MD_AUTODETECT=y kernel option is obsolete (boot delay and udev/mdadm (raid) disturbance)

[ceg]

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.

[ceg (ceg)]

To disable raid assembly by the kernel (before it is disabled in the kernel config) add the boot option "raid=noautodetect".

[Jeremy Foshee (jeremyfoshee)]

Hi ceg,

Please be sure to confirm this issue exists with the latest development release of Ubuntu. ISO CD images are available from http://cdimage.ubuntu.com/releases/ . If the issue remains, please run the following command from a Terminal (Applications->Accessories->Terminal). It will automatically gather and attach updated debug information to this report.

apport-collect -p linux 551719

Also, if you could test the latest upstream kernel available that would be great. It will allow additional upstream developers to examine the issue. Refer to https://wiki.ubuntu.com/KernelMainlineBuilds . Once you've tested the upstream kernel, please remove the 'needs-upstream-testing' tag. This can be done by clicking on the yellow pencil icon next to the tag located at the bottom of the bug description and deleting the 'needs-upstream-testing' text. Please let us know your results.

Thanks in advance.

    [This is an automated message. Apologies if it has reached you inappropriately; please just reply to this message indicating so.]

[ceg (ceg)]

I don't see any change in the CONFIG_MD_AUTODETECT setting.

[ceg (ceg)]

This is simpy an unneeded and adverse option enabled in the default kernel configuration. Not much to to with upstream.

[ceg (ceg)]

Please nominate this quick correction of the kernel config for natty.

[donarntz (donarntz)]

This is a pretty big pain, as all my music files are on a raid10.

[ceg (ceg)]

This can not be a duplicate of bug #942106. This is about an obsolete kernel option for very old style kernel raid assembly. It slows mdadm assembly down and can grab devices.

In case a patch to another bug fixes this bug too, please mark the patch as closing multiple bugs.

[Jean-Philippe Guérard (fevrier)]

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.

[ceg (ceg)]

Same symptoms do not mean the same cause (bug).

Many thanks for hinting to the module restriction. This bug can be closed then. :)