Re: mdadm named array (metadata 1.2) and udev when assembling an array in initramfs

[Michael Tokarev <mjt@tls.msk.ru>]

01.11.2014 21:53, Stephen Kent wrote:
> I have set up a machine to use an md array as the root device. I have
> created the array with metadata type 1.2, name "root," on host
> "optimus" (so the full metadata name is "optimus:root").
> 
> I'm using Gentoo Linux (kernel 3.16.5-gentoo), and previously I had no
> initramfs so I wrote a short one to assemble the array. My initramfs
> sets the hostname and runs "mdadm -As," which correctly auto-assembles
> the root device array on /dev/md/root (even without an
> /etc/mdadm.conf).
> 
> Once the initramfs script is finished, init starts which then starts
> udev. Now, at this point the array is already assembled, but the
> /dev/md/* name is lost (and so is /run/mdadm/map) since those files
> were on the initramfs' tempfs root.

This is your problem.  You should not lose neither /dev nor /run
when switching root.  Usual thing to do is to mount tmpfs /dev and
tmpfs /run in initramfs and do mount --move /dev /root/dev (and
the same for /run) before switching root.

Thanks,

/mjt

>   udev creates a /dev/mdXXX node
> (ex. /dev/md127) and then runs a rule to try to figure out the
> /dev/md/* name, which should be /dev/md/root. The default udev rule
> calls mdadm --detail --export /dev/md127, which should print
> MD_DEVNAME=root for udev, but instead it prints
> MD_DEVNAME=optimus:root, and the symlink in /dev/md gets created as
> /dev/md/optimus:root.
> 
> I think this is a bug in mdadm, but I'm not sure how to dig into it. I
> have made sure that my main OS init sets the hostname before starting
> udev, and I have tried with and without "HOMEHOST optimus" in
> /etc/mdadm.conf.
> 
> The workaround I am using right now is to define the array name
> manually in /etc/mdadm.conf like this:
> 
> ARRAY /dev/md/root UUID=782a671c:e646f2d5:848baad0:5668ee84
> 
> This works by avoiding automatic name detection and using the defined
> name (/dev/md/root), but it really seems that mdadm should know how to
> determine MD_DEVNAME correctly for an already-started array if the
> hostname (or homehost) is correctly set. Is this a bug in mdadm? If
> so, is this the correct place to report it or is there a bug tracking
> system I should use? I am using mdadm v3.3.1.
> 
> (Please cc me on replies as I am not currently subscribed to the
> linux-raid list.)
> 
> Thanks,
> 
> Stephen
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