Re: [gentoo-amd64] Re: not amd64 specific - disk failure

[Raffaele BELARDI]

Duncan wrote:
> If you have a spare drive of the same size or larger, you can try dd, or
> probably better yet, merge dd-rescue and try it.  They copy a file or
> part of one, in this case an entire block device, from one location to
> another, "raw".  What you want to do is copy the entire bad device, /dev/
> sdc above, to the new device.  Then you have a copy to play around with
> without worrying about making the bad device worse before you get
> whatever you were trying to get off of it, off.
>
Duncan,

thanks for the ddrescue explanation, I will surely give it a try.

Yesterday evening I got a new drive double the size of the damaged one,
created a 250Gb partition on it and tried:
# dd if=/dev/hdb of=/mnt/disk_500/sdb.img

It stopped after few kb due to read errors. So I modified to
dd conv=noerror if=/dev/hdb of=/mnt/disk_500/sdb.img

and after some time I got a 250Gb sdb.img on the new drive.
Then started the fun (it was already past midnight). When I created the
new partition I noted down the superblock backup locations.
Unfortunately, every:
# e2fsck -b xxx -B 4096 /mnt/disk_500/sdb.img

returned 'bad superblock'. After googling for some utility to scan disc
for superblocks, I ended up with testdisk (it's ~amd64). To my
understanding this works on real HW only, so I had to reconnect the
damaged HD and let it do its job. testdisk found the superblocks, but
according to it they were at the exact locations I had already noted, so
no help. I also tried to let it search for partitions because I read it
has an option to parse the directory. It worked, it let me see the list
of lost files, but that's all, it has no option to recover. But at least
it told me there is some good superblock somewhere.

Finally I went back to the sdg.img and used "od | less" to look at what
was present at the superblock location. What I saw was, I believe, a
part of the superblock (an almost regular patter of numbers, increasing,
which could be a list of blocks? I need to study ext2) but the point is
that this pattern began well before the 'theoretical address' of the
superblock.

So my hypothesis is that the bad blocks or sectors at the beginning of
the partition were not copied, or only partly copied, by dd, and due to
this the superblocks are all shifted down. Although I don't like to
access again the hw, maybe I should try:
# dd conv=noerror,sync bs=4096 if=/dev/hdb of=/mnt/disk_500/sdb.img

to get an aligned image. Problem is I don't know what bs= should be.
Block size, so 4k?

Any other option I might have?

thanks,

raffaele
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