MDADM

mdadm is a utility to create and manage raid devices.

Usage

Get information about a raid

To get an info for a running raid (assuming it is /dev/md0) run mdadm -D /dev/md0.

Add disk/partition to raid device

You can add a disk or partition to a raid device by running the following. It is assumed you are adding a partition called /dev/sdd1 but it could also be a whole drive and the mdadm drive is called /dev/md0.

mdadm --add /dev/md0 /dev/sdd1

Raid 1

Raid 1 creates a mirror with even amount of drives.

Create raid 1 device

You can create a Raid 1 device with mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sda /dev/sdb where sda and sdb are the disks you want to include in your raid. You can increase the number of raid devices, but they have to be at least 2. If it is an uneven amount of disks in the raid, a disk will act as a spare disk.

Convert raid 1 to raid 5

Assuming the raid device is called /dev/md0. All other drives are part of the md0 raid device.

Remove all drives but 2 by running mdadm /dev/md0 --fail /dev/sda1 and mdadm /dev/md0 --remove /dev/sda1 where sda1 is the drive to remove
Make sure your raid 1 array has only 2 active drives by running mdadm --grow /dev/md0 -n 2
Now convert your raid 1 to a raid 5 device with mdadm --grow /dev/md0 -l5
Add the disks you removed back to the raid with mdadm /dev/md0 --add /dev/sda1
Finally grow the active drive number to your needs (in this example 4) mdadm --grow /dev/md0 -n4
mdadm now reshapes the raid. You can monitor it by running watch cat /proc/mdstat

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