Re: raid5 to raid6 reshape never appeared to start, how to cancel/revert

[Roger Heflin <rogerheflin@gmail.com>]

On Mon, May 22, 2017 at 2:33 PM, Andreas Klauer
<Andreas.Klauer@metamorpher.de> wrote:
> On Mon, May 22, 2017 at 01:57:44PM -0500, Roger Heflin wrote:
>> I had a 3 disk raid5 with a hot spare.  I ran this:
>> mdadm --grow /dev/md126 --level=6 --backup-file /root/r6rebuild
>>
>> I suspect I should have changed the number of devices in the above command to 4.
>
> It doesn't hurt to specify, but that much is implied.
> Growing 3 device raid5 + spare to raid6 results in 4 device raid6.
>

Yes.

>> The backup-file was created on a separate ssd.
>
> Is there anything meaningful in this file?
>

16MB in size, but od -x indicates all zeros, so no, there is nothing
meaningful in the file.

>> trying assemble now gets this:
>>  mdadm --assemble /dev/md126 /dev/sd[abe]1 /dev/sdd
>> --backup-file=/root/r6rebuild
>> mdadm: Failed to restore critical section for reshape, sorry.
>>
>> examine shows this (sdd was the spare when the --grow was issues)
>>  mdadm --examine /dev/sdd
>> /dev/sdd1:
>
> You wrote /dev/sdd above, is it sdd1 now?
>
>>         Version : 0.91.00
>
> Ancient metadata. You could probably update it to 1.0...
>

I know.

>>   Reshape pos'n : 0
>
> So maybe nothing at all changed on disk?
>
> You could try your luck with overlay
>
> https://raid.wiki.kernel.org/index.php/Recovering_a_failed_software_RAID#Making_the_harddisks_read-only_using_an_overlay_file
>
> mdadm --create /dev/md42 --metadata=0.90 --level=5 --chunk=64 \
>       --raid-devices=3 /dev/overlay/{a,b,c}
>
>> It does appear that I added sdd rather than sdd1 but I don't believe
>> that is anything critical to the issue as it should still work fine
>> with the entire disk.
>
> It is critical because if you use the wrong one the data will be shifted.
>
> If the partition goes to the very end of the drive, I think the 0.90
> metadata could be interpreted both ways (as metadata for partition
> as well as whole drive).
>
> If possible you should find some way to migrate to 1.2 metadata.
> But worry about it once you have access to your data.
>

I deal with others messing up partition/no partition recoveries often
enough to not be worried about how to debug and/or fix that mistake.

I found a patch from Neil from 2016 that may be solution to this
issue, I am not clear if it is an exact match to my issue, it looks
pretty close.

http://comments.gmane.org/gmane.linux.raid/51095

> Regards
> Andreas Klauer