title: what exactly does the `--save_resume` option in vowpal wabbit do
body: <p>I'm training a neural net with data streamed from two separate sources, heuristically:</p>
  
  <pre><code>cat train1.vw |vw --oaa 10 --nn 20 --save_resume -f /tmp/weights
  cat train2.vw |vw --oaa 10 --nn 20 --save_resume -i /tmp/weights -f /tmp/weights
  </code></pre>
  
  <p>However this raises an error to the effect that "--oaa cannot be passed multiple times." Fine, <code>--save_resume</code> holds on to config arguments, so I revised:</p>
  
  <pre><code>cat train1.vw |vw --oaa 10 --nn 20 --save_resume -f /tmp/weights
  cat train2.vw |vw --save_resume -i /tmp/weights -f /tmp/weights
  </code></pre>
  
  <p>and everything worked. Out of curiosity I repeated the second approach without the <code>--save_resume</code> option, everything <em>still</em> worked, except that the model performance was slightly worse. </p>
  
  <p>I would speculate that <code>--save_resume</code> saves the learning rate and number of examples seen in the weights file. This is what appeared to be happening from the output, and corroborates better performance. Is there anything else?</p>
  
  <p><strong>Edit:</strong> After a little experimentation I see that passing the initial regressor via <code>-i</code> is what raises the "option '--oaa' cannot be specified more than once error," not <code>--save_resume</code>. </p>