mdadm: Creating a RAID Array

Document Scope

The mdadm utility can be used to create and manage storage arrays using Linux’s software RAID capabilities. Administrators have great flexibility in coordinating their individual storage devices and creating logical storage devices that have greater performance or redundancy characteristics.

In this guide, we will describe some different configurations you can use to create a software-based RAID array using mdadm under Linux. We will be focusing on RAID levels 0, 1, and 5 as those are the most common RAID levels we implement and see our customers utilize. Keep in mind the steps here, if followed, will give you working RAID but may not match your original configuration, nor do these steps cover ALL the various ways you can use this tool to setup your RAID.

Prerequisites

To follow the steps in this guide, you will need:

• A non-root user with sudo privileges
• A Linux operating system (like Rocky Linux or Ubuntu) with the mdadm utility installed
• A basic understanding of RAID terminology and concepts. (To learn more about RAID and what RAID level is right for you, read our introduction to RAID article.)
• Multiple raw storage devices available on your server. The examples in this tutorial demonstrate how to configure various types of arrays on the server. As such, you will need some drives to configure. While this guide will have examples, you should ensure any commands you run use YOUR devices and you should NOT just copy and paste the commands as seen here.
• Depending on the array type, you will need two to four storage devices. These drives do not need to be formatted prior to following this guide.

Resetting Existing RAID Devices (Optional)

You can skip this section for now if you have not yet set up any arrays. This guide will introduce a number of different RAID levels. If you wish to follow along and complete each RAID level for your devices, you will likely want to reuse your storage devices after each section. This specific section Resetting Existing RAID Devices can be referenced to reset your component storage devices prior to testing a new RAID level.

Warning: This process will completely destroy the array and any data written to it. Make sure that you are operating on the correct array and that you have copied any data you need to retain prior to destroying the array.

Begin by finding the active arrays in the /proc/mdstat file:

cat /proc/mdstat

Output

Personalities : [raid0] [linear] [multipath] [raid1] [raid6] [raid5] [raid4] [raid10] 
 md0 : active raid0 sdc[1] sdd[0]
      209584128 blocks super 1.2 512k chunks

unused devices: <none>

Then unmount the array from the filesystem:

sudo umount /dev/md0

Now stop and remove the array:

sudo mdadm --stop /dev/md0

Find the devices that were used to build the array with the following command:

Warning: Keep in mind that the /dev/sd* names can potentially change any time you reboot. Check them every time to make sure you are operating on the correct devices.

lsblk -o NAME,SIZE,FSTYPE,TYPE,MOUNTPOINT

Output

NAME    SIZE FSTYPE            TYPE MOUNTPOINT
sda     100G linux_raid_member disk 
sdb     100G linux_raid_member disk 
sdc     100G                   disk 
sdd     100G                   disk 
vda     25G                    disk 
├─vda1  24.9G ext4             part /
├─vda14 4M                     part 
└─vda15 106M  vfat             part /boot/efi
vdb     466K  iso9660          disk

After discovering the devices used to create an array, zero their superblock which holds metadata for the RAID setup. Zeroing this removes the RAID metadata and resets them to normal:

sudo mdadm --zero-superblock /dev/sda
sudo mdadm --zero-superblock /dev/sdb

It’s recommended to also remove any persistent references to the array. Edit the /etc/fstab file and comment out or remove the reference to your array using nano or your preferred text editor.

sudo nano /etc/fstab

You can comment it out by inserting a hashtag symbol # at the beginning of the line, using nano or your preferred text editor:

# /dev/md0 /mnt/md0 ext4 defaults,nofail,discard 0 0

From here, you should be ready to reuse the storage devices individually, or as components of a different array.

Creating a RAID 0 Array

The RAID 0 array works by breaking up data into chunks and striping it across the available disks. This means that each disk contains a portion of the data and that multiple disks will be referenced when retrieving information.

• Requirements: Minimum of 2 storage devices.
• Primary benefit: Performance in terms of read/write and capacity.
• Things to keep in mind: Make sure that you have functional backups. A single device failure will destroy all data in the array.

