[Bug 551719] Re: MD_AUTODETECT=y kernel option is obsolete (boot delay and udev/mdadm (raid) disturbance)

[Jean-Philippe Guérard]

The problem this bug report is about is "boot delay and udev/mdadm
(raid) disturbance". As such, it is a duplicate of #942106.

Then the bug report offers an explanation and a solution.

The MD autodetection only occurs when md drivers are statically compiled
(see https://www.kernel.org/doc/Documentation/md.txt). The Ubuntu MD
drivers are compiled as mdules, so will not be automatically detected.

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https://bugs.launchpad.net/bugs/551719

Title:
  MD_AUTODETECT=y kernel option is obsolete (boot delay and udev/mdadm
  (raid) disturbance)

Status in “linux” package in Ubuntu:
  Confirmed
Status in “mdadm” package in Ubuntu:
  Confirmed

Bug description:
  Ubuntu still uses the kernel option
  CONFIG_MD_AUTODETECT=y
  thus enables the kernel's RAID autodetection during boot.

  However, aside from causing a delay for everyone, this often also disturbs the udev/mdadm setup when raid devices are present in the system.
  (i.e. devices are partly busy/set up by the kernel, without an entry in the mapfile used by mdadm --incremental)

  Here is what README.initrams from mdadm tarball says about the
  kernel's raid autodetection features:

  "while useful, [they] do not provide complete functionality
  and are unlikely to be extended.  The preferred way to assemble md
  arrays at boot time is using 'mdadm' "

  Ubuntu does assemble arrays using udev rules that run mdadm. (In
  initramfs as well as later on.)

  The kernel's raid autodetection can and should therefore be disabled
  in ubuntu kernels.

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