title: attempt to change docker data-root fails - why
body: <p>I am trying to set my docker storage dir as other than default, something I've done on other machines:</p>
  
  <p>/etc/docker/daemon.json:</p>
  
  <pre><code>{
      "data-root": "/mnt/x/y/docker_data"
  }
  </code></pre>
  
  <p>where the storage dir looks like </p>
  
  <pre><code>jeremyr@snorble:~$ ls -ltr /mnt/x/y
  total 4
  drwxrwxrwx 11 jeremyr  5001  122 Mar 19 08:14 docker_data
  </code></pre>
  
  <p>with the daemon.json file in place, <code>sudo systemctl restart docker</code>  hits <code>Job for docker.service failed</code> (without that daemon.json, docker restarts fine and <code>docker run hello-world</code> runs fine) . with the daemon.json in place,  <code>journalctl -xn</code> shows </p>
  
  <pre><code>Mar 25 14:20:33 bolt88 systemd[1]: docker.service start request repeated too quickly, refusing to start.
  Mar 25 14:20:33 bolt88 systemd[1]: Failed to start Docker Application Container Engine.
  -- Subject: Unit docker.service has failed
  -- Defined-By: systemd
  -- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
  -- 
  -- Unit docker.service has failed.
  -- 
  -- The result is failed.
  Mar 25 14:20:33 bolt88 systemd[1]: Unit docker.service entered failed state.
  Mar 25 14:20:34 bolt88 sudo[23961]: jeremyr : TTY=pts/18 ; PWD=/home/jeremyr ; USER=root ; COMMAND=/bin/journalctl -xn
  Mar 25 14:20:34 bolt88 sudo[23961]: pam_unix(sudo:session): session opened for user root by jeremyr(uid=0)
  </code></pre>
  
  <p>while <code>systemctl status docker.service</code> just shows <code>code=exited, status=1/FAILURE</code></p>
  
  <p>and in dmesg I see this:</p>
  
  <pre><code>1547:[Mon Mar 25 14:21:41 2019] aufs au_opts_verify:1570:dockerd[20714]: dirperm1 breaks the protection by the permission bits on the lower branch
  1548-[Mon Mar 25 14:21:41 2019] device veth34d1dfd entered promiscuous mode
  1549-[Mon Mar 25 14:21:41 2019] IPv6: ADDRCONF(NETDEV_UP): veth34d1dfd: link is not ready
  1550-[Mon Mar 25 14:21:41 2019] IPv6: ADDRCONF(NETDEV_CHANGE): veth34d1dfd: link becomes ready
  1551:[Mon Mar 25 14:21:41 2019] docker0: port 1(veth34d1dfd) entered forwarding state
  1552:[Mon Mar 25 14:21:41 2019] docker0: port 1(veth34d1dfd) entered forwarding state
  1553:[Mon Mar 25 14:21:41 2019] docker0: port 1(veth34d1dfd) entered disabled state
  1554-[Mon Mar 25 14:21:41 2019] device veth34d1dfd left promiscuous mode
  1555:[Mon Mar 25 14:21:41 2019] docker0: port 1(veth34d1dfd) entered disabled state
  1556-[Mon Mar 25 14:21:59 2019] systemd-sysv-generator[20958]: Ignoring creation of an alias umountiscsi.service for itself
  </code></pre>
  
  <p>Docker version 17.05.0-ce, build 89658be, on a debian 8.8 setup . </p>
  
  <p>Does anyone know why docker isn't allowing use of that dir as data-root?</p>