HP Serviceguard Extended Distance Cluster for Linux A.11.20.10 Deployment Guide, December 2012

A.4 Removing and Adding an MD Mirror Component Disk
There are certain failure scenarios, where you would need to manually remove the mirror component
of an MD device and add it again later. For example, if links between two data centers fail, you
would need to remove and add the disks that were marked as failed disks.
When a disk within an MD device fails, the /proc/mdstat file of the MD array displays a
message. For example:
[root@dlhct1 dev]# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sde[2](F) sdf[0]
9766784 blocks [2/1] [U_]
unused devices: <none>
In the message, the (F) indicates which disk has failed. In this example, the sde[2] disk has
failed.
In such a scenario, you must remove the failed disk from the MD array. You need to determine the
persistent name of the failed disk before you remove it from the MD array. For this example, run
the following command to determine the persistent name of the disk:
# udevinfo -q symlink -n sdc1
Following is a sample output:
hpdev/mylink-sdc \
disk/by-id/scsi-3600805f3000b9510a6d7f8a6cdb70054-part1 \ disk/by
-path/pci-0000:06:01.0-scsi-0:0:1:30-part1
Run the following command to remove a failed component device from the MD array:
# mdadm - -remove <md device name> <md_mirror_component_persistent_name>
In this example:
# mdadm --remove /dev/md0 /dev/hpdev/mylink-sdc1
This command removes the failed mirrored disk from the array.
Example 3 Removing a failed MD component disk from /dev/md0 array
To remove a failed MD component disk from /dev/md0, run the following command:
# mdadm –-remove /dev/md0 /dev/hpdev/sde
Following is an example of the status message that is displayed when a failed component is
removed from the MD array:
[root@dlhct1 dev]# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdf[0]
9766784 blocks [2/1] [U_]
unused devices: <none>
A.4.1 Adding a Mirror Component Device
As mentioned earlier, in certain failure scenarios, you need to remove a failed mirror disk
component, repair it and then add it back into an MD array. Run the following command to add
a mirror component back into the MD array:
# mdadm - -add <md device name> <md_mirror_component_persistent_name>
A.4 Removing and Adding an MD Mirror Component Disk 47