/usr/share/initramfs-tools/conf.d/zfs is in zfs-initramfs 0.7.5-1ubuntu15.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | for x in $(cat /proc/cmdline)
do
case $x in
root=ZFS=*)
BOOT=zfs
;;
esac
done
|