Hello,

I have a fileserver running CentOS 6.2 that uses mdadm for RAID. A drive failed this week and I replaced it. However, when replacing it, I did not first mark the drive as failed before it was removed from the array. The new drive was added without issue, however, when I review the data in /proc/mdstat, the device number for the new drive is 2, instead of 1.

Here's the output; the part I'm referring to is bolded:

How can I get my array to see the sdb drive as device 1 again?

134 views and I thought this was going to be an easy one! :^ D

The 2 will mark the role of the drive in the array, i.e. it looks like it’s treated like a hot-spare. So the question is, how to convert a hot-spare to a normal member.

You might want to have a look at
http://bugs.centos.org/view.php?id=5549
This behaviour of incrementing the Number instead of making it 0 as per the following example is new for mdadm.
I'm unsure whether it is intentional or a minor bug.

[root@localhost ~]# mdadm -V
mdadm - v3.2.2 - 17th June 2011
[root@localhost ~]# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdb1[1]
204788 blocks super 1.0 [2/1] [_U]

md2 : active raid1 sdb3[1] sda3[0]
308266876 blocks super 1.1 [2/2] [UU]
bitmap: 2/3 pages [8KB], 65536KB chunk

md1 : active raid1 sdb2[1] sda2[0]
4094968 blocks super 1.1 [2/2] [UU]

unused devices: <none>
[root@localhost ~]# mdadm /dev/md0 --add /dev/sda1
mdadm: /dev/sda1 reports being an active member for /dev/md0, but a --re-add fails.
mdadm: not performing --add as that would convert /dev/sda1 in to a spare.
mdadm: To make this a spare, use "mdadm --zero-superblock /dev/sda1" first.
[root@localhost ~]# mdadm --zero-superblock /dev/sda1
[root@localhost ~]# mdadm /dev/md0 --add /dev/sda1
mdadm: added /dev/sda1
[root@localhost ~]# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sda1[2] sdb1[1]
204788 blocks super 1.0 [2/2] [UU]

md2 : active raid1 sdb3[1] sda3[0]
308266876 blocks super 1.1 [2/2] [UU]
bitmap: 2/3 pages [8KB], 65536KB chunk

md1 : active raid1 sdb2[1] sda2[0]
4094968 blocks super 1.1 [2/2] [UU]

It's related to mdadm version 3.2.2

Same test with version 3.2.1
[root@localhost ~]# mdadm -V
mdadm - v3.2.1 - 28th March 2011
[root@localhost ~]# cat /proc/mdstat
Personalities : [raid1]
md2 : active raid1 sda3[0] sdb3[1]
308266876 blocks super 1.1 [2/2] [UU]
bitmap: 1/3 pages [4KB], 65536KB chunk

md1 : active raid1 sda2[0] sdb2[1]
4094968 blocks super 1.1 [2/2] [UU]

md0 : active raid1 sda1[0] sdb1[1]
204788 blocks super 1.0 [2/2] [UU]

unused devices: <none>
[root@localhost ~]# mdadm /dev/md0 --fail /dev/sdb1
mdadm: set /dev/sdb1 faulty in /dev/md0
[root@localhost ~]# mdadm /dev/md0 --remove /dev/sdb1
mdadm: hot removed /dev/sdb1 from /dev/md0
[root@localhost ~]# cat /proc/mdstat
Personalities : [raid1]
md2 : active raid1 sda3[0] sdb3[1]
308266876 blocks super 1.1 [2/2] [UU]
bitmap: 0/3 pages [0KB], 65536KB chunk

md1 : active raid1 sda2[0] sdb2[1]
4094968 blocks super 1.1 [2/2] [UU]

md0 : active raid1 sda1[0]
204788 blocks super 1.0 [2/1] [U_]

unused devices: <none>
[root@localhost ~]# mdadm /dev/md0 --add /dev/sdb1
mdadm: re-added /dev/sdb1
You have new mail in /var/spool/mail/root
[root@localhost ~]# cat /proc/mdstat
Personalities : [raid1]
md2 : active raid1 sda3[0] sdb3[1]
308266876 blocks super 1.1 [2/2] [UU]
bitmap: 1/3 pages [4KB], 65536KB chunk

md1 : active raid1 sda2[0] sdb2[1]
4094968 blocks super 1.1 [2/2] [UU]

md0 : active raid1 sdb1[1] sda1[0]
204788 blocks super 1.0 [2/2] [UU]

unused devices: <none>

I don't think it's being marked as a hot spare. The /proc/mdstat output indicates that the mirror is operating normally. When I replaced the drive, I didn't remove the old drive from the array first. Had I done that, I'm thinking the new drive would have been listed as 1 when it was added.