title: Why "Data Space Used" + "Data Space Available" != "Data Space Total" in docker?
body: <p>The <code>docke info</code> command output on my system is as follows:  </p>
  
  <pre><code># docker info
  Containers: 0
  Images: 0
  Storage Driver: devicemapper
   .....
   Data Space Used: 1.821 GB
   Data Space Total: 107.4 GB
   Data Space Available: 32.92 GB
   Metadata Space Used: 1.479 MB
   Metadata Space Total: 2.147 GB
   Metadata Space Available: 2.146 GB
  ......
  </code></pre>
  
  <p>The "<code>Metadata Space Used</code>" + "<code>Metadata Space Available</code>" = <code>Metadata Space Total</code>, but why "<code>Data Space Used</code>" + "<code>Data Space Available</code>" != "<code>Data Space Total</code>"?</p>