Re: Re-add not selecting drive for correct slot?

[Thomas Fjellstrom <thomas@fjellstrom.ca>]

On Mon 10 Aug 2015 07:10:55 PM Wols Lists wrote:
> On 10/08/15 18:44, Thomas Fjellstrom wrote:
> > On Mon 10 Aug 2015 11:35:13 AM Mikael Abrahamsson wrote:
> >> On Sat, 8 Aug 2015, Thomas Fjellstrom wrote:
> >>> I did try that :( It fails to assemble because it only sees sdc as a
> >>> spare.
> >>> Maybe because I did things with the old mdadm first, and did a --remove?
> >>> That seems to have wiped out the "slot" information (it's -1) so the
> >>> assemble force magic can't figure things out? Just a guess on my part.
> >> 
> >> Unless someone else has a better idea, I'd say you're right. If you would
> >> have unplugged the failed drive (so it disappeared completely), it could
> >> probably have been re-added. So unless you have a copy of the old
> >> superblock, your only way to proceed now is to use --create
> >> --assume-clean
> >> and get all the parameters right (order, offsets etc). There are lots of
> >> examples in the mailing list archives of people trying this and some
> >> actually suceeding.
> > 
> > I think the only thing that would stop that from working is that there is
> > data in the bitmap. So if a assume clean is done, it might ignore that
> > and cause some extra corruption?
> 
> Which is why you use loopback devices. You'll need to look back at
> previous posts to see how to do it, but you put a pseudo-layer over the
> real disks (which never actually get written to), and you can then fsck
> your array. If that comes up clean, you know you got the assemble
> parameters right, and you can shut down the pseudo-array and assemble
> the real array.
> 
> > It'd be interesting to figure out if i can set that slot number manually
> > or
> > with a tool. That might be a smarter/safer way of doing it.
> 
> Better the pseudo way (which will definitely allow you to recover IF the
> disk isn't corrupted) than trying your own stuff which might write to
> the disk and make life harder/impossible to recover.

Yeah, I did that once previously for a recovery. It was quite handy. I backed 
everything up to a different machine. And re-created the array.

I may do that again. But then I actually have a mostly full backup, about the 
only things i care about is some pictures I added to the array before it went 
down, that I still have a copy of, but would have to copy them all back off of 
various devices.

> Cheers,
> Wol

-- 
Thomas Fjellstrom
thomas@fjellstrom.ca