Re: RAID5 disk failure during rebuild of spare, any chance of recovery when one of the failed devices is suspected to be intact?

[Nicolas Jungers <nicolas@jungers.net>]

On 08/16/2010 07:54 AM, Tor Arne Vestbø wrote:
> Hey Nicolas!
>
> On Mon, Aug 16, 2010 at 7:29 AM, Nicolas Jungers<nicolas@jungers.net>  wrote:
>> I'd try to recreate the array with a copy of the disks
>>
>> do a ddrescue of the 3 good disks and then try a mdadm -C with the exact
>> parameters of the array creation with the same mdadm version (use missing
>> for slot 2).  It saved my raid10 array.
>
> You mean you sdc and sde plus either sdb or sdd, depending on which
> one I think is more sane a this point?

I'd try both.  Do a ddrescue of the failing one and try that (with copy 
of the others) and check what's coming out.

>
> mdadm -E before http://pastebin.com/f46EWXEA vs after
> http://pastebin.com/Kp145Mkx indicates sdb1 is missing a few events,
> dunno if that's too much to be usable at this point?

Did write happen during the reconstruction?  You'll lose not only what 
was on the write but also everything on the same strip, including 
directories.  That's a reason to try with a copy of sdd.

> Can the create-process be done "read-only", so that I don't have to
> use extra disks? If I've understood things correctly --create will
> only overwrite the superblocks, not touch the data?

yes, but I'd not do it.  And beside, you should not trust the failing 
one (sdd).

>
> So I could do:
>
> mdadm -C /dev/md0 /dev/sdb1 /dev/sdc1 misssing /dev/sde1

or even
mdadm -C /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1

with copies.

>
> I suppose the 33% restored spare sdf1 won't help any in this situation?

I suppose not.

N.
>
> Tor Arne

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