title: the size of qcow2 disk file grows rapidly
body: <p>Hello everyone！I am learning OpenStack、qemu and kvm.</p>
  
  <p>As we know, QCOW2 is a disk file format of kvm virtual machine.
   I found a phenomenon, that is the size of QCOW2 disk file grows rapidly.
  I created a kvm virtual machine which used QCOW2 disk file and win7 operating system. I didn't do any action for it. <strong>But a day later, the size of its QCOW2 disk file was increased by more than 1G.</strong></p>
  
  <p>I know that shutting down the operating system's automatically updates will slow the growth of the QCOW2 disk file size. Can you tell me whether you have other more effective ways? I would be appreciated if I can get your help.</p>