Re: questions about softraid limitations

"Janos Haar" <djani22@xxxxxxxxxxxx> · Thu, 15 May 2008 01:29:57 +0200

Hello David,

----- Original Message ----- 

From: "David Greaves" <david@xxxxxxxxxxxx>

To: "Janos Haar" <djani22@xxxxxxxxxxxx>
Cc: <linux-raid@xxxxxxxxxxxxxxx>
Sent: Wednesday, May 14, 2008 12:45 PM
Subject: Re: questions about softraid limitations

Janos Haar wrote:

Hello list, Neil,

Hi Janos

I have worked on a faulty hw raid card data recovery some days before.

The project is already successfully done, but i run into some 

limitations.

Firstly, are you aware that Linux SW raid will not understand disks 

written by

hardware raid.

Yes, i know, but the linux raid is a great tool to try it, and if the user 

know what he is doing, it is safe too. :-)

Than try to build an "old fashion" linear arrays from each disks + 64k
another blockdevice. (for store the superblock)
But the mdadm refused to _build_ the array, because the source scsi
drive is jumpered to readonly. Why? :-)

This will not allow md to write superblocks to the disks.

I think exactly for this steps:

dd if=/dev/zero of=suberblock.bin bs=64k count=1
losetup /dev/loop0 superblock.bin
blockdev --setro /dev/sda
mdadm --build -l linear /dev/md0 /dev/sda /dev/loop0

The superblock area is writable.

And this is enough to try to assemble the array to do the recovery, but this 

step is refused.

I try to build the array with --readonly option, but the mdadm still
dont understand what i want. (yes, i know, rtfm...)

This will start the array in readonly mode - you've not created an array 

yet

because you haven't written any superblocks...

Yes, i only want to build, not to create.

Its OK, but what about building a readonly raid 5 array for recovery
usage only? :-)

That's fine. If they are md raid disks. Yours aren't yet since you haven't
written the superblocks.

I only want to help for some people to get back the data.
I only need to build, not to create.

Thanks,

Janos Haar

David

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