Re: How to fix mistake on raid: mdadm create instead of assemble?

[Shaohua Li <shli@kernel.org>]

On Fri, Oct 21, 2016 at 10:45:10AM +0200, Santiago DIEZ wrote:
> 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 <antlists@youngman.org.uk> wrote:
> >
> > On 08/10/16 13:30, Andreas Klauer wrote:
> > > On Fri, Oct 07, 2016 at 05:37:32PM +0200, Santiago DIEZ wrote:
> > >> > 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?
> > >
> > >> > 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