Re: RAID5 superblock and filesystem recovery after re-creation

[NeilBrown <neilb@suse.de>]

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On Mon, 9 Jul 2012 00:45:08 +0200 Alexander Schleifer
<alexander.schleifer@googlemail.com> wrote:

> 2012/7/9 NeilBrown <neilb@suse.de>:
> > On Sun, 8 Jul 2012 23:47:16 +0200 Alexander Schleifer
> > <alexander.schleifer@googlemail.com> wrote:
> >
> >> Hi,
> >>
> >> after a new installation of Ubuntu, my RAID5 device was set to
> >> "inactive". All devices were set to spare device and the level was
> >> unknown. So I tried to re-create the array by the following command.
> >
> > Sorry about that.  In case you haven't seen it,
> >    http://neil.brown.name/blog/20120615073245
> > explains the background
> >
> >>
> >> mdadm --create /dev/md0 --assume-clean --level=5 --raid-disk=6
> >> --chunk=512 --metadata=1.2 /dev/sde /dev/sdd /dev/sda /dev/sdc
> >> /dev/sdg /dev/sdh
> >>
> >> I have a backup of the mdadm -Evvvvs output, so I could recover the
> >> chunk size, metadata and offset (2048) from this information.
> >>
> >> The partially output of mdadm --create... shows this output:
> >>
> >> ...
> >> mdadm: /dev/sde appears to be part of a raid array:
> >>     level=raid5 devices=6 ctime=Sun Jul  8 23:02:51 2012
> >> mdadm: partition table exists on /dev/sde but will be lost or
> >>        meaningless after creating array
> >> ...
> >>
> >> The array is recreated, but no valid filesystem is found on /dev/md0
> >> (dumpe2fs: Filesystem revision too high while trying to open /dev/md0.
> >> Couldn't find valid filesystem superblock.). Also fdisk /dev/sde shows
> >> no partition.
> >> My next step would be creating Linux RAID type partitions on the 6
> >> devices with fdisk and call mdadm --create with /dev/sde1 /dev/sdd1
> >> and so on.
> >> Is this step a possible solution for recovering the filesystem?
> >
> > Depends.. Was the original array created on partitions, or on whole devices?
> > The saved '-E' output should show that.
> >
> > Maybe you have the devices in the wrong order.  The order you have looks odd
> > for a recently created array.
> >
> > NeilBrown
> 
> The original array was created on whole devices, as the saved output
> starts with e.g. "/dev/sde:".

Right, so you definitely don't want to create partitions.  Maybe when mdadm
reported "partition table exists' it was a false positive, or maybe old
information - creating a 1.2 array doesn't destroy the partition table.

> I used the order of the 'Device UUID' from the saved output to
> recreate the order in the new system (the ports changed due to a new
> mainboard).

When you say "the order", do you mean the numerical order?

If you looked at the old "mdadm -E" output matching the "Device Role" with
"Device UUID" to determine the order of the UUIDs, then looked at the
"mdadm -E" output after the metadata got corrupted and used the "Device UUID"
to determine the correct "Device Role", then ordered the devices by that Role,
then that should have worked.

I assume you did have a filesystem directly on /dev/md0, and hadn't
partitioned it or used LVM on it?

NeilBrown


>              After the installation I had a degraded array in
> initramfs, but I was able to simply "exit" the debug shell and the
> array was accessible. I will now skip the step of creating raid type
> partitions and try every possible order of devices.
> 
> Thanks,
> -Alex


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