[Bug 964052] Re: mdadm: created as /dev/md0, becomes /dev/md127 after reboot.

[TJ]

I dealt with this issue recently and initially looked at it as a bug,
but having explored it some more, I have come to the conclusion that it
is not - it is by design, and for a purpose.

During early userspace when the initrd.img's /init shell script is
running mdadm is triggered. It examines /etc/mdadm/mdadm.conf (in the
initrd.img) for ARRAY entries and will assemble those, but it will also
assemble other arrays it finds that are not defined in the conf file.

Since arrays defined using ARRAY usually have device numbers starting at
0, the dynamically discovered arrays are numbered starting at 127 to
avoid them using the device number of MD devices defined and assembled
later when Upstart has taken over.

With an entry in the root file-sytem's /etc/mdadm/mdadm.conf of, for
example:

ARRAY /dev/md0 UUID=...

the update-intramfs hook scripts installed by mdadm will copy that into
the initrd when it is built and the device will receive its correct
designation.

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Title:
  mdadm: created as /dev/md0, becomes /dev/md127 after reboot.

Status in “mdadm” package in Ubuntu:
  Confirmed

Bug description:
  -- lsb_release -rd
  Description:    Ubuntu precise (development branch)
  Release:        12.04
  --

  -- apt-cache policy mdadm
  mdadm:
    Installed: 3.2.3-2ubuntu1
    Candidate: 3.2.3-2ubuntu1
    Version table:
   *** 3.2.3-2ubuntu1 0
          500 http://us.archive.ubuntu.com/ubuntu/ precise/main amd64 Packages
          100 /var/lib/dpkg/status
  --

  -- Environment
  Ubuntu 12.04 Server Edition 64bit (installed from the 20120323 daily ISO)
  Installed into a guest running on VirtualBox 4.1.10, hosted on Windows 7.
  Disk configuration: 
  1 SATA controller
  1 20GB HDD (used at / mount point)
  5x 512MB HDD (used to simulate RAID5 array)
  --

  -- What action I took
  On an untouched installation, I install mdadm, and attempt to create a RAID5 array with this command:

  sudo mdadm --create /dev/md0 --level=5 --raid-devices=5 /dev/sdb
  /dev/sdc /dev/sdd /dev/sde /dev/sdf

  After confirming that I wanted to create the array, mdadm reported success.  I initialized a file system on it, mounted it, and created a simple text file containing 'Hello World' on it.  Then, I reboot.
  --

  -- What I expected to happen
  mdadm would reassemble the array at boot time as /dev/md0.
  --

  -- What happened instead
  mdadm reassemled the array at boot time as /dev/md127.
  --

  It took a great deal if hunting around to discover that the array had
  actually been assembled.  I forget what command I issued that finally
  tipped me off to the existence of the assembled array, but running
  'sudo mdadm --detail /dev/md127' confirmed that it was the array I had
  just built.  I mounted the array and viewed the contents of the text
  file to confirm.

  This did not occur when I built my array on my test and production
  servers running Ubuntu 10.04 Server 64bit, so I am calling this new
  and unexpected behavior, and filing it as a bug.

  ProblemType: Bug
  DistroRelease: Ubuntu 12.04
  Package: mdadm 3.2.3-2ubuntu1
  ProcVersionSignature: Ubuntu 3.2.0-20.32-generic 3.2.12
  Uname: Linux 3.2.0-20-generic x86_64
  ApportVersion: 1.95-0ubuntu1
  Architecture: amd64
  CurrentDmesg:
   
  Date: Sat Mar 24 11:05:07 2012
  InstallationMedia: Ubuntu-Server 12.04 LTS "Precise Pangolin" - Alpha amd64 (20120323)
  MDadmExamine.dev.sda: Error: command ['/sbin/mdadm', '-E', '/dev/sda'] failed with exit code 1: mdadm: cannot open /dev/sda: Permission denied
  MDadmExamine.dev.sda1: Error: command ['/sbin/mdadm', '-E', '/dev/sda1'] failed with exit code 1: mdadm: cannot open /dev/sda1: Permission denied
  MDadmExamine.dev.sda2: Error: command ['/sbin/mdadm', '-E', '/dev/sda2'] failed with exit code 1: mdadm: cannot open /dev/sda2: Permission denied
  MDadmExamine.dev.sda5: Error: command ['/sbin/mdadm', '-E', '/dev/sda5'] failed with exit code 1: mdadm: cannot open /dev/sda5: Permission denied
  MDadmExamine.dev.sdb: Error: command ['/sbin/mdadm', '-E', '/dev/sdb'] failed with exit code 1: mdadm: cannot open /dev/sdb: Permission denied
  MDadmExamine.dev.sdc: Error: command ['/sbin/mdadm', '-E', '/dev/sdc'] failed with exit code 1: mdadm: cannot open /dev/sdc: Permission denied
  MDadmExamine.dev.sdd: Error: command ['/sbin/mdadm', '-E', '/dev/sdd'] failed with exit code 1: mdadm: cannot open /dev/sdd: Permission denied
  MDadmExamine.dev.sde: Error: command ['/sbin/mdadm', '-E', '/dev/sde'] failed with exit code 1: mdadm: cannot open /dev/sde: Permission denied
  MDadmExamine.dev.sdf: Error: command ['/sbin/mdadm', '-E', '/dev/sdf'] failed with exit code 1: mdadm: cannot open /dev/sdf: Permission denied
  MachineType: innotek GmbH VirtualBox
  ProcEnviron:
   TERM=xterm
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-3.2.0-20-generic root=/dev/mapper/hostname-root ro
  SourcePackage: mdadm
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 12/01/2006
  dmi.bios.vendor: innotek GmbH
  dmi.bios.version: VirtualBox
  dmi.modalias: dmi:bvninnotekGmbH:bvrVirtualBox:bd12/01/2006:svninnotekGmbH:pnVirtualBox:pvr1.2:
  dmi.product.name: VirtualBox
  dmi.product.version: 1.2
  dmi.sys.vendor: innotek GmbH
  etc.blkid.tab: Error: [Errno 2] No such file or directory: '/etc/blkid.tab'

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