2019-kvm-forum/4300-block-status-impact.html

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   5 <h1>Block status: does it change the status quo?</h1>
   6
   7 <p>
   8   Checking whether a block is already zero will eliminate the need to
   9   write zeroes to that block, but at what cost?
  10
  11   <p>
  12   <span style="background-color: green; color: white;">block status reports hole</span>
  13   <span style="background-color: red; color: white;">block status reports data</span>
  14   <span style="background-color: blue; color: white;">write zeroes</span>
  15
  16   <p>
  17   <table id="border" style="width:100%">
  18     <tr>
  19       <th></th>
  20       <th>O(1) server</th>
  21       <th>O(n) server</th>
  22     </tr>
  23     <tr>
  24       <td>no block status</td>
  25       <td><span style="background-color: blue; padding-right:20px;">&nbsp;</span></td>
  26       <td><span style="background-color: blue; padding-right:200px;">&nbsp;</span></td>
  27     </tr>
  28     <tr>
  29       <td>block status sees zero</td>
  30       <td><span style="background-color: green; padding-right:20px;">&nbsp;</span></td>
  31       <td><span style="background-color: green; padding-right:20px;">&nbsp;</span></td>
  32     </tr>
  33     <tr>
  34       <td>block status sees data</td>
  35       <td><span style="background-color: red; padding-right:20px;">&nbsp;</span>
  36         <span style="background-color: blue; padding-right:20px;">&nbsp;</span></td>
  37       <td><span style="background-color: red; padding-right:20px;">&nbsp;</span>
  38         <span style="background-color: blue; padding-right:200px;">&nbsp;</span></td>
  39     </tr>
  40     <tr>
  41       <td>block status is slow</td>
  42       <td><span style="background-color: red; padding-right:250px;">&nbsp;</span>
  43         <span style="background-color: blue; padding-right:20px;">&nbsp;</span></td>
  44       <td><span style="background-color: red; padding-right:250px;">&nbsp;</span>
  45         <span style="background-color: blue; padding-right:200px;">&nbsp;</span></td>
  46     </tr>
  47   </table>
  48
  49   <p>
  50     If checking once per block is bad, can we instead check once per
  51     image?
  52   <p>Or why even check - what if we ensure the image starts
  53     with all zeroes?
  54 </p>