Identifying the Component Devices

To start, find the identifiers for the raw disks that you will be using:

lsblk -o NAME,SIZE,FSTYPE,TYPE,MOUNTPOINT

Output

NAME    SIZE  FSTYPE  TYPE MOUNTPOINT
sda     100G          disk
sdb     100G          disk
vda     25G           disk 
├─vda1  24.9G ext4    part /
├─vda14 4M            part 
└─vda15 106M  vfat    part /boot/efi
vdb     466K  iso9660 disk

In this example, you have two disks without a filesystem, each 100G in size. These devices have been given the /dev/sda and /dev/sdb identifiers for this guide and will be the raw components used to build the array.

Creating the Array

To create a RAID 0 array with these components, pass them into the mdadm --create command. You will have to specify the device name you wish to create, the RAID level, and the number of devices. In this command example, you will be naming the device /dev/md0, and include the two disks that will build the array:

sudo mdadm --create --verbose /dev/md0 --level=0 --raid-devices=2 /dev/sda /dev/sdb

Confirm that the RAID was successfully created by checking the /proc/mdstat file:

cat /proc/mdstat

Output:

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
 md0 : active raid0 sdb[1] sda[0]
      209584128 blocks super 1.2 512k chunks

unused devices: <none>

This output reveals that the /dev/md0 device was created in the RAID 0 configuration using the /dev/sda and /dev/sdb devices.

Creating and Mounting the Filesystem

Next, create a filesystem on the array:

sudo mkfs.ext4 -F /dev/md0

Then, create a mount point to attach the new filesystem:

sudo mkdir -p /mnt/md0

You can mount the filesystem with the following command:

sudo mount /dev/md0 /mnt/md0

After, check whether the new space is available:

df -h -x devtmpfs -x tmpfs

Output:

Filesystem Size Used Avail Use% Mounted on
/dev/vda1  25G  1.4G 23G   6%   /
/dev/vda15 105M 3.4M 102M  4%   /boot/efi
/dev/md0   196G 61M  186G  1%   /mnt/md0

The new filesystem is now mounted and accessible. Next, we want to add the new filesystem mount options to the /etc/fstab file for automatic mounting at boot. To do this, we'll need the UUID for our RAID:

sudo blkid | grep md

This output should provide the UUID we'll need for the next step

$ sudo blkid | grep md
/dev/md0: UUID="a0b9226c-23e8-4a77-a7d8-3c6c7f57c412" TYPE="ext4"

Note this UUID. Now edit your /etc/fstab file and add the following:

UUID=YOUR-UUID /mnt/md0 ext4 defaults 0 0

In a virtual machine of Rocky Linux we created for this guide, it looked like this:

# /etc/fstab
# Created by anaconda on Wed Jan 10 00:34:36 2024
#
# Accessible filesystems, by reference, are maintained under '/dev/disk/'.
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info.
#
# After editing this file, run 'systemctl daemon-reload' to update systemd
# units generated from this file.
#
UUID=625c16e9-4b40-467c-b47d-0029556a3b34 / xfs defaults 0 0
UUID=e1166107-d6d2-408a-9198-bd87d724386c /boot xfs defaults 0 0
UUID=6186-1CD4 /boot/efi vfat umask=0077,shortname=winnt 0 2
UUID=0f8cd2bd-acb5-4ed3-a92f-5492b56c0288 none swap defaults 0 0
UUID=a0b9226c-23e8-4a77-a7d8-3c6c7f57c412 /mnt/md0 ext4 defaults 0 0

Your RAID 0 array will now automatically mount at each boot.

You’re now finished with your RAID 0 set up. If you want to try a different RAID, follow the resetting instructions at the beginning of this tutorial to proceed with creating a new RAID array type from one of the other options below!

Creating a RAID 1 Array

The RAID 1 array type is implemented by mirroring data across all available disks. Each disk in a RAID 1 array gets a full copy of the data, providing redundancy in the event of a device failure.

• Requirements: Minimum of 2 storage devices.
• Primary benefit: Redundancy between two storage devices
• Things to keep in mind: Since every disk maintains a copy of the data, your raid "size" will never be larger than a single disk

Identifying the Component Devices

To start, find the identifiers for the raw disks that you will be using:

lsblk -o NAME,SIZE,FSTYPE,TYPE,MOUNTPOINT

Output

NAME    SIZE  FSTYPE  TYPE MOUNTPOINT
sda     100G          disk
sdb     100G          disk
vda     25G           disk 
├─vda1  24.9G ext4    part /
├─vda14 4M            part 
└─vda15 106M  vfat    part /boot/efi
vdb     466K  iso9660 disk

In this example, you have two disks without a filesystem, each 100G in size. These devices have been given the /dev/sda and /dev/sdb identifiers for this guide and will be the raw components used to build the array.

