Comment 3 for bug 1801349

Also checking on a non-juju serverstack cosmic instance:

# cat /etc/fstab
LABEL=cloudimg-rootfs / ext4 defaults 0 0
LABEL=UEFI /boot/efi vfat defaults 0 0
/dev/vdb /mnt auto defaults,nofail,x-systemd.requires=cloud-init.service,comment=cloudconfig 0 2

# umount /mnt
# grep vdb /etc/mtab
# grep vdb /proc/mounts

# zpool create -f lxd /dev/vdb
/dev/vdb is in use and contains a ext4 filesystem.

Then edit out /dev/vdb from /etc/fstab and reboot:

# vim /etc/fstab
LABEL=cloudimg-rootfs / ext4 defaults 0 0
LABEL=UEFI /boot/efi vfat defaults 0 0
#/dev/vdb /mnt auto defaults,nofail,x-systemd.requires=cloud-init.service,comment=cloudconfig 0 2

# reboot

# zpool create -f lxd /dev/vdb
(no error)

Definitely *something* is caching that it was mounted? Kernel, ZFS, something else?