[Bug 1681244] Re: mdadm fails to assemble IMSM - Cannot assemble mbr metadata on /dev/md126

[François]

I can confirm bug still present in Ubuntu 17.04 (mdadm version
unchanged)

** Also affects: mdadm
   Importance: Undecided
       Status: New

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Title:
  mdadm fails to assemble IMSM - Cannot assemble mbr metadata on
  /dev/md126

Status in mdadm:
  New
Status in mdadm package in Ubuntu:
  New

Bug description:
  /* General Description
  ******************************************************/

  I'm having a problem using mdadm in a "fakeraid IMSM" environment.  It
  seems like the container Volume0_0 is found and assembled but the
  partitions (x3) are never put together.  The setup works properly when
  manually using a mix of dmraid for IMSM and mdadm for my other RAID1
  regular linux software raid devices.

  
  /* System *******************************************************************/

  Ubuntu 16.10
  mdadm 3.4-4

  
  /* Debug Information ********************************************************/

  Please consider the following debug trace during initramfs boot up:

  /dev/sdb is identified as a member of /dev/md/imsm0, slot -1.
  /dev/sda is identified as a member of /dev/md/imsm0, slot -1.
  added /dev/sda to /dev/md/imsm0 as -1
  added /dev/sdb to /dev/md/imsm0 as -1
  Container /dev/md/imsm0 has been assembled with 2 drives
  looking for devices for further assembly
  [...]
  looking in container /dev/md/imsm0
  found match on member /md127/0 in /dev/md/imsm0
  Started /dev/md/Volume0_0 with 2 devices
  looking for devices for further assembly
  [...]
  looking in container /dev/md/imsm0
  member /md127/0 in /dev/md/imsm0 is already assembled
  looking for devices for further assembly
  [...]
  looking for devices for further assembly
  Cannot assemble mbr metadata on /dev/md126
  [...]

  Addtionnal info : GParted is complaining that "Not all of the space
  available to /dev/sda appears to be used, you can fix the GPT to use
  all of the space (an extra 50016680 blocks) or continue with the
  current setting?", and I also click ignore.  Same applies for
  /dev/sdb.

  
  /* Disks information ********************************************************/

  IMSM RAID1 on /dev/sda and /dev/sdb SSDs (GPT with protective MBR)

  DMRAID output (working, for reference only)

  isw_hggicbcjd_Volume0
  - isw_hggicbcjd_Volume0p1 - mounted on /boot/efi - vfat
  - isw_hggicbcjd_Volume0p2 - mounted on /boot - ext4
  - isw_hggicbcjd_Volume0p3 - mounted on / - LVM on luks

  
  /* Workaround ***************************************************************/

  To boot my system, I go through these steps to fallback on dmraid :
  kernel option break=mount

  from initramfs prompt
  - mdadm --stop /dev/md126
  - mdadm --stop /dev/md127
  - dmraid -ay
  - cryptsetup luksOpen /dev/mappper/isw_hggicbcjd_Volume0p3 isw_hggicbcjd_Volume0p3_crypt
  - CTRL-D

  
  ##############################################################################
  INITIALLY REPORTED HERE
  https://ubuntuforums.org/showthread.php?t=2353942

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