Hello!

I have a question about mdadm usage. One of me customers killed a
raid5+1 by accidently removing an ide bus with two disk on it (dont't
ask, I know ;-))

Assuming that the data is on the disks I'm looking for a way to convinc=
e
linux to mark the superblocks (of the missing disk only?) as "not
failed" so that the raid comes up again. mdadm seems to have what I am
looking for, like "madm --assemble --update=3D? --force" can write the
superblocks freshly without destorying the data on it? Do I have to use=

--update=3Dsuper=E2=80=90minor or --update=3Dsuper=E2=80=90minor on ia3=
2?
Or is there an recovery tool for such a case (i.e. mddump?)?

If this does not work, can anyone point me out where I can find an
overview about the structure of a superblock, so that I may fix the
problem via hex-editor (my skills point back to C64 times ... :-))
I've seen the source but some kind of overview/scheme would be nice.

I'm scared. :)

Rgds,
Andreas


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On Friday August 20, lists [at] aj.net-lab.net wrote:
> Hello!
>
> I have a question about mdadm usage. One of me customers killed a
> raid5+1 by accidently removing an ide bus with two disk on it (dont't
> ask, I know ;-))
>
> Assuming that the data is on the disks I'm looking for a way to convince
> linux to mark the superblocks (of the missing disk only?) as "not
> failed" so that the raid comes up again. mdadm seems to have what I am
> looking for, like "madm --assemble --update=? --force" can write the
> superblocks freshly without destorying the data on it? Do I have to use
> --update=super$,1rp(Bminor or --update=super$,1rp(Bminor on ia32?
> Or is there an recovery tool for such a case (i.e. mddump?)?

You don't need any --update. Just --assemble --force. mdadm will
then pick the best available drives to assemble a degraded array (one
drive missing). If the result seems good (e.g. fsck reports ok), then
you can add the other drive (--add) and it will be reconstructed with
consistent data.

NeilBrown

>
> If this does not work, can anyone point me out where I can find an
> overview about the structure of a superblock, so that I may fix the
> problem via hex-editor (my skills point back to C64 times ... :-))
> I've seen the source but some kind of overview/scheme would be nice.
>
> I'm scared. :)
>
> Rgds,
> Andreas
>
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On Tue, 2 Mar 2010 19:15:04 -0600 (CST)
Robert Minvielle <robert [at] lite3d.com> wrote:

>
> I am trying to setup a new raid array on a debian box, and I have not run into
> this problem before. I could not find a mdadm list, so I will ask here. If this
> is totally off topic, please disregard.

Perfectly on-topic.

>
>
> I am attempting to setup a raid 6 array, but this is failing so I backing down
> to a no-frills raid 5 array. The system is Debian5, stock kernel, stock everything.
> The mdadm is the stock debian, pulled with apt-get, version 2.6.7.
> The machine in question has one IDE drive for linux, and 45 SATA drives. Debian
> sees all of the drives, and I have fdisked all of them with one partition of type
> fd (Linux autodetect raid). fdisk -l /dev/sd[a-z] /dev/sdaa[a-s] shows them all
> with no problems.

Using old software on new hardware....
When using Debian, I would recommend the -testing version for new hardware....
(not that I am prepared to back-up that recommendation with support).

However I suspect Debian5 should be able to be made to work with your setup.

>
> The issue is that when I do a
>
> mdadm --create /dev/md0 --level=5 --raid-devices=45 /dev/sd[a-z]1 /dev/sda[a-s]1
>
> to create a raid array with no spares, all defaults, it returns with
>
> invalid number of raid devices.

The default metadata layout has a maximum of 28 devices. If you want more,
add
--metadata=1.0

You won't be able to use in-kernel autodetect, but you shouldn't need to with
Debian, even at that vintage.

>
> I have searched the web to no avail. --verbose does not increase verbosity. There
> are no debug switches (that I know of) to mdadm. log files show nothing. Leaving off
> --raid-devices=45 does nothing. Changing the number of devices just for fun does
> nothing. (45,44,43,2,whatever). I am not sure if this is a problem with this version
> in debian, the number of drives that I have, or the setup. I have done this before
> (with a few less drives) with no problems.

I'm surprised it didn't work with '2', or did you mean "42" ?

NeilBrown

>
> Any suggestions would be appreciated.
>
> Thanks.
>
>
>
>
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> the body of a message to majordomo [at] vger.kernel.org
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----- "Neil Brown" <neilb [at] suse.de> wrote:

> On Tue, 2 Mar 2010 19:15:04 -0600 (CST)
> Using old software on new hardware....
> When using Debian, I would recommend the -testing version for new
> hardware....
> (not that I am prepared to back-up that recommendation with support).
>
> However I suspect Debian5 should be able to be made to work with your
> setup.

Hrmm, will look into that.

>
> The default metadata layout has a maximum of 28 devices. If you want
> more,
> add
> --metadata=1.0
>
> You won't be able to use in-kernel autodetect, but you shouldn't need
> to with
> Debian, even at that vintage.
>

Argh, I see the note now in the man page. I missed it. Funny that google
did not return that/man page/etc when I searched for things like "maximum number of
drives in mdadm array" etc.



> I'm surprised it didn't work with '2', or did you mean "42" ?
>

Yes, sorry for the confusion.

Thanks.
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