* martin f krafft <madd...@debian.org> [2010-05-26 10:31]:
> also sprach Neil Brown <ne...@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/
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