Re: Is it possible to restart --add?

[Wols Lists <antlists@youngman.org.uk>]

On 10/12/2022 19:59, Chris Dunlop wrote:
> Hi,
> 
> When replacing a failed disk with a new one using --add, is it possible 
> to restart a partially-complete --add, e.g. after a reboot?
> 
> I have a raid-6 with a failed disk, and used --add to add a new disk as 
> a replacement. From /proc/mdstat, "finish" told me it would take around 
> 24 hours to complete the add.
> 
> The machine was rebooted some hours into the add, and on restart the md 
> was missing the new disk (and the failed disk). I tried to --re-add the 
> new disk again, but mdadm told me it's "not possible":
> 
> mdadm: --re-add for /dev/sdh1 to /dev/md0 is not possible
> 
> I ended up --add'ing the disk again, so the 24 hours to complete started 
> again.
> 
> Is this expected, and/or is there a way to restart the --add rather than 
> starting from the beginning again?
> 
Raid is supposed to be robust, so this surprises me. When it rebooted it 
should have known it was part-way through a rebuild. Was it a controlled 
reboot, or a crash and restart?

What I would expect is that the array would be rebuilt including sdh1, 
and the rebuild would just carry on. So I suspect that whatever went 
wrong, it was a bit further back than that - somehow md forgot that sdh1 
was now part of the array.

Weird.

Cheers,
Wol