Re: Unable to add journal device to RAID 6 array

[Guoqing Jiang <guoqing.jiang@linux.dev>]

On 5/6/22 6:42 PM, James Stephenson wrote:
> Hi,
>
> First time using this (and indeed any) mailing list, so apologies if I
> violate any etiquette I'm not aware of!
>
> I'm trying to add a write-back journal device to an existing RAID 6
> array, and it's proving difficult. Essentially I did this:
>
> 1. Put the array in read-only
> 2. Attempt to add a journal device to the array
> 3. md said no because the array has a bitmap
> 4. I tried to remove the bitmap, and it said no: 'md: couldn't update
> array info. -16'
> 5. I rebooted
> 6. The array wouldn't start, and to my surprise was listed as having a
> journal device. Here's what it looked like:
> https://gist.github.com/jstephenson/1db2008c4243c2539d029f1f4706dc14
> 7. It was _very_ unhappy, and refused to do anything: 'md/raid:md126:
> array cannot have both journal and bitmap'
> 8. The only thing I could do was to zero the superblock on the journal
> device, and then fortunately everything assemble again nicely (with
> bitmap still in place)
>
> So, after a bit of messing around the array is back to where I
> started—RAID 6 with internal bitmap. However, I still cannot remove
> its bitmap.
>
> 1. sudo mdadm --grow /dev/md126 --bitmap=none
> 2. md: couldn't update array info. -16

-16 means EBUSY, so probably the array needs resync or recovery per

https://elixir.bootlin.com/linux/v5.10.106/source/drivers/md/md.c#L7374

Thanks,
Guoqing