Linux Software RAID Setup and Configuration

Last modified: Friday December 1st, 2023

This article describes how to use a software RAID for organizing the interaction of multiple drives in a Linux operating system, and without using a hardware RAID controller.

Display the state of a software RAID

To do this, just use this command:

cat /proc/mdstat

If no RAID is active, the output looks like this:

Personalities : [raid1]
unused devices: <none>

Here’s the output for a configured RAID1:

Personalities : [raid1]
md2 : active raid1 sda3[2] sdb3[1]
      234405504 blocks super 1.2 [2/2] [UU]
      bitmap: 0/2 pages [0KB], 65536KB chunk

md1 : active raid1 sda2[2] sdb2[1]
      523712 blocks super 1.2 [2/2] [UU]

md0 : active raid1 sda1[2] sdb1[1]
      33521664 blocks super 1.2 [2/2] [UU]

unused devices: <none>

Here’s the output for a configured RAID0 (You need to always configure the /boot partition md0 as RAID1 to allow the server to boot from it):

Personalities : [raid1] [raid0]
md2 : active raid0 sda3[0] sdb3[1]
      883956736 blocks super 1.2 512k chunks

md1 : active raid0 sda2[0] sdb2[1]
      52393984 blocks super 1.2 512k chunks

md0 : active raid1 sda1[0] sdb1[1]
      523264 blocks super 1.2 [2/2] [UU]

unused devices: <none>

If there is a progress bar displayed under one of the partitions, a RAID resync is currently running:

md0 : active raid1 sdb1[0] sdc1[1]
     2095040 blocks super 1.2 [2/2] [UU]
     	[====>................]  resync = 32.7% (418656/2095040) finish=4.2min speed=131219K/sec

Add a software RAID array

In our example scenario, the drives /dev/sda and /dev/sdb are already combined in multiple RAID1 arrays, which contain the operating system:

cat /proc/mdstat
Personalities : [raid1]
md2 : active raid1 sda3[2] sdb3[1]
      234405504 blocks super 1.2 [2/2] [UU]
      bitmap: 0/2 pages [0KB], 65536KB chunk

md1 : active raid1 sda2[2] sdb2[1]
      523712 blocks super 1.2 [2/2] [UU]

md0 : active raid1 sda1[2] sdb1[1]
      33521664 blocks super 1.2 [2/2] [UU]

unused devices: <none>

But we have two more drives (/dev/sdc and /dev/sdd), which we would also like to set up for data storage with RAID1 array. So we need to add the RAID array first:

mdadm --create --verbose /dev/md3 --level=1 --raid-devices=2 /dev/sdc /dev/sdd

The RAID configuration should now look like this:

cat /proc/mdstat
Personalities : [raid1]
md3 : active raid1 sdc1[0] sdd1[1]
     2095040 blocks super 1.2 [2/2] [UU]
     	[====>................]  resync = 32.7% (418656/2095040) finish=4.2min speed=131219K/sec

md2 : active raid1 sda3[2] sdb3[1]
      234405504 blocks super 1.2 [2/2] [UU]
      bitmap: 0/2 pages [0KB], 65536KB chunk

md1 : active raid1 sda2[2] sdb2[1]
      523712 blocks super 1.2 [2/2] [UU]

md0 : active raid1 sda1[2] sdb1[1]
      33521664 blocks super 1.2 [2/2] [UU]

unused devices: <none>

Now we can format the new partition (here with EXT4) and mount them:

mkfs.ext4 /dev/md3
mount /dev/md3 /mnt

Email notification when a drive in software RAID fails

Requirement: You must first install and configure a mail server of your choice (e.g. Sendmail).

Debian/Ubuntu/CentOS

Edit /etc/mdadm/mdadm.conf or /etc/mdadm.conf (CentOS) and change the following line:

MAILADDR holu@example.com

Here you can directly specify a destination address. Or, you can forward all emails sent to root to a specific email address using /etc/aliases.

You can also optionally configure the sending email address:

MAILFROM mdadm@example.com

For Debian and Ubuntu, it is important that you set AUTOCHECK in the file /etc/default/mdadm to true:

# grep AUTOCHECK= /etc/default/mdadm
AUTOCHECK=true

For CentOS, you must enable the RAID check in the file /etc/sysconfig/raid-check:

# grep ENABLED /etc/sysconfig/raid-check
ENABLED=yes

openSUSE

Edit /etc/sysconfig/mdadm and add the email address where you would receive the notification next to the variable MDADM_MAIL:

MDADM_MAIL="holu@example.com"

Test the configuration

You can verify your configuration by letting mdadm send a test mail to the mail address using this command:

mdadm --monitor --test --oneshot /dev/md0

You should also ensure that the file /etc/cron.daily/mdadm contains the following line, which performs the daily monitoring of your RAID:

exec --monitor --scan --oneshot

Removing a software RAID

To remove a software RAID, you can use the following commands.

mdadm --remove /dev/md0
mdadm --remove /dev/md1
mdadm --remove /dev/md2
mdadm --stop /dev/md0
mdadm --stop /dev/md1
mdadm --stop /dev/md2

After that, you can formatt the drives normally again (for example, with EXT4):

mkfs.ext4 /dev/sda
mkfs.ext4 /dev/sdb

You can check the result with the commands…

cat /proc/mdstat

…and…

fdisk -l

And now you should remove the software RAID.

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