title: What does rows_merged mean in compactionhistory?
body: <p>When I issue</p>
  
  <pre><code>$ nodetool compactionhistory
  </code></pre>
  
  <p>I get</p>
  
  <pre><code>. . . compacted_at        bytes_in       bytes_out      rows_merged
  . . . 1404936947592       8096           7211           {1:3, 3:1}
  </code></pre>
  
  <p><strong>What does <code>{1:3, 3:1}</code> mean?</strong> The only documentation I can find is <a href="http://www.datastax.com/documentation/cassandra/2.1/cassandra/tools/toolsCompactionHistory.html">this</a> which states</p>
  
  <blockquote>
    <p>the number of partitions merged</p>
  </blockquote>
  
  <p>which does not explain why multiple values and what the colon means.</p>