title: Starting Gnome 3 in a read-only environment
body: <p>First off, my intention is to create a portable, bootable USB drive containing a GNU/Linux distribution. Specifically, I want to use Arch Linux with a squashfs <strong>read-only</strong> root filesystem.</p>
  
  <p>The squashfs image is based on a snapshot of a working VM. The base system with it's services like ssh work out of the box as expected. But when trying to launch gnome via systemd (systemctl start gdm), all I see is a black screen (supposedly the X-Server started but gdm fails to load). I already tried to figure out whats happening, but failed to identify the exact problem.</p>
  
  <ul>
  <li>Home directories are writeable</li>
  <li>/tmp is writeable</li>
  <li>/var/log is writeable</li>
  <li>/var/run &amp; /run are writeable anyway</li>
  </ul>
  
  <p>/var/log/gdm gets created but stays empty.
  Which modules may require write access to any other files? Is there any documentation? What would make sense to strace or similar?</p>
  
  <p>My desire is to know the root of the problem and fix it, instead of using workarounds like unionfs. Thanks for any help or hints!</p>
  
  <p>Although it's not relevant, for those who might wonder why I want to do this, here are some points to consider:</p>
  
  <ul>
  <li><strong>Stability</strong> - as you cannot modify system files, you cannot mess up the system (unless you write bogus directly to the drive of course)</li>
  <li><strong>Storage</strong> - as files are compressed, more data fits on the drive</li>
  <li><strong>Performance</strong> - as I/O on most USB drives is slow, compression gives you higher I/O speed</li>
  <li><strong>Portability</strong> - no special treatment for read-only storage, you might copy it on a CD or any other read-only technology and it will still work the same way as it would on a writeable disk</li>
  </ul>
  
  <p><strong>Update</strong></p>
  
  <p>I figured out that the problem was actually at /var/lib/gdm. GDM tried to access files in there an (silently) failed doing so giving me a black screen.</p>