This bug was fixed in the package cloud-init - 0.7.8-3-g80f5ec4-0ubuntu1

---------------
cloud-init (0.7.8-3-g80f5ec4-0ubuntu1) yakkety; urgency=medium

  * New upstream snapshot.
    - Adjust mounts and disk configuration for systemd. (LP: #1611074)
    - dmidecode: run dmidecode only on i?86 or x86_64 arch. [Robert Schweikert]

 -- Scott Moser <smo...@ubuntu.com>  Tue, 20 Sep 2016 13:59:20 -0400

** Changed in: cloud-init (Ubuntu)
       Status: Confirmed => Fix Released

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https://bugs.launchpad.net/bugs/1611074

Title:
  Reformatting of ephemeral drive fails on resize of Azure VM

Status in cloud-init:
  Confirmed
Status in cloud-init package in Ubuntu:
  Fix Released

Bug description:
  === Begin SRU Template ===
  [Impact] 
  In some cases, cloud-init writes entries to /etc/fstab, and on azure it will
  even format a disk for mounting and then write the entry for that 'ephemeral'
  disk there.

  A supported operation on Azure is to "resize" the system.  When you do this
  the system is shut down, resized (given larger/faster disks and more CPU) and
  then brought back up.  In that process, the "ephemeral" disk re-initialized
  to its original NTFS format.  The designed goal is for cloud-init to recognize
  this situation and re-format the disk to ext4.

  The problem is that the mount of that disk happens before cloud-init can
  reformat.  Thats because the entry in fstab has 'auto' and is automatically
  mounted.  The end result is that after resize operation the user will be left
  with the ephemeral disk mounted at /mnt and having a ntfs filesystem rather
  than ext4.

  [Test Case]
  The text in comment 3 describes how to recreate by the original reporter.
  Another way to do this is to just re-format the ephemeral disk as
  ntfs and then reboot.  The result *should* be that after reboot it
  comes back up and has an ext4 filesystem on it.

  1.) boot system on azure
    (for this, i use https://gist.github.com/smoser/5806147, but you can
     use web ui or any other way).

  2.) unmount the ephemeral disk
     $ umount /mnt

  3.) repartition it so that mkfs.ntfs does less and is faster
     This is not strictly necessary, but mkfs.ntfs can take upwards of
     20 minutes.  shrinking /dev/sdb2 to be 200M means it will finish
     in < 1 minute.

     $ disk=/dev/disk/cloud/azure_resource
     $ part=/dev/disk/cloud/azure_resource-part1
     $ echo "2048,$((2*1024*100)),7" | sudo sfdisk "$disk"
     $ time mkfs.ntfs --quick "$part"

  4.) reboot
  5.) expect that /proc/mounts has /dev/disk/cloud/azure_resource-part1 as ext4
      and that fstab has x-systemd.requires in it.

      $ awk '$2 == "/mnt" { print $0 }' /proc/mounts 
      /dev/sdb1 /mnt ext4 rw,relatime,data=ordered 0 0

      $ awk '$2 == "/mnt" { print $0 }' /etc/fstab
      /dev/sdb1 /mnt auto 
defaults,nofail,x-systemd.requires=cloud-init.service,comment=cloudconfig 0 2

  [Regression Potential] 
  Regression is unlikely.  Likely failure case is just that the problem is not
  correctly fixed, and the user ends up with either an NTFS formated disk that
  is mounted at /mnt or there is nothing mounted at /mnt.

  === End SRU Template ===

  After resizing a 16.04 VM on Azure, the VM is presented with a new
  ephemeral drive (of a different size), which initially is NTFS
  formatted. Cloud-init tries to format the appropriate partition ext4,
  but fails because it is mounted. Cloud-init has unmount logic for
  exactly this case in the get_data call on the Azure data source, but
  this is never called because fresh cache is found.

  Jun 27 19:07:47 azubuntu1604arm [CLOUDINIT] handlers.py[DEBUG]: start: 
init-network/check-cache: attempting to read from cache [trust]
  Jun 27 19:07:47 azubuntu1604arm [CLOUDINIT] util.py[DEBUG]: Reading from 
/var/lib/cloud/instance/obj.pkl (quiet=False)
  Jun 27 19:07:47 azubuntu1604arm [CLOUDINIT] util.py[DEBUG]: Read 5950 bytes 
from /var/lib/cloud/instance/obj.pkl
  Jun 27 19:07:47 azubuntu1604arm [CLOUDINIT] stages.py[DEBUG]: restored from 
cache: DataSourceAzureNet [seed=/dev/sr0]
  Jun 27 19:07:47 azubuntu1604arm [CLOUDINIT] handlers.py[DEBUG]: finish: 
init-network/check-cache: SUCCESS: restored from cache: DataSourceAzureNet 
[seed=/dev/sr0]
  ...
  Jun 27 19:07:48 azubuntu1604arm [CLOUDINIT] cc_disk_setup.py[DEBUG]: Creating 
file system None on /dev/sdb1
  Jun 27 19:07:48 azubuntu1604arm [CLOUDINIT] cc_disk_setup.py[DEBUG]:      
Using cmd: /sbin/mkfs.ext4 /dev/sdb1
  Jun 27 19:07:48 azubuntu1604arm [CLOUDINIT] util.py[DEBUG]: Running command 
['/sbin/mkfs.ext4', '/dev/sdb1'] with allowed return codes [0] (shell=False, 
capture=True)
  Jun 27 19:07:48 azubuntu1604arm [CLOUDINIT] util.py[DEBUG]: Creating fs for 
/dev/disk/cloud/azure_resource took 0.052 seconds
  Jun 27 19:07:48 azubuntu1604arm [CLOUDINIT] util.py[WARNING]: Failed during 
filesystem operation#012Failed to exec of '['/sbin/mkfs.ext4', 
'/dev/sdb1']':#012Unexpected error while running command.#012Command: 
['/sbin/mkfs.ext4', '/dev/sdb1']#012Exit code: 1#012Reason: -#012Stdout: 
''#012Stderr: 'mke2fs 1.42.13 (17-May-2015)\n/dev/sdb1 is mounted; will not 
make a filesystem here!\n'

  $ lsb_release -rd
  Description:    Ubuntu 16.04.1 LTS
  Release:        16.04
  $ cat /etc/cloud/build.info
  build_name: server
  serial: 20160721
  ~$ dpkg -l cloud-init
  Desired=Unknown/Install/Remove/Purge/Hold
  | Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
  |/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
  ||/ Name                             Version               Architecture       
   Description
  
+++-================================-=====================-=====================-=====================================================================
  ii  cloud-init                       0.7.7~bzr1256-0ubuntu all                
   Init scripts for cloud instances

  We're seeing ~100% repro of this bug on resize, where the only success
  cases are caused by another bug that messes up fstab and prevents
  mounting of the drive.

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