4150-initial-differential.html

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   5 <h1>Initial qemu pull-mode differential backup</h1>
   6
   7 <ul>
   8   <li>expose the bitmap over NBD for third-party access</li>
   9   <li>create a new bitmap on each backup</li>
  10   <li>transfer which bitmap is enabled; all others are disabled</li>
  11   <li>merge sequence of bitmaps to perform differential backup</li></li>
  12 </ul>
  13
  14 <p>Differential backup
  15   <table>
  16     <tr>
  17       <td style="font-size:20px;">Image.qcow2</td>
  18       <td style="font-size:20px;">BackupFull.qcow2</td>
  19     </tr>
  20     <tr>
  21       <td>
  22         <table class="disk">
  23           <tr>
  24             <th style="visibility:hidden;">b0</th>
  25             <th>A</th><th>-</th><th>A</th><th>B</th>
  26             <th>B</th><th>B</th><th>-</th><th>-</th>
  27           </tr>
  28           <tr style="color:red;">
  29             <td style="font-style: italic">b0</td>
  30             <td>-</td><td>-</td><td>-</td><td>X</td>
  31             <td>X</td><td>X</td><td>-</td><td>-</td>
  32           </tr>
  33           <tr style="color:green;">
  34             <td style="font-style: italic">b1</td>
  35             <td>-</td><td>-</td><td>X</td><td>X</td>
  36             <td>-</td><td>-</td><td>-</td><td>-</td>
  37           </tr>
  38           <tr style="color:blue;">
  39             <td style="font-style: italic">t</td>
  40             <td>-</td><td>-</td><td>X</td><td>X</td>
  41             <td>X</td><td>X</td><td>-</td><td>-</td>
  42           </tr>
  43         </table>
  44       </td>
  45       <td>
  46         <table class="disk">
  47           <tr>
  48             <th>A</th><th>-</th><th>A</th><th>A</th>
  49             <th>-</th><th>A</th><th>-</th><th>-</th>
  50           </tr>
  51         </table>
  52       </td>
  53     </tr>
  54     <tr>
  55       <td></td>
  56       <td style="font-size:20px;">IncBackup1.qcow2</td>
  57     </tr>
  58     <tr>
  59       <td></td>
  60       <td>
  61         <table class="disk">
  62           <tr>
  63             <th>-</th><th>-</th><th>-</th><th>B</th>
  64             <th>B</th><th>B</th><th>-</th><th>-</th>
  65           </tr>
  66         </table>
  67       </td>
  68     </tr>
  69     <tr>
  70       <td></td>
  71       <td style="font-size:20px;">DiffBackup.qcow2</td>
  72     </tr>
  73     <tr>
  74       <td></td>
  75       <td>
  76         <table class="disk">
  77           <tr>
  78             <th>-</th><th>-</th><th>C</th><th>C</th>
  79             <th>B</th><th>B</th><th>-</th><th>-</th>
  80           </tr>
  81         </table>
  82       </td>
  83     </tr>
  84   </table>
  85 </p>
  86
  87 <p class="attribution">See also this 2018 KVM Forum presentation:
  88   <a href="https://www.youtube.com/watch?v=zQK5ANionpU">Facilitating
  89     Incremental Backup</a>, Eric Blake</p>