On 23/02/2020 17:38, Aymeric wrote:

Hello,

I've a system with a RAID1 using two disks (sda + sdb) + 1 spare (sdc).

For some stupid reason (cable) the spare (sdc) became active and sda was
out the array.

I then added back sda to the RAID1 but now I've RAID1 with three active
disks and zero spare.
I don't understand why the spare device didn't switch back as spare
after sda have been rebuild.

Because rebuilding sda is a lot of work. The sensible option would have 
been to leave sdb and sdc active, and left sda as spare.

I used the following command to add sda to the RAID1:
# mdadm  /dev/md0 --add /dev/sda3


What did I do wrong here? And how to fix it? Do I have to set sdc to
faulty and add it back?

I'd just leave it as a 3-disk raid. But no, if you do want to set it 
back to a 2-disk raid don't do a "faulty and re-add" - that'll just make 
re-sync it again. Somehow you've got active devices set to 3, so just 
set that back to two. If you look on the wiki it covers the scenario of 
converting a three-disk raid to two.

The only thing I will add about a 3-disk raid is that if you want to 
change it to anything else, you'll have to revert to 2 first, but do 
that when you want to change things, leave it at 3 for the moment.

Thanks!

Cheers,

Wol