Hotplug
From Linux Raid Wiki
HW issues of the disk hotplugging are described in the Hotswap chapter of theHardware issues page.
The Linux RAID supports hotplug operations fully from Hot-unplug branch of the mdadm version 3.1.2.
mdadm versions < 3.1.2
Let's have the following RAID configuration:
# cat /proc/mdstat Personalities : [raid1] md0 : active raid1 sda1[0] sdb1[1] 3903680 blocks [2/2] [UU] md1 : active raid1 sda2[0] sdb2[1] 224612672 blocks [2/2] [UU]
If we hot-unplug the disk /dev/sda, the /proc/mdstat will show:
# cat /proc/mdstat Personalities : [raid1] md0 : active raid1 sda1[2](F) sdb1[1] 3903680 blocks [2/1] [_U] md1 : active raid1 sda2[0] sdb2[1] 224612672 blocks [2/2] [UU]
We see that sda1 has role [2]. Since RAID1 needs only 2 components - [0] and [1], the [2] means "Spare disk". And it is (F)ailed.
What we can do is remove the failed disk:
# mdadm /dev/md3 --fail detached --remove detached mdadm: hot removed 8:1
mdadm versions > 3.1.2
(to be finished)