Bug#569359: mdadm -Ds insists on /dev/md/X device presence

[Martin Michlmayr]

* martin f krafft <madduck at debian.org> [2010-05-26 10:31]:
> also sprach Neil Brown <neilb at suse.de> [2010.05.26.1016 +0200]:
> > The most likely explanation for this is that /var/run/mdadm/map existed and
> > contained and entry for 'md0' declaring that it could be found at /dev/md/0.
> > 
> > Normally mdadm will write such a name there when auto-assembling an array,
> > then udev will notice that the array has appeared, will use mdadm to find the
> > path name in /var/run/mdadm/map and will create the node in /dev.
> > 
> > So it would seem that udev is not running, but /dev/.udev exists.
> > If udev was running, it would have created /dev/md/0.
> > If /dev/.udev didn't exist, then mdadm would have created it when assembling
> > the array.
> > 
> > If this is correct, udev is not running but /dev/.udev exists, then you can
> > force mdadm to still create the device with
> >   export MDADM_NO_UDEV=1
> 
> Martin, could you please investigate this further? I don't have much
> experience with d-i, nor really the time to attain that right now.

Thanks for your explanation, Neal.  So here's a description of what's
going on:

We boot into Debian installer.  The installer creates a RAID1 device with:
    mdadm --create /dev/md0 --auto=yes [...]
udev is running.  /dev/md0 exists.  /dev/md/0 does not exist.

There's a file /var/run/map (not /var/run/mdadm/map) which contains
this:
    md0 1.2 dcfdb5af:3b385a1e:def86b24:09a2527f /dev/md0

When I run:
    mdadm --examine --scan --config=partitions
I get:
    ARRAY /dev/md/0 metadata=1.2 UUID=afb5fddc:1e5a383b:246bf8de:7f52a209 name=debian:0
i.e. the /dev/md/0 form.  (Also note that that the UUID in the map
file is different to that reported by --examine --scan; not sure
why.)

After creating the RAID0 and formating it, Debian installer will then
install Debian to disk (in a chroot).  At some point, mdadm is
installed in the chroot.  /dev from the real system is bind mounted
in the chroot.

Debian's mdadm sees that there's no mdadm config file and tries to
generate one.  It runs:
    mdadm --examine --scan --config=partitions
and writes this to /etc/mdadm/mdadm.conf.  The output is:
    ARRAY /dev/md/0 metadata=1.2 UUID=afb5fddc:1e5a383b:246bf8de:7f52a209 name=debian:0
There's no map file in /var/run at this point.

/dev/md0 exists but /dev/md/0 does not.

Debian then generates a ramdisk.  It looks at /etc/mdadm/mdadm.conf
and sees that the RAID device is /dev/md/0 and then does:
    mdadm --detail /dev/md/0
to determine the RAID level.  This fails because /dev/md/0 does not
exist.  As a consequence, the ramdisk won't contain the RAID modules
and will fail to boot.

So my questions (the first one primarily for Neil, the second one
for madduck):

 - Why does mdadm --examine --scan output the /dev/md/X form rather
   than /dev/mdX when no config and map file exists.  Is /dev/md/X
   prefered over /dev/mdX?  If so, maybe Debian installer should
   use
       mdadm --create /dev/md/X
   instead of
       mdadm --create /dev/mdX
   ?

 - Why is /dev/md/X not created?  Well, I guess it's not generated
   because we use /dev/mdX and not even the map file mentions
   /dev/md/X but given that "mdadm --examine --scan" prefers /dev/md/X
   maybe we should add a udev rule to the installer that always
   ensures that /dev/md/X is created?

-- 
Martin Michlmayr
http://www.cyrius.com/