[Bug 158918] Re: [->UUIDudev] installing mdadm (or outdated mdadm.conf) breaks bootup

[mark glanville]

This is still a problem in 12.04.1 Server AMD64.
OS is installed on a single drive and the machine also contains 6 drives that I want to configure as RAID0. Installed and configured OS fine, installed mdadm and now can't boot.
The 6 drives destined for the array are brand new so have never been part of a RAID array. The OS drive was previously part of a RAID array, but obviously was formatted during installation.

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Title:
  [->UUIDudev] installing mdadm (or outdated mdadm.conf) breaks bootup

Status in “mdadm” package in Ubuntu:
  Triaged

Bug description:
  Original Report (also confirmed with 9.10 and 10.04 beta1):

  On a freshly installed ubuntu-7.10-alternate, with latest apt-get
  update.

  When the 'mdadm' package is installed, the system fails to boot
  successfully, and ends up at the initrd '(busybox)' prompt.

  Hardware: DELL 1950 - 1RU Server
  HDD: SAS

  To get the server booting again you need to revert to the old
  initramfs:

  - Boot with ubuntu-7.10-alternate, and go through install steps up to 'partitioning'.
  - ALT-F2 to start other shell
  - 'fdisk -l' to see details of available drives.
  - mkdir /mnt/disk
  - mount -t ext3 /dev/sdb1 /mnt/disk
  - cd /mnt/disk/boot
  - mv initrd-<version>.img initrd-<version>.img-new
  - cp initrd-<version>.img.bak initrd-<version>.img
  - sync
  - reboot

  ---
  Diagnose:

  -> This is mdadm setting up arrays according to unreliable superblock
  information (device "minor" numbers, labels, hostnames) combined with
  the idea of fixing the unreliability by limiting array assembly with
  information from mdadm.conf (PARTITIONS, ARRAY, HOMEHOST lines) which
  just reassigns the unsolvable conflict handling problem to setup
  tools, admins and installers. It forces them to create mdadm.conf
  files. And of course they fail.

  In cases where old superblocks are found on the disks during mdadm
  install, they are added to ARRAY definitions  (that really shouldn't
  need to be there at all) in mdadm.conf, and copied over into the
  initramfs. During next boot the system can not assemble these
  (incomplete) arrays.

  Cure:

  Systematically prevent conflicts from arising instead of relying on
  mdadm.conf maintanance. -> Do not depend on mdadm.conf definitions but
  use UUID-based array assembly as described in comment #33

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