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[Launchpad Bug Tracker <1854981@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package lvm2 - 2.02.176-4.1ubuntu3.18.04.2

---------------
lvm2 (2.02.176-4.1ubuntu3.18.04.2) bionic; urgency=medium

  * d/p/fix-auto-activation-at-boot.patch: (LP: #1854981)
    Allow LV auto-activation (e.g. /usr on it's separate LV)

 -- Eric Desrochers <eric.desrochers@xxxxxxxxxxxxx>  Thu, 05 Dec 2019
15:15:56 +0000

** Changed in: lvm2 (Ubuntu Bionic)
       Status: Fix Committed => Fix Released

** Changed in: lvm2 (Ubuntu Disco)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1854981

Title:
  system doesn't properly boot as expected if /usr is on its own LV

Status in initramfs-tools package in Ubuntu:
  Won't Fix
Status in lvm2 package in Ubuntu:
  Fix Released
Status in initramfs-tools source package in Xenial:
  Won't Fix
Status in lvm2 source package in Xenial:
  Fix Released
Status in initramfs-tools source package in Bionic:
  Won't Fix
Status in lvm2 source package in Bionic:
  Fix Released
Status in initramfs-tools source package in Disco:
  Won't Fix
Status in lvm2 source package in Disco:
  Fix Released

Bug description:
  [Impact]
  In summary, the problem is a LVM 'auto-activation' issue. Basically the LVM activation doesn't occurs during the boot process (except for LVM root partition) and since /usr is an important component before the 'pivot'[0], the init inside the initramfs space tries to mount /usr. If /usr is a LVM volume then it will try to mount it while its backend device is not activated yet (due to the actual auto-activation issue), thus the mounting will fails and bring the user to the initramfs prompt for debugging.

  Workaround:
  At the initramfs prompt: lvm vgchange -ay and then 'ctrl-d' to resume the boot process.

  [Test Case]

  - Install Bionic or Disco
  - Create a VG with /usr on its on LV
  - Complete the installation
  - At reboot the system will be stuck at mounting /usr file system

  Extra notes:
  Since this problem is ONLY related to LVM activation. If one create a non-LVM /usr separate partition, everything will work as expected.

  Other separate LVM (e.g. /home, /var/, /srv) if on its own separate
  LVM volume will not be activate either, but since they don't need to
  be mounted in the initramfs space, it's not a problem, they can be
  activated later on (after the pivot) by systemd.

  So the only combination possible is when /usr is on its separate LV
  only.

  [Regression potential]

  I don't foresee any regression, the fix will instruct udev rule to
  pvscan activate volume based on their major/minor number if LVM is
  scanned.

  +RUN+="(LVM_EXEC)/lvm pvscan --cache --activate ay --major $major
  --minor $minor", ENV{LVM_SCANNED}="1"

  The current situation doesn't auto-activate at all.

  One downside, for users who will see this fixed, will still experience
  the situation in standard ISO, since I'm afraid Disco won't produces
  new ISO. (it is not recommended to do new disco installs anyway since
  it's going EOL shortly) One would need to rely on the mini.iso for
  fetching the latest lvm2 package in the archive.

  IIRC Bionic will have a new point release somewhere in Feb 2020 but
  until then one would need to rely on the mini.iso for fetching the
  latest lvm2 package in the archive as well.

  or use the workaround in the [Impact] section and then apt-get dist-
  upgrade in order to get the latest lvm2.

  [Other information]

  This problem only exhibit in Bionic and Disco.
  Xenial and Eoan and late didn't exhibit the situation.

  https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge/
  https://wiki.debian.org/UsrMerge

  [Original Description]

  Only the lv for root volume get activated, because of the grub
  parameter that specifies/enforce it "root=/dev/mapper/ubuntu-vg-ubuntu
  --lv"

  http://man7.org/linux/man-pages/man7/bootparam.7.html
  'root=...'
                This argument tells the kernel what device is to be used as
                the root filesystem while booting.

