[Bug 1801349] Re: zpool create -f lxd /dev/vdb fails on cosmic (18.10) -- func27

I added debug into the kernel and observed /dev/vdb is being mounted by
systemd:

[ 9.859895] mount: 1624 mount, parent 1 systemd
[ 9.866398] mount: 1624 mount /dev/vdb

and also snap-confine is mounting mount too:

[ 9.966827] mount: 1663 snap-confine /mnt

Curiously, I've not seen any associated umounts.

I wonder if the namespace/confinement is causing the issue of not being
able to see the mount. I need to debug this further.

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Title:
zpool create -f lxd /dev/vdb fails on cosmic (18.10) -- func27

Status in OpenStack LXD Charm:
New
Status in linux package in Ubuntu:
In Progress

Bug description:
Test: tests/gate-basic-cosmic-rocky

As part of the config, the lxd charm creates a pool device depending
on the config. The test config is:

lxd_config = {
'block-devices': '/dev/vdb',
'ephemeral-unmount': '/mnt',
'storage-type': 'zfs',
'overwrite': True
}

The config drive is normally mounted on /mnt, and the lxd charm
umounts it as part of the start up. The /etc/fstab on the unit is:

# 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
/dev/vdc none swap sw,comment=cloudconfig 0 0


However, even after umount-ing the /mnt off of /dev/vdb, the zpool create command still fails:

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


If the /etc/fstab is edited so that /dev/vdb is *never* mounted and then rebooted, then the zpool create command succeeds:

# zpool list
NAME SIZE ALLOC FREE EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT
lxd 14.9G 106K 14.9G - 0% 0% 1.00x ONLINE -

# zpool status lxd
pool: lxd
state: ONLINE
scan: none requested
config:

NAME STATE READ WRITE CKSUM
lxd ONLINE 0 0 0
vdb ONLINE 0 0 0

errors: No known data errors

Something odd is going on with cosmic (18.10) and the combination of
lxd, zfs and the kernel

lxd version: 3.6
zfsutils-linux/cosmic,now 0.7.9-3ubuntu6
Linux: 4.18.0-10-generic

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