2000-introduction-cbt.html

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   5 <h1>Change Block Tracking</h1>
   6
   7 <p>
   8   Two common approaches to tracking changed blocks:
   9 </p>
  10 <table id="border">
  11   <tr>
  12     <th style="width: 30%;">Approach</th>
  13     <th>Trade-offs</th>
  14   </tr>
  15   <tr>
  16     <td style="text-align:center;">Generation tag</td>
  17     <td>
  18       <ul>
  19         <li>Monotonically increasing tag tracked for each
  20           cluster</li>
  21         <li>Set of changes since a point in time is all clusters with
  22           generation id larger than the value at that point</li>
  23         <li>Multiple points in time tracked by same amount of metadata</li>
  24       </ul>
  25     </td>
  26   </tr>
  27   <tr>
  28     <td style="text-align:center;">Dirty bitmap</td>
  29     <td>
  30       <ul>
  31         <li>Bitmap that gets updated with each changed cluster</li>
  32         <li>Less metadata storage required for all changes since a
  33           single point in time</li>
  34         <li>Multiple bitmaps required to track changes since multiple
  35         points in time</li>
  36       </ul>
  37     </td>
  38   </tr>
  39 </table>
  40 <p>Both approaches also have to balance granularity: more precision on
  41   what changed requires more metadata but can produce smaller
  42   incremental backups.</p>