From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eli Ben-Shoshan <eli@benshoshan.com>
Subject: Re: Accidentally resized array to 9
Date: Fri, 29 Sep 2017 10:47:57 -0400
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 <22990.16096.295333.439814@quad.stoffel.home>
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To: John Stoffel <john@stoffel.org>
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On 09/29/2017 08:38 AM, John Stoffel wrote:
>>>>>> "Eli" == Eli Ben-Shoshan <eli@benshoshan.com> writes:
> 
> Eli> I need to add another disk to my array (/dev/md128) when I accidentally
> Eli> did an array resize to 9 with the following command:
> 
> Eli> First I add the disk to the array with the following:
> 
> Eli> mdadm --manage /dev/md128 --add /dev/sdl
> 
> Eli> This was a RAID6 with 8 devices. Instead of using --grow with
> Eli> --raid-devices set to 9, I did the following:
> 
> Eli> mdadm --grow /dev/md128 --size 9
> 
> Eli> This happily returned without any errors so I went to go look at
> Eli> /proc/mdstat and did not see a resize operation going. So I shook my
> Eli> head and read the output of --grow --help and did the right thing which is:
> 
> Eli> mdadm --grow /dev/md128 --raid-devices=9
> 
> Eli> Right after that everything hit the fan. dmesg reported a lot of
> Eli> filesystem errors. I quickly stopped all processes that were using this
> Eli> device and unmounted the filesystems. I then, stupidly, decided to
> Eli> reboot before looking around.
> 
> 
> I think you *might* be able to fix this with just a simple:
> 
>     mdadm --grow /dev/md128 --size max
> 
> And then try to scan for your LVM configuration, then fsck your volume
> on there.  I hope you had backups.
> 
> And maybe there should be a warning when re-sizing raid array elements
> without a --force option if going smaller than the current size?

I just tried that and got the following error:

mdadm: Cannot set device size in this type of array

Trying to go further down this path, I also tried to set the size 
explicitly with:

mdadm --grow /dev/md150 --size 1953383512

but got:

mdadm: Cannot set device size in this type of array

I am curious if my data is actually still there on disk.

What does the --size with --grow actually do?