docs/python/tf/contrib/data/Dataset#shuffle</a></p>
  
  <p>For example, if <code>buffer_size = 100</code>, seems like only the first 100 records are ever used.</p>
  
  <h3>Question</h3>
  
  <p>Should <code>buffer_size</code> always be the length of the dataset? Would that impact training performance?</p>