title: btrfs across multiple non-RAID devices
body: <p>I have the following layout on an Arch Linux system:</p>
  <pre><code>/dev/sda (HDD)
         |_sda5
         |    |_mountpoint: /var
         |
         |_sda6
         |    |_mountpoint: /tmp
         |
         |_sda7
              |_mountpoint: /data
  
  /dev/sdb  (SSD)
         |_sdb1
         |    |_mountpoint: /boot
         |
         |_sdb2
              |_mountpoint: /
  </code></pre>
  <p>I have two questions.
  Is there a way to, or should I even want to, make /var, /tmp, and /data subvolumes of the root volume?
  Can I use /data to hold all the snapshots of the root volume and/or other (sub)volumes?</p>