Comment 12 for bug 1680197

Something I'm looking further into is:

1) kernel command line using overlayroot=tmpfs

Kernel command line: BOOT_IMAGE=ubuntu/amd64/ga-17.04/zesty/daily/boot-kernel nomodeset iscsi_target_name=iqn.2004-05.com.ubuntu:maas:ephemeral-ubuntu-amd64-ga-17.04-zesty-daily iscsi_target_ip=172.24.124.1 iscsi_target_port=3260 iscsi_initiator=kzanol ip=::::kzanol:BOOTIF ip6=off ro root=/dev/disk/by-path/ip-172.24.124.1:3260-iscsi-iqn.2004-05.com.ubuntu:maas:ephemeral-ubuntu-amd64-ga-17.04-zesty-daily-lun-1 overlayroot=tmpfs cc:{datasource_list: [MAAS]}end_cc cloud-config-url=http://172.24.124.1:5240/MAAS/metadata/latest/by-id/xr36pd/?op=get_preseed log_host=172.24.124.1 log_port=514 BOOTIF=01-f0:92:1c:5e:1e:70

2) issues with systemd mount, likely triggered after partitioning

Apr 6 13:26:53 kzanol cloud-init[2457]: curtin: Installation started. (0.1.0~bzr470-0ubuntu1~16.04.1)
Apr 6 13:26:57 kzanol kernel: [ 81.464816] sda: sda1 sda15
Apr 6 13:26:57 kzanol systemd[1]: Stopped target Local File Systems.
Apr 6 13:26:57 kzanol systemd[1]: Unmounting /media/root-ro...
Apr 6 13:26:57 kzanol systemd[1]: Unmounted /media/root-ro.
Apr 6 13:26:58 kzanol systemd[1]: Mounting /media/root-ro...
Apr 6 13:26:58 kzanol kernel: [ 82.109869] EXT4-fs (sda1): mounted filesystem with ordered data mode. Opts: (null)
Apr 6 13:26:58 kzanol systemd[1]: Mounted /media/root-ro.
Apr 6 13:26:58 kzanol cloud-init[2457]: Failed to exclusively open path: /dev/sda1
Apr 6 13:26:58 kzanol cloud-init[2457]: Traceback (most recent call last):
Apr 6 13:26:58 kzanol cloud-init[2457]: File "/curtin/curtin/block/__init__.py", line 773, in exclusive_open
Apr 6 13:26:58 kzanol cloud-init[2457]: fd = os.open(path, os.O_RDWR | os.O_EXCL)
Apr 6 13:26:58 kzanol cloud-init[2457]: OSError: [Errno 16] Device or resource busy: '/dev/sda1'
Apr 6 13:26:58 kzanol cloud-init[2457]: Device holders with exclusive access: []
Apr 6 13:26:58 kzanol cloud-init[2457]: Device mounts: ['/dev/sda1 /media/root-ro ext4 ro,relatime,data=ordered 0 0']

It's likely that curtin is racying with systemd.mount files/entries related to initramfs configuring overlayroot.

Some details for that here:
https://bugs.launchpad.net/ubuntu/+source/open-iscsi/+bug/1666573