Re: using dd (or dd_rescue) to salvage array

[Stefan *St0fF* Huebner <st0ff@gmx.net>]

On 05.02.2012 20:10, Keith Keller wrote:
> On 2012-02-04, Stefan /*St0fF*/ Hübner<stefan.huebner@stud.tu-ilmenau.de>  wrote:
>> actually, ddrescue is THE WAY TO GO in this case.  Don't use the old
>> ddrescue, but the GNU version.  Some distros call it gddrescue, on
>> gentoo the old one is called dd-rescue and the gnu-one ddrescue.  Just
>> check it out: http://www.gnu.org/software/ddrescue/ddrescue.html
> Thanks for the advice, Stefan.  Frustratingly enough, I will get a
> chance to try GNU ddrescue despite my impatience--I originally used
> dd_rescue to try to get an image of the failing drive, and while that
> succeeded just fine (only lost 8k), the target ended up reporting ECC
> errors during the rebuild!  So I've taken a new image with ddrescue
> to a tested drive (again, losing 8k), and am hoping that it goes better.
> (At the moment I'm just attempting a one-spare rebuild, which I'm hoping
> will go faster than a two-disk build, and therefore report any problems
> sooner.)
>
> I realized after reading my initial post that I wasn't 100% clear what I
> was asking.  I knew that some sort of dd would work, but I'd only done
> it before in a filesystem context, and didn't know how mdraid would
> react.  So I am curious, does anyone know what I might expect when the
> rebuild gets to the part on the new image where the data was lost?  Will
> it just create a problem on the filesystem, or might something worse
> happen?  Should I run a check if the rebuild completes successfully?
> And will mismatch_cnt get populated by the rebuild, or would I need a
> check to expose mismatches?
>
> --keith
>
 From the logical point of view those lost 8k would create bad data - 
i.e. a filesystem problem OR simply corrupted data.  That depends on 
which blocks exactly are bad.  If you were using lvm it could even be 
worse, like broken metadata.

It would be good if those 8k were "in a row" - that way at max 3 
fs-blocks (when using 4k fs-blocksize) would be corrupted.  If you're 
lucky, you won't even notice - like me: my system SSD broke down 
lately.  I ddrescued as much as I could, but around 250k are gone.  I'm 
dual-booting windows and gentoo and I have not yet encountered a problem 
from the missing data.  Lucky me...

Cheers,
Stefan
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