title: elasticsearch: how to free store size after deleting documents
body: <p>On my elasticsearch server:
  total documents: 3 million, total size: 3.6G
  Then, I delete about 2.8 millions documents:
  total documents: about 0.13 million, total size: 3.6G</p>
  
  <p>I have deleted the documents, how should I free the size of the documents?</p>