Re: mdadm RAID1 with 3 disks active on 2 disks configuration

[Aymeric <mulx@aplu.fr>]

Le 23 février 2020 21:40:44 GMT+01:00, antlists <antlists@youngman.org.uk> 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..

>> 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