[Bug 2147295] Re: Add rd.auto=1 to grub for storage with md RAID

[Dan Bungert]

** Also affects: subiquity (Ubuntu)
   Importance: Undecided
       Status: New

** Changed in: subiquity (Ubuntu)
    Milestone: None => ubuntu-26.04

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Title:
  Add rd.auto=1 to grub for storage with md RAID

Status in subiquity:
  New
Status in subiquity package in Ubuntu:
  New

Bug description:
  A fresh Ubuntu 26.04 Server (beta) installation using autoinstall with
  the root filesystem on md RAID1 fails to boot after installation. The
  system drops to a dracut emergency shell with an error indicating the
  root device UUID (the md device) cannot be found.

  Steps to reproduce:

  1. Prepare an autoinstall user-data with md RAID1 as the root filesystem, e.g.:
  storage:
    config:
      - {type: disk, id: nvme0, ptable: gpt, match: {path: /dev/nvme0n1}, ...}
      - {type: disk, id: nvme1, ptable: gpt, match: {path: /dev/nvme1n1}, ...}
      - {type: partition, id: nvme0p3, device: nvme0, number: 3, size: 32G}
      - {type: partition, id: nvme1p3, device: nvme1, number: 3, size: 32G}
      - {type: raid, id: md0, name: md0, devices: [nvme0p3, nvme1p3], raidlevel: raid1}
      - {type: format, id: fs-root, volume: md0, fstype: ext4}
      - {type: mount, id: mount-root, device: fs-root, path: /}

  2. Boot the Ubuntu 26.04 Server ISO and launch the autoinstall via GRUB:
  linux /casper/vmlinuz ip=dhcp autoinstall 'ds=nocloud;s=http://SERVER/path/' ---
  3. Installation completes successfully.
  4. On first reboot, the system drops to a dracut emergency shell.

  Expected result:

  The system boots normally after installation.

  Actual result:

  Dracut fails with Failed to mount sysroot.mount - /sysroot. The root
  device UUID is not found because the md RAID arrays have not been
  assembled. From the dracut emergency shell, running mdadm --assemble
  --scan makes the UUID appear, confirming the arrays are present but
  not assembled.

  Workaround:

  Adding rd.auto=1 to the kernel command line via the GRUB editor allows
  the system to boot. Making it permanent:

  echo 'GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX rd.auto=1"' > /etc/default/grub.d/50-md.cfg
  update-grub

  Understanding of the potential root cause:

  Ubuntu 26.04 switched the initramfs generator from initramfs-tools to
  dracut. initramfs-tools automatically assembles all md arrays during
  early boot. Dracut does not. It requires the rd.auto=1 kernel
  parameter (or explicit rd.md.uuid= entries) to assemble md arrays
  before mounting root.

  Subiquity does not add this parameter when configuring GRUB for the
  target system. The relevant code is in subiquity/models/filesystem.py,
  FilesystemModel.render(). The curtin config it generates contains a
  grub section but no mechanism to add kernel command line parameters.
  Curtin's BootCfg (in curtin/config.py) also has no field for custom
  kernel arguments.

  A minimal fix is to have FilesystemModel.render() detect when md RAID
  arrays are present in the storage configuration and add a write_files
  entry that creates /etc/default/grub.d/50-curtin-md.cfg with
  GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX rd.auto=1". This file is
  written before curtin runs setup_grub / update-grub, so the parameter
  is picked up and baked into grub.cfg.

  A pull request with this fix is available at:
  https://github.com/canonical/subiquity/pull/2355

  Affected version: Ubuntu 26.04 beta, Subiquity from the 26.04 server
  ISO

  Not affected: Ubuntu 24.04 and earlier (which use initramfs-tools)

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