Got raid10 assembled wrong - how to fix?

Got raid10 assembled wrong - how to fix?

[George Spelvin]

I just discovered (the hard way, sigh, but not too much data loss) that a
4-drive RAID 10 array had the mirroring set up incorrectly.

Given 4 drvies A, B, C and D, I had intended to mirror A<->C and B<->D,
so that I could split the mirror and run on either (A,B) or (C,D).

However, it turns out that the mirror pairs are A<->B and C<->D.  So
pulling both A and B off-line results in a non-functional array.

So basically what I need to do is to decommission B and C, and rebuild
the array with them swapped: A, C, B, D.

Can someone tell me if the following incantation is correct?

mdadm /dev/mdX -f /dev/B -r /dev/B
mdadm /dev/mdX -f /dev/C -r /dev/C
mdadm --zero-superblock /dev/B
mdadm --zero-superblock /dev/C
mdadm /dev/mdX -a /dev/C
mdadm /dev/mdX -a /dev/B

I'm assuming that fresh spares will be assigned to the lowest available
slot.  I just get nervous about commands with names like --zero-superblock
when I have data I'd rather not lose.

Thanks!

Re: Got raid10 assembled wrong - how to fix?

[Michael Tokarev]

George Spelvin wrote:
> I just discovered (the hard way, sigh, but not too much data loss) that a
> 4-drive RAID 10 array had the mirroring set up incorrectly.
> 
> Given 4 drvies A, B, C and D, I had intended to mirror A<->C and B<->D,
> so that I could split the mirror and run on either (A,B) or (C,D).
> 
> However, it turns out that the mirror pairs are A<->B and C<->D.  So
> pulling both A and B off-line results in a non-functional array.
> 
> So basically what I need to do is to decommission B and C, and rebuild
> the array with them swapped: A, C, B, D.
> 
> Can someone tell me if the following incantation is correct?
> 
> mdadm /dev/mdX -f /dev/B -r /dev/B
> mdadm /dev/mdX -f /dev/C -r /dev/C
> mdadm --zero-superblock /dev/B
> mdadm --zero-superblock /dev/C
> mdadm /dev/mdX -a /dev/C
> mdadm /dev/mdX -a /dev/B

That should work.

But I think you'd better just physically swap the drives instead -
this way, no rebuilding the array will be necessary, and your data
will be safe all the time.

/mjt