Creating the Array

To create a RAID 1 array with these components, pass them into the mdadm --create command. You will have to specify the device name you wish to create, the RAID level, and the number of devices. In this command example, you will be naming the device /dev/md0, and include the disks that will build the array:

sudo mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sda /dev/sdb

If the component devices you are using are not partitions with the boot flag enabled, you will likely receive the following warning. It is safe to respond with y and continue:

mdadm: Note: this array has metadata at the start and
may not be suitable as a boot device. If you plan to
store '/boot' on this device please ensure that
your boot-loader understands md/v1.x metadata, or use
--metadata=0.90
mdadm: size set to 104792064K
Continue creating array? y

The mdadm tool will start to mirror the drives. This can take some time to complete, but the array can be used during this time. You can monitor the progress of the mirroring by checking the /proc/mdstat file:

cat /proc/mdstat

Output:

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
 md0 : active raid1 sdb[1] sda[0]
      104792064 blocks super 1.2 [2/2] [UU]
       [====>................] resync = 20.2% (21233216/104792064) finish=6.9min speed=199507K/sec

unused devices: <none>

In the first highlighted line, the /dev/md0 device was created in the RAID 1 configuration using the /dev/sda and /dev/sdb devices. The second highlighted line reveals the progress on the mirroring. You can continue to the next step while this process completes.

Creating and Mounting the Filesystem

Next, create a filesystem on the array:

sudo mkfs.ext4 -F /dev/md0

Then, create a mount point to attach the new filesystem:

sudo mkdir -p /mnt/md0

You can mount the filesystem with the following command:

sudo mount /dev/md0 /mnt/md0

After, check whether the new space is available:

df -h -x devtmpfs -x tmpfs

Output:

Filesystem Size Used Avail Use% Mounted on
/dev/vda1  25G  1.4G 23G   6%   /
/dev/vda15 105M 3.4M 102M  4%   /boot/efi
/dev/md0   99G  60M  94G   1%   /mnt/md0

The new filesystem is now mounted and accessible. Next, we want to add the new filesystem mount options to the /etc/fstab file for automatic mounting at boot. To do this, we'll need the UUID for our RAID:

sudo blkid | grep md

This output should provide the UUID we'll need for the next step

$ sudo blkid | grep md
/dev/md0: UUID="a0b9226c-23e8-4a77-a7d8-3c6c7f57c412" TYPE="ext4"

Note this UUID. Now edit your /etc/fstab file and add the following:

UUID=YOUR-UUID /mnt/md0 ext4 defaults 0 0

In a virtual machine of Rocky Linux we created for this guide, it looked like this:

# /etc/fstab
# Created by anaconda on Wed Jan 10 00:34:36 2024
#
# Accessible filesystems, by reference, are maintained under '/dev/disk/'.
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info.
#
# After editing this file, run 'systemctl daemon-reload' to update systemd
# units generated from this file.
#
UUID=625c16e9-4b40-467c-b47d-0029556a3b34 / xfs defaults 0 0
UUID=e1166107-d6d2-408a-9198-bd87d724386c /boot xfs defaults 0 0
UUID=6186-1CD4 /boot/efi vfat umask=0077,shortname=winnt 0 2
UUID=0f8cd2bd-acb5-4ed3-a92f-5492b56c0288 none swap defaults 0 0
UUID=a0b9226c-23e8-4a77-a7d8-3c6c7f57c412 /mnt/md0 ext4 defaults 0 0

Your RAID 1 array will now automatically mount at each boot.

You’re now finished with your RAID 1 set up. If you want to try a different RAID, follow the resetting instructions at the beginning of this tutorial to proceed with creating a new RAID array type.

Creating a RAID 5 Array

The RAID 5 array type is implemented by striping data across the available devices. One component of each stripe is a calculated parity block. If a device fails, the parity block and the remaining blocks can be used to calculate the missing data. The device that receives the parity block is rotated so that each device has a balanced amount of parity information.

