When I boot into single user mode (i.e., kernel /boot/vmlinuz-linux root=/dev/sda6 ro single) the LVM volumes in my fstab are not found (systemd times out while trying to access them - "Timed out waiting for device dev-vg1-lvabs.device"), fail to mount, and cause the boot process to fail.

Apparently making the following changes to the systemd configuration fixes the issue:

sudo mkdir /etc/systemd/system/sysinit.target.wants
cd /etc/systemd/system/sysinit.target.wants/
sudo ln -s /usr/lib/systemd/system/lvmetad.socket

This causes the lvmeta daemon to be active at system init, and so makes the lvm volumes available on a single user boot.

Perhaps these changes should be part of the LVM package? Booting into single user mode seems to me to be a common enough use case, and I would think it shouldn't fail just because you have LVM volumes.