Fantastic! I ran the following command:

mdadm --assemble /dev/md10 --verbose --force --run /dev/loop0
/dev/loop1 /dev/loop2

and it returned

mdadm: looking for devices for /dev/md10
mdadm: /dev/loop0 is identified as a member of /dev/md10, slot 0.
mdadm: /dev/loop1 is identified as a member of /dev/md10, slot 1.
mdadm: /dev/loop2 is identified as a member of /dev/md10, slot 2.
mdadm: forcing event count in /dev/loop0(0) from 81589 upto 81626
mdadm: forcing event count in /dev/loop2(2) from 81589 upto 81626
mdadm: added /dev/loop1 to /dev/md10 as 1
mdadm: added /dev/loop2 to /dev/md10 as 2
mdadm: no uptodate device for slot 6 of /dev/md10
mdadm: added /dev/loop0 to /dev/md10 as 0
mdadm: /dev/md10 has been started with 3 drives (out of 4).

I can mount /dev/md10 and I have access to my data.

Now that I know my data is safe.
I need to restart the original array on the server.
The array is completely deactivated on the original server.
The company that I rent the dedicated server from is waiting for my GO
to replace disk sdd with a new one.
I'm giving then the GO now.
After I reboot the server, what is it I need to do to rebuild sdd as
the fourth disk of the array?

Regards
-------------------------
Santiago DIEZ
Quark Systems & CAOBA
23 rue du Buisson Saint-Louis, 75010 Paris
-------------------------


On Sat, Oct 22, 2016 at 12:35 AM, Shaohua Li [off-list ref] wrote:

On Fri, Oct 21, 2016 at 10:45:10AM +0200, Santiago DIEZ wrote:

quoted

Hi,

Thanks Andreas,

Yes apparently, 3/4 of the original disks seem to be safe. But I'm
terrified at the idea of doing something wrong assembling them.
Incidentally, I indeed did a mistake trying to assemble the ddrescue
images of the 3 safe disks. I tried to create again with proper
metadata and chunck but it did not work. I'm still scared at the idea
of restarting the original raid. I'm currently ddrescuing again the 3
partitions to then try and *assemble* them rather than *create*.


Thanks Wol,

I use loop devices because I work on partition images, not on actual partitions:
I use ddrescue to copy data from /dev/sd[abc]10 to
some.other.server:/home/sd[abc].img
Then I go to some.other.server and turn the images into loop devices :
losetup /dev/loop0 /home/sda10.img
losetup /dev/loop1 /home/sdb10.img
losetup /dev/loop2 /home/sdc10.img
Then I tried to created the raid, it worked but as I said, the
filesystem was unreadable.
I know the idea of using loop devices works because I tested it before.
I'm doing the whole procedure all over again (takes 5 days to ddrescue
the 3 partitions to another server) and then I will use the command
you recommended :
mdadm --assemble /dev/md0 /dev/loop0 /dev/loop1 /dev/loop2 --force


Will keep you posted

-------------------------
Santiago DIEZ
Quark Systems & CAOBA
23 rue du Buisson Saint-Louis, 75010 Paris
-------------------------

On Mon, Oct 10, 2016 at 12:39 AM, Wols Lists [off-list ref] wrote:

quoted

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

quoted

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

quoted

quoted

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?

quoted

quoted

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.

Hmm, those commands work for me. I'm adding Song and Jes if they have ideas.

Thanks,
Shaohua