Re: Another RAID-5 problem

On Wed, 9 May 2012 21:31:19 +0200 Piergiorgio Sartor
[off-list ref] wrote:

Actually, we solved this issue in a "creative" way.

Looking at:

https://raid.wiki.kernel.org/index.php/RAID_superblock_formats

we identify the proper address and look-up way for the
component order and, using "od -Ax -tx4 /dev/sdXi | less"
we were able to understand the device order.
For the fresh men, please note that address 0xA0 has
the device number and this is a pointer added to 0x100,
where the device role is stored.
While at 0xA0 are stored 4 bytes int, at 0x100 are
stored 2 bytes int (short int), so the "-tx4" of "od"
swaps (due to CPU endianess) each pair of short int,
so the order will be 1 0 3 2 5 4 ...
If "-tx2" is used, than the 0x100 will be correct,
but 0xA0 will have bytes swapped pair wise.

Well done!!

Fortunately, the superblock seemed OK for the data.
Other information, like raid level, was completely
wiped out...

The only itch left is the data offset.
We plan to try to use mdadm 2.6.7.1 (which originally
created the array) using Ubuntu 10.10 desktop (live).

Still, I would like to know about backing up the
first few MB of each component with "dd" and about
switching to read only, in order to avoid damage
by LVM/mount.

Takes a backup of the first few MB certainly wouldn't hurt, and might help if
something goes terribly wrong. (Just as long as you don't restore a backup to
the wrong device :-)

Yes, starting in --readonly mode, or switching to --readonly mode after you
have started the array, is probably a good idea.
Some filesystems sometimes try to write even if the device is marked as
read-only (e.g. they try to replay the journal).  If that happen md will BUG
and machine will crash, which might be better than corrupting data.

NeilBrown