If you setup a Laptop with 15.XX and a luks encrypted SSD, you did set on older OS „tmpfs“ for /tmp. Now under „systemd“ the the boot hangs cause systemd „automount“ tmpfs to /tmp by default!!! If you enter tmpfs into fstab like on ubuntu 14.10, the OS boots into the „RECOVERY RESCUE MODE“
Here some details, from a forum post:
Disable automatic mount
Under systemd, /tmp may be automatically mounted as a tmpfs even though you have no entry for that in your /etc/fstab.
To disable the automatic mount, run:
# systemctl mask tmp.mount
Files will no longer be stored in a tmpfs, but your block device instead. The /tmp contents will now be preserved between reboots, which you might not want. To regain the previous behavior and clean the /tmp folder automatically when restarting your machine, consider using tmpfiles.d:
# see tmpfiles.d # always enable /tmp folder cleaning D! /tmp 1777 root root 0 # remove files in /var/tmp older than 10 days D /var/tmp 1777 root root 10d # namespace mountpoints (PrivateTmp=yes) are excluded from removal x /tmp/systemd-private-* x /var/tmp/systemd-private-* X /tmp/systemd-private-*/tmp X /var/tmp/systemd-private-*/tmp