Le 23 février 2020 21:40:44 GMT+01:00, antlists [off-list ref] a écrit :

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

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.

Well, for some reason that didn't happen that way..

quoted

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.

I would prefer to revert to sdc as a spare, it's a really a slow disk (5400rpm with a sata1 while others are 7200 at sata3). For a spare disk it's good enough to give me time to get a new disk in case of failure but not for daily.

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.

So I tried with the --grow option to change the number disk in the array but it doesn't want to change...

# mdadm --grow /dev/md0 --raid-devices=2
mdadm: /dev/md0: no change requested
# mdadm --grow /dev/md0 --raid-devices=3
mdadm: Need 1 spare to avoid degraded array, and only have 0.
       Use --force to over-ride this check.

When looking with --examine, sdc3 is marked a replacement device.

# mdadm --examine /dev/sda3 /dev/sdb3 /dev/sdc3|grep 'dev\|Role\|State'
/dev/sda3:
          State : clean
   Device Role : Active device 1
   Array State : AR ('A' == active, '.' == missing, 'R' == replacing)
/dev/sdb3:
          State : clean
   Device Role : Active device 0
   Array State : AR ('A' == active, '.' == missing, 'R' == replacing)
/dev/sdc3:
          State : clean
   Device Role : Replacement device 1
   Array State : AR ('A' == active, '.' == missing, 'R' == replacing)

Thanks,
Aymeric
-- 
Aymeric P.
Phone: +33 685 674 843
Aymeric