• Requirements: Minimum of 3 storage devices.
• Primary benefit: Redundancy with more usable capacity.
• Things to keep in mind: While the parity information is distributed, one disk’s worth of capacity will be used for parity. RAID 5 can suffer from very poor performance when in a degraded state

Identifying the Component Devices

To start, find the identifiers for the raw disks that you will be using:

lsblk -o NAME,SIZE,FSTYPE,TYPE,MOUNTPOINT

Output

NAME    SIZE  FSTYPE  TYPE MOUNTPOINT
sda     100G          disk
sdb     100G          disk
sdc     100G          disk
vda     25G           disk 
├─vda1  24.9G ext4    part /
├─vda14 4M            part 
└─vda15 106M  vfat    part /boot/efi
vdb     466K  iso9660 disk

You have three disks without a filesystem, each 100G in size. These devices have been given the /dev/sda, /dev/sdb, and /dev/sdc identifiers for this session and will be the raw components you use to build the array.

Creating the Array

To create a RAID 5 array with these components, pass them into the mdadm --create command. You will have to specify the device name you wish to create, the RAID level, and the number of devices. In this command example, you will be naming the device /dev/md0, and include the disks that will build the array:

sudo mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sda /dev/sdb /dev/sdc

The mdadm tool will start to configure the array. It uses the recovery process to build the array for performance reasons. This can take some time to complete, but the array can be used during this time. You can monitor the progress of the mirroring by checking the /proc/mdstat file:

cat /proc/mdstat

Output:

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
 md0 : active raid5 sdc[3] sdb[1] sda[0]
      209582080 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/2] [UU_]
       [>....................] recovery = 0.9% (957244/104791040) finish=18.0min speed=95724K/sec

unused devices: <none>

In the first highlighted line, the /dev/md0 device was created in the RAID 5 configuration using the /dev/sda, /dev/sdb and /dev/sdc devices. The second highlighted line shows the progress of the build. You can continue the guide while this process completes. 

Creating and Mounting the Filesystem

Next, create a filesystem on the array:

sudo mkfs.ext4 -F /dev/md0

Then, create a mount point to attach the new filesystem:

sudo mkdir -p /mnt/md0

You can mount the filesystem with the following command:

sudo mount /dev/md0 /mnt/md0

After, check whether the new space is available:

df -h -x devtmpfs -x tmpfs

Output:

Filesystem Size Used Avail Use% Mounted on
/dev/vda1  25G  1.4G 23G   6%   /
/dev/vda15 105M 3.4M 102M  4%   /boot/efi
/dev/md0   197G 60M  187G  1%   /mnt/md0

The new filesystem is now mounted and accessible. Next, we want to add the new filesystem mount options to the /etc/fstab file for automatic mounting at boot. To do this, we'll need the UUID for our RAID:

sudo blkid | grep md

This output should provide the UUID we'll need for the next step

$ sudo blkid | grep md
/dev/md0: UUID="a0b9226c-23e8-4a77-a7d8-3c6c7f57c412" TYPE="ext4"

Note this UUID. Now edit your /etc/fstab file and add the following:

UUID=YOUR-UUID /mnt/md0 ext4 defaults 0 0

In a virtual machine of Rocky Linux we created for this guide, it looked like this:

# /etc/fstab
# Created by anaconda on Wed Jan 10 00:34:36 2024
#
# Accessible filesystems, by reference, are maintained under '/dev/disk/'.
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info.
#
# After editing this file, run 'systemctl daemon-reload' to update systemd
# units generated from this file.
#
UUID=625c16e9-4b40-467c-b47d-0029556a3b34 / xfs defaults 0 0
UUID=e1166107-d6d2-408a-9198-bd87d724386c /boot xfs defaults 0 0
UUID=6186-1CD4 /boot/efi vfat umask=0077,shortname=winnt 0 2
UUID=0f8cd2bd-acb5-4ed3-a92f-5492b56c0288 none swap defaults 0 0
UUID=a0b9226c-23e8-4a77-a7d8-3c6c7f57c412 /mnt/md0 ext4 defaults 0 0

Your RAID 5 array will now automatically mount at each boot.

You’re now finished with your RAID 5 set up. If you want to try a different RAID, follow the resetting instructions at the beginning of this tutorial to proceed with creating a new RAID array type.