Removing a disk from a RAID 1 array

I’m in the process of building a new NAS server to replace our existing one which is getting a bit long in the tooth. As usual I’ve opted to install Ubuntu Server LTS as the OS. In my current NAS I’ve got the system partition RAID 1’d across all the drives, this means that should one fail I’ve still got 4 copies.

Setting up the new NAS I’ve put the system partition on a separate SSD from the main data drives. I’ve currently only got the one SSD but I wanted the option of RAID 1’ing the system drive for redundancy should I decide to get another SSD in future.

The Ubuntu installer will quite happily let you create a RAID 1 MD device with 2 disks but only select a single drive to belong to it. This would be the equivalent of the following mdadm command:

mdadm --create /dev/md2 --level=1 --raid-devices=2 /dev/loop1 missing

However, when you boot the OS will consider that array degraded:

mdadm -D /dev/md2
/dev/md2:
Version : 1.2
Creation Time : Fri Jan 16 21:24:44 2015
Raid Level : raid1
Array Size : 32704 (31.94 MiB 33.49 MB)
Used Dev Size : 32704 (31.94 MiB 33.49 MB)
Raid Devices : 2
Total Devices : 1
Persistence : Superblock is persistent

Update Time : Fri Jan 16 21:26:58 2015
State : clean, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0

Name : lupin:2 (local to host lupin)
UUID : eccabf13:879f4196:1c47de11:3059e0b8
Events : 6

Number Major Minor RaidDevice State
0 7 1 0 active sync /dev/loop1
1 0 0 1 removed

It seems that you can’t remove the “missing” disk from the array even with

mdadm /dev/md2 -r detached

I eventually found the solution to this problem. It seems (counter-intuitively) that to remove a device from the array you have to “grow” that array as follows:

mdadm /dev/md2 --grow --force --raid-devices=1

This resulted in the missing / removed / non-existent device being removed from the array and it being considered clean:

mdadm -D /dev/md2
/dev/md2:
Version : 1.2
Creation Time : Fri Jan 16 21:24:44 2015
Raid Level : raid1
Array Size : 32704 (31.94 MiB 33.49 MB)
Used Dev Size : 32704 (31.94 MiB 33.49 MB)
Raid Devices : 1
Total Devices : 1
Persistence : Superblock is persistent

Update Time : Fri Jan 16 21:32:17 2015
State : clean
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0

Name : lupin:2 (local to host lupin)
UUID : eccabf13:879f4196:1c47de11:3059e0b8
Events : 11

Number Major Minor RaidDevice State
0 7 1 0 active sync /dev/loop1

This took me a while to figure out so I hope this might help someone else out there who’s trying to achieve the same thing.