  If one add a separate LV for /usr, the system will go straight to
  initramfs prompt failling to mount /usr.

  At initramfs prompt, we notice that 'lv-usr' status is 'NOT
  available'.

  Performing 'lvm vgchange -ay' at initramfs prompt workaround the
  problem and allow one to successfully boot.

  Adding 'debug' parameter, we clearly we see /root being detected and mounted:
  initramfs.debug:
  => + mount -r -t ext4 /dev/mapper/ubuntu--vg-ubuntu--lv /root
  => + mountroot_status=0
  + [ ]
  + log_end_msg
  + _log_msg done.\n
  + [ n = y ]
  + printf done.\n
  done.
  + read_fstab_entry /usr
  + found=1
  + [ -f /root/etc/fstab ]
  + read MNT_FSNAME MNT_DIR MNT_TYPE MNT_OPTS MNT_FREQ MNT_PASS MNT_JUNK
  + [ / = /usr ]
  + read MNT_FSNAME MNT_DIR MNT_TYPE MNT_OPTS MNT_FREQ MNT_PASS MNT_JUNK
  + [ /usr = /usr ]
  + [ -n ]
  + found=0
  + break 2
  + return 0
  + log_begin_msg Mounting /usr file system
  + _log_msg Begin: Mounting /usr file system ...

  then the code read /etc/fstab and specifically search for /usr (most
  likely because of the /usr binary merged) and try to mount if if
  found.

  initramfs-tools:init
  271 maybe_break mount
  272 log_begin_msg "Mounting root file system"
  273 # Always load local and nfs (since these might be needed for /etc or
  274 # /usr, irrespective of the boot script used to mount the rootfs).
  275 . /scripts/local
  276 . /scripts/nfs
  277 . /scripts/${BOOT}
  278 parse_numeric "${ROOT}"
  279 maybe_break mountroot
  280 mount_top
  281 mount_premount
  282 mountroot
  283 log_end_msg
  284
  => 285 if read_fstab_entry /usr; then
  => 286 log_begin_msg "Mounting /usr file system"
  => 287 mountfs /usr
  => 288 log_end_msg
  => 289 fi

  In this case, /usr is present in /etc/fstab but the logical volume is
  not available, so it is mounting a filesystem without his backend
  device, thus goes straight to the initramfs prompt because /usr
  couldn't be mounted.

  It clearly seems to be an 'auto-activation' issue at boot.

  For other such as /home, /var, it's not a big deal cause they can be
  activated later on in the process and they are (I haven't check but I
  guess systemd or systemd unit/generator is taking care of it at some
  point), but /usr is a important piece to have mounted before the pivot
  since it contains most of the crucial binary, especially that nowadays
  /sbin, /bin, and /lib are pointing to /usr:

  lrwxrwxrwx   1 root root         10 Aug 26 13:51 libx32 -> usr/libx32
  lrwxrwxrwx   1 root root          9 Aug 26 13:51 lib64 -> usr/lib64
  lrwxrwxrwx   1 root root          9 Aug 26 13:51 lib32 -> usr/lib32
  lrwxrwxrwx   1 root root          7 Aug 26 13:51 lib -> usr/lib
  lrwxrwxrwx   1 root root          7 Aug 26 13:51 bin -> usr/bin
  lrwxrwxrwx   1 root root          8 Aug 26 13:51 sbin -> usr/sbin

  NOTE:

  * That doesn't affect /usr found in /etc/fstab on its separate
  partition when no LVM involve (e.g. /dev/vdb /usr ext4 ....). It only
  cause issue when /usr is in a LVM context.

  * I was able to reproduce on Bionic and Disco so far.
  Eoan doesn't seem to exhibit the situation so far in my testing.

  * While certain release such as Bionic, Xenial doesn't come implicitly
  with the /usr merge approach. One can install package 'usrmerge' and
  convert their system /usr merged. I don't think removing the
  read_fstable_entry for /usr is an option here, as some user could
  potentially decide to convert their system with 'usrmerge' pkg.

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