title: btrfs raid1 with multiple devices
body: <p>I have 6 devices: 4TB, 3TB, 2TB, 2TB, 1.5TB, 1TB (/dev/sda to /dev/sdf).</p>
  
  <p><strong>First question:</strong></p>
  
  <p>With RAID-1 I'd have:</p>
  
  <ul>
  <li>2TB mirrored in 2TB</li>
  <li>1TB mirrored in 0.5@4TB + 0.5@3TB</li>
  <li>1.5TB mirrored in 1.25@4TB + 0.25@3TB</li>
  <li>the rest 2.25 of 3TB mirrored in the rest 2.25TB of 4TB.</li>
  </ul>
  
  <p>My total size would be in that case (4 + 3 + 2 + 2 + 1.5 + 1) = 13.5/2 = 6.75TB</p>
  
  <p>Will <code>$ mkfs.btrfs --data raid1 --metadata raid1 /dev/sda /dev/sdb /dev/sdc /dev/sdd /dev/sde /dev/sdf</code> provide me with approximately 6.75TB? If yes, how many disks (and which?) can I afford losing?</p>
  
  <p><strong>Second question:</strong></p>
  
  <p>With the RAID-1 I can afford, for example, losing three disks:</p>
  
  <ul>
  <li>one 2TB disk, </li>
  <li>the 1TB disk and </li>
  <li>the 1.5TB disk, </li>
  </ul>
  
  <p>without losing data. </p>
  
  <p>How can I have the same freedom in losing the same disks with btrfs?</p>
  
  <p>Thanks!</p>