title: qemu-img info: disk size vs virtual size - misunderstanding
body: <p>please note following log: </p>
  
  <pre><code>[user@machine]$ qemu-img info kvm_1
  image: kvm_1
  file format: qcow2
  virtual size: 146G (157286400000 bytes)
  disk size: 101G
  cluster_size: 65536
  backing file: /xyz.qcow2
  backing file format: qcow2
  Format specific information:
      compat: 0.10
  
  
  [user@machine]$ qemu-img info kvm_2
  image: kvm_2
  file format: qcow2
  virtual size: 146G (157286400000 bytes)
  disk size: 146G
  cluster_size: 65536
  backing file: /xyz.qcow2
  backing file format: qcow2
  Format specific information:
      compat: 0.10
  </code></pre>
  
  <p>The second machine (<code>kvm_2</code>) was fallocated using <code>fallocate</code> tool. 
  Can anyone explain a difference? What are difference in performance between these two config?
  Does it matter for <code>qcow2</code> format? What about <code>raw</code> format? What is root cause of difference in performance (unless any exists)?</p>