title: BTRFS raid-1: which device gets the reads?
body: <p>I have a raid-1 with the following configuration:</p>
  
  <pre><code>$ btrfs fi show
  Total devices 2 FS bytes used 203.31GiB
          devid    1 size 224.00GiB used 206.03GiB path /dev/sda
          devid    2 size 224.00GiB used 206.03GiB path /dev/mmcblk0p4
  </code></pre>
  
  <p><code>/dev/mmcblk0p4</code> is fast and <code>/dev/sda</code> is slow</p>
  
  <p>What determines what device would get the IO reads and is there a way to control that?</p>