[NCLUG] Need help with ether md raid or possibly ext4 corrupted group descriptors
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.
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
> 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
> 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
> 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
> Indeed. As you say, overwrite the superblocks, meaning your existing
> 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
> an array without a superblock. With these arrays there is no
> between initially creating the array and subsequently assembling
> array, except that hopefully there is useful data there in the
>> 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 --
> 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
>> 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
> 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 Reifschneider, Member of Technical Staff <jafo at tummy.com>
> tummy.com, ltd. - Linux Consulting since 1995: Ask me about High
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