I have previously installed Ubuntu on my hard drive with one big encrypted (LUKS) partition, containing an LVM volume group. The thinking was that when I would later want to install a new version of Ubuntu or some other distro, I could just create a logical volume and install it there.
Yesterday I wanted to make a clean install of Ubuntu 18.04. From the old installation, i created new logical volumes for / and /home. Using an 18.04 live USB, it seemed that the installer did not recognize the LUKS partition. I unlocked it in «Disks», and now Ubiquity found the logical volumes. I placed /, /home, /boot and /boot/efi (the last two unencrypted) where I wanted them and finished the installation.
The system would not boot, but instead put me in a BusyBox prompt. It turned out that my logical volumes couldn't be found. It seemed the installer had not realized that some decryption would be necessary to access the / volume i chose. After a while of trial and error, I was able to find and follow these instructions to create an /etc/crypttab and update initramfs and grub, which was successful.
So my question is: How should I have done this? Was there some option in the installer that I didn't see? Should the crypttab have been generated automatically during the installation process (that is, is it a bug)? Or is installing to an existing LUKS setup like this simply not supported?