Re: unable to recover RAID 5 due to bad block [solved]

[Chris Purves <chris@northfolk.ca>]

On 2011-11-10 16:11, NeilBrown wrote:
> On Thu, 10 Nov 2011 12:50:46 -0400 Chris Purves<chris@northfolk.ca>  wrote:
>
>> On 2011-11-10 12:20, Alexander Kühn wrote:
>>> ddrescue to the rescue!
>>> Get a another new disk, then ddrescue the one with the read error to the new disk.
>>> Assemble the array using the new disk instead of the one with the read error.
>>> You will loose the blocks that can't be read of course.
>>> And in the future do run raid check/scrubbing at regular intervals. ;)
>>
>> I have tried this already.  After cloning the disk with errors, I replaced it with the clone and tried to re-start the array using
>>
>> mdadm --assemble --force /dev/md1
>>
>> mdadm assigned the new disk as a spare and said there were only three disks to start the array and so couldn't start it.
>>
>> After I clone the disk with the error, how precisely should I re-start the array?
>>
>>
>
> This should have worked.  So presumably some unstated requirement wasn't met.
>
> Give details.  Names of devices, sizes of devices, "mdadm --examine" of
> device.  Are you using partitions or whole disk ....
>
> The best assemble command would be:
>    mdadm --assemble /dev/md1 -vv ...list...of.devices..you...want.it.to.include.
>
>
> NeilBrown

After ddrescue finished, I unplugged the disk with the bad sector (all 4 KB worth), and ran mdadm --assemble --force.

This time the array started up with four disks (three old, one new).  I'm not sure why it didn't work the previous time; I must have done something differently.

In any case, it's working now.  Thanks to everyone for your assistance.  It was a big help knowing that replacing the damaged disk with a clone should work.  It saved me a lot of hassle restoring from backup.  Now I just need to add a fifth disk and I'll be back to normal.


-- 
Chris Purves
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