[NCLUG] Need help with ether md raid or possibly ext4 corrupted group descriptors

Mike Jensen jent at afkfoo.com
Sat Jan 31 23:49:03 MST 2009

Thank you that is all very valuable information.

I had forgot to neglect that I tried --assemble (mdadm --assemble /dev/md0
/dev/sda2 /dev/sdb1 /dev/sdc1....I thought it should maybe be mdadm
--assemble /dev/md0 only but it complained about being unaware of that
raid, I think assemble may require the mdadm.conf??).  But it did not
work, I believe it gave an error like "no raid devices found" or something
similar.  But I did not understand why because the output of the --examine
for each raid partition that was supplied in the command (and with the
output supplied in the first email), it all seemed correctly to me.

Please correct me if I am miss understanding anything.  I definitely want
to have a better understanding for the future.  Thanks for your
time/effort.  Honestly I wonder if I may have found some bug,possibly
related to ext4.  But I can't be certain enough as I am also currently
over clocking my computer and am not 100% confident in my stability.

Mike Jensen
jent at afkfoo.com

On Sat, January 31, 2009 23:36, Sean Reifschneider wrote:
> Mike Jensen wrote:
>> I did not think I needed a super block because it should use the ones
>> that
> Of course you needed a superblock.  The only time you don't need a
> superblock is if you either are trying to access a (very) old array that
> was created using a format before they had superblocks.  You didn't want
> to
> do a build, you probably wanted to do an assemble:
>        -A, --assemble
>               Assemble a pre-existing array.
> As far as why the boot disc didn't detect the array, I wonder if you had
> the partitions set up as type "fd" in fdisk.  That means "RAID
> Autodetect",
> the kernel will look in the partitions for md superblocks and assemble the
> arrays it finds.
>> array, and overwrite the superblocks thus forgetting the previous raid
>> configuration.
> Indeed.  As you say, overwrite the superblocks, meaning your existing
> array
> had superblocks.  The hint that you might have been going the wrong
> direction with --build is that it says that it operates on an array that
> has no superblocks.  From the mdadm man page:
>        This  usage  is similar to --create.  The difference is that it
> creates
>        an array without a superblock. With these arrays there is no
> difference
>        between  initially  creating  the array and subsequently assembling
> the
>        array, except that hopefully there is useful data there in  the
> second
>        case.
>> I think the reason the rescue cd could not find and mount the array is
>> because /etc/mdadm.conf was contained on the array, so it could not
> The rescue disc and the booting should not use /etc/mdadm.conf --
> otherwise
> you couldn't have /etc on the array.  They use the superblocks and the
> partition-type of "fd" to assemble the array.
>> remember how to rebuild the previous configuration.  If I told it to
>> mount
>> the drives it would say "no linux partitions found".
> You can't just ask to mount a single device of a multi-disc array.  Mount
> knows nothing about the array, and the array knows nothing about the
> file-system.  Mount just works on block devices, and md just turns
> multiple
> block devices into a single (larger and/or more redundant) block device.
> With the exception of a RAID-1 array, directly mounting a component
> partition of the array will probably mess things up pretty badly.  With a
> RAID-1 array, you might be able to mount it read-only, but mounting it
> read-write may result in file-system corruption or keeping only the
> original data.
> Sean
> --
> Sean Reifschneider, Member of Technical Staff <jafo at tummy.com>
> tummy.com, ltd. - Linux Consulting since 1995: Ask me about High
> Availability
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