Hello Neil,
(sorry Neil for duplication, but first i forget to cc)
----- Original Message -----
From: "Neil Brown" <neilb@xxxxxxx>
To: "Janos Haar" <djani22@xxxxxxxxxxxx>
Cc: "David Greaves" <david@xxxxxxxxxxxx>; <linux-raid@xxxxxxxxxxxxxxx>
Sent: Friday, May 16, 2008 3:39 AM
Subject: Re: questions about softraid limitations
On Thursday May 15, djani22@xxxxxxxxxxxx wrote:
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. :-)
As long as the user also knows what the kernel is doing .....
If you build an md array on top of a read-only device, the array is
still writable, and the device gets written too!!
Yes, it is a bug. I hadn't thought about that case before. I will
look into it.
Ooops. :-)
>
>> 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
^p
losetup /dev/loop0 superblock.bin
blockdev --setro /dev/sda
mdadm --build -l linear /dev/md0 /dev/sda /dev/loop0
^ --raid-disks=2
The superblock area is writable.
And this is enough to try to assemble the array to do the recovery, but
this
step is refused.
What error message do you get? It worked for me (once I added
--raid-disks=2).
The previous example is just an on the fly typing, in real readonly jumpered
scsi sda gives az error message!
You probably want superblock.bin to be more than 64K. The superblock
is located between 64K and 128K from the end of the device, depending
on device size. It is always a multiple of 64K from the start of the
device.
Usually i used a 128MB! disk partitions for this.
This is safe enough.
And sometimes we need more loop devices than 8.... :-)
>
>>
>> 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.
And this you can do ... but not with mdadm at the moment
unfortunately.
What carefully :-)
--------------------------------------------------------------
/tmp# cd /sys/block/md0/md
/sys/block/md0/md# echo 65536 > chunk_size
/sys/block/md0/md# echo 2 > layout
/sys/block/md0/md# echo raid5 > level
/sys/block/md0/md# echo none > metadata_version
/sys/block/md0/md# echo 5 > raid_disks
/sys/block/md0/md# ls -l /dev/sdb
brw-rw---- 1 root disk 8, 16 2008-05-16 11:13 /dev/sdb
/sys/block/md0/md# ls -l /dev/sdc
brw-rw---- 1 root disk 8, 32 2008-05-16 11:13 /dev/sdc
/sys/block/md0/md# echo 8:16 > new_dev
/sys/block/md0/md# echo 8:32 > new_dev
/sys/block/md0/md# echo 8:48 > new_dev
/sys/block/md0/md# echo 8:64 > new_dev
/sys/block/md0/md# echo 8:80 > new_dev
/sys/block/md0/md# echo 0 > dev-sdb/slot
/sys/block/md0/md# echo 1 > dev-sdc/slot
/sys/block/md0/md# echo 2 > dev-sdd/slot
/sys/block/md0/md# echo 3 > dev-sde/slot
/sys/block/md0/md# echo 4 > dev-sdf/slot
/sys/block/md0/md# echo 156250000 > dev-sdb/size
/sys/block/md0/md# echo 156250000 > dev-sdc/size
/sys/block/md0/md# echo 156250000 > dev-sdd/size
/sys/block/md0/md# echo 156250000 > dev-sde/size
/sys/block/md0/md# echo 156250000 > dev-sdf/size
/sys/block/md0/md# echo readonly > array_state
/sys/block/md0/md# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
[multipath] [faulty]
md0 : active (read-only) raid5 sdf[4] sde[3] sdd[2] sdc[1] sdb[0]
624999936 blocks super non-persistent level 5, 64k chunk, algorithm 2
[5/5] [UUUUU]
unused devices: <none>
----------------------------------------------------------
Did you catch all of that?
sysfs! :-)
Wow! :-)
This is what really helps for me, thanks! :-)
But what about the other people?
Mdadm will know this too?
The per-device 'size' is in K - I took it straight from
/proc/partitions.
The chunk_size is in bytes.
Have fun.
Thank you!
Next time i will try it. :-)
Janos
NeilBrown
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