title: Elasticsearch free storage space drastical reduce
body: <p>I executed a <code>update_by_query</code> to copy value from a field to another field that target ~80 million documents. I hosted Elasticsearch on AWS. During execution, I monitored the metrics and found an interesting problem. </p>
  
  <p>During the execution, I saw drastically decline graph for free storage space and it went up back after the execution is finished. </p>
  
  <p><a href="https://i.stack.imgur.com/qcbhD.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/qcbhD.png" alt="enter image description here"></a></p>
  
  <p>What was happened during that time?  </p>