On 08/10/16 13:30, Andreas Klauer wrote:

On Fri, Oct 07, 2016 at 05:37:32PM +0200, Santiago DIEZ wrote:

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First thing I did is ddrescue the remaining partitions sd[abc]10 .
ddrescue did not stumble into any read error so I assume all remaining
partitions are perfectly safe.

So ... don't you still have a good copy?

You only killed one of them, right? Did not make same mistake twice?

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There comes my mistake: I ran the --create command instead of --assemble :

================================================================================
# mdadm --create --verbose /dev/md1 --raid-devices=4 --level=raid5
--run --readonly /dev/loop0 /dev/loop1 /dev/loop2 missing

One oddity I've noticed. You've created the array using loop devices.
What are these?

The reason I ask is that using loopback devices is a standard technique
for rebuilding a damaged array, specifically to prevent md from actually
writing to the drive. So is it possible that "mdadm --create" only wrote
to ram, and a reboot will recover your ddrescue copies untouched?

My raid-fu isn't enough to tell me whether I'm right or not ... :-)

If necessary you'll have to do another ddrescue from the original
drives, and you should then be able to assemble the array from the
copies. Don't use "missing", use "--force" and you should get a working,
degraded, array to which you can add a new drive and rebuild the array.

mdadm --assemble /dev/md0 /dev/sd[efg]10 --force

if I'm right ... so long as it's the copies, you can always recover
again from the original disks, and if there's a problem with the copies
mdadm should complain when it assembles the array.

Cheers,
Wol