How To Create RAID Arrays with mdadm on Ubuntu 18.04

Introduction

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, you will perform different RAID configurations that can be set up using an Ubuntu 18.04 server.

Prerequisites

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

• A non-root user with sudo privileges on an Ubuntu 18.04 server. To learn how to set up an account with these privileges, follow our Ubuntu 18.04 initial server setup guide.
• 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.
• 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.

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:

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

sudo nano /etc/fstab

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

Also, comment out or remove the array definition from the /etc/mdadm/mdadm.conf file:

sudo nano /etc/mdadm/mdadm.conf

. . .
# ARRAY /dev/md0 metadata=1.2 name=mdadmwrite:0 UUID=7261fb9c:976d0d97:30bc63ce:85e76e91

Finally, update the initramfs again so that the early boot process does not try to bring an unavailable array online:

sudo update-initramfs -u

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 session 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.

Saving the Array Layout

To make sure that the array is reassembled automatically at boot, you will have to adjust the /etc/mdadm/mdadm.conf file. You can automatically scan the active array and append the file with the following:

sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf

Afterwards, you can update the initramfs, or initial RAM file system, so that the array will be available during the early boot process:

sudo update-initramfs -u

Add the new filesystem mount options to the /etc/fstab file for automatic mounting at boot:

echo '/dev/md0 /mnt/md0 ext4 defaults,nofail,discard 0 0' | sudo tee -a /etc/fstab

Your RAID 0 array will now automatically assemble and mount each boot.

You’re now finished with your RAID 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 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 two copies of the data are maintained, only half of the disk space will be usable.

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 session and will be the raw components you use 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:

Output
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 by running the following:

sudo mount /dev/md0 /mnt/md0

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 mounted and accessible.

Saving the Array Layout

To make sure that the array is reassembled automatically at boot, you have to adjust the /etc/mdadm/mdadm.conf file. You can automatically scan the active array and append the file with the following:

sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf

Afterward, you can update the initramfs, or initial RAM file system, so that the array will be available during the early boot process:

sudo update-initramfs -u

Add the new filesystem mount options to the /etc/fstab file for automatic mounting at boot:

echo '/dev/md0 /mnt/md0 ext4 defaults,nofail,discard 0 0' | sudo tee -a /etc/fstab

Your RAID 1 array will now automatically assemble and mount each boot.

You’re now finished with your RAID 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

Create a mount point to attach the new filesystem:

sudo mkdir -p /mnt/md0

You can mount the filesystem with the following:

sudo mount /dev/md0 /mnt/md0

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 mounted and accessible.

Saving the Array Layout

To make sure that the array is reassembled automatically at boot, you have to adjust the /etc/mdadm/mdadm.conf file.

You can monitor the progress of the mirroring by checking the /proc/mdstat file:

cat /proc/mdstat

Output
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] 
md0 : active raid5 sdc[3] sdb[1] sda[0]
      209584128 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU]
      
unused devices: <none>

This output reveals that the rebuild is complete. Now, you can automatically scan the active array and append the file:

sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf

Afterwards, you can update the initramfs, or initial RAM file system, so that the array will be available during the early boot process:

sudo update-initramfs -u

Add the new filesystem mount options to the /etc/fstab file for automatic mounting at boot:

echo '/dev/md0 /mnt/md0 ext4 defaults,nofail,discard 0 0' | sudo tee -a /etc/fstab

Your RAID 5 array will now automatically assemble and mount each boot.

You’re now finished with your RAID 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 6 Array

The RAID 6 array type is implemented by striping data across the available devices. Two components of each stripe are calculated parity blocks. If one or two devices fail, the parity blocks and the remaining blocks can be used to calculate the missing data. The devices that receive the parity blocks are rotated so that each device has a balanced amount of parity information. This is similar to a RAID 5 array, but allows for the failure of two drives.

• Requirements: Minimum of 4 storage devices.
• Primary benefit: Double redundancy with more usable capacity.
• Things to keep in mind: While the parity information is distributed, two disks worth of capacity will be used for parity. RAID 6 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 
sdd      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 four disks without a filesystem, each 100G in size. These devices have been given the /dev/sda, /dev/sdb, /dev/sdc, and /dev/sdd identifiers for this session and will be the raw components used to build the array.

Creating the Array

To create a RAID 6 array with these components, pass them into the mdadm --create command. You have to specify the device name you wish to create, the RAID level, and the number of devices. In this following 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=6 --raid-devices=4 /dev/sda /dev/sdb /dev/sdc /dev/sdd

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 raid6 sdd[3] sdc[2] sdb[1] sda[0]
      209584128 blocks super 1.2 level 6, 512k chunk, algorithm 2 [4/4] [UUUU]
      [>....................]  resync =  0.6% (668572/104792064) finish=10.3min speed=167143K/sec
      
unused devices: <none>

In the first highlighted line, the /dev/md0 device has been created in the RAID 6 configuration using the /dev/sda, /dev/sdb, /dev/sdc and /dev/sdd 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

Create a mount point to attach the new filesystem:

sudo mkdir -p /mnt/md0

You can mount the filesystem with the following:

sudo mount /dev/md0 /mnt/md0

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 mounted and accessible.

Saving the Array Layout

To make sure that the array is reassembled automatically at boot, you will have to adjust the /etc/mdadm/mdadm.conf file. You can automatically scan the active array and append the file by typing:

sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf

Afterwards, you can update the initramfs, or initial RAM file system, so that the array will be available during the early boot process:

sudo update-initramfs -u

Add the new filesystem mount options to the /etc/fstab file for automatic mounting at boot:

echo '/dev/md0 /mnt/md0 ext4 defaults,nofail,discard 0 0' | sudo tee -a /etc/fstab

Your RAID 6 array will now automatically assemble and mount each boot.

You’re now finished with your RAID 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 Complex RAID 10 Array

The RAID 10 array type is traditionally implemented by creating a striped RAID 0 