mdadm is a tool that allows for creation and + management of software RAID arrays on Linux. Creating an array + is a rather straightforward process.

+ +

Install packages

The only package needed is mdadm itself

apt install mdadm

+ +

Partition disks

We'll need to parition the disks to be used in the array before + creating it. This isn't anything complicated, we will just be + creating a single partition using all the space on each disk.

+ +

Use lsblk to get a list of disks attached to your system.

+ +

+ +

Then use fdisk to edit the partition table of the + first disk. In my case this would be /dev/sdb. + Be sure that you are selecting the correct disk as selecting the + wrong one can result in data being lost.

+ +

fdisk /dev/sdb

+ +

Use g to create a new GUID partition table. + Use n to create a new partition, and then just press + enter at all of the prompts to accept the defaults. Finally use + w to write the changes. You can use lsblk again + to verify the change and you should see that /dev/sdb now has a + partition /dev/sdb1.

+ +

+ +

Repeat this process for your other disks before continuing.

+ + +

Create the array

Creating the array is done with a single command, but takes just a + bit of planning.

Define an ID for the array, which is just a + number identifier. I use 0 in the command.
Determine the RAID level you want to use. I am going to + use RAID5 in this example. The argument to --level is the + number of the RAID level
Determine the devices that will be used in the array. + Somewhat contrary to the argument name, these devices + will actually be the partitions of the disks and not the + disks themselves. In the command, give the number of + partitions (3) and then a space-separated list of the + partitions that will be active in the array + (/dev/sdb1 /dev/sdc1 /dev/sdd1).
If you want to have any spare devices in the array you + will define them with the --spare-devices argument. These are + defined in the exact same way as the active RAID devices. In + the example I use 1 spare /dev/sde1. Spare + devices are hot-spares that will automatically be inserted into + the array if one of the disks fails.

+ +

mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1 --spare-devices=1 /dev/sde1

+ +

After creating the array run the following command to get details + and the status of the array. It will take a bit to initialize the + array, you will know this is done when the state is clean. You do not + need to wait for the array to completely intialize to continue.

+ +

mdadm --detail /dev/md0

+ +

Take note of the UUID and name values as we will need them in the + next step.

+ +

Before moving on we need to make a filesystem on the array. I'm + going to make a simple EXT4 filesystem here.

+ +

mkfs.ext4 /dev/md0

+ + +

Configuration Files

+ +

Open the mdadm configuration file at + /etc/mdadm/mdadm.conf and append this line. Replace + uuid and name with the values you got when running + mdadm --detail, replace 0 whatever ID you chose.

+ +

ARRAY /dev/md0 metadata=1.2 UUID=uuid name=name

+ +

Optionally, create an /etc/fstab entry for + automounting of the array. Replace /mnt/raid with the directory + where you want to mount the array. If you made a filesystem other than + ext4 make sure to change that value.

+ +

/dev/md0 /mnt/raid ext4 defaults 0 1

+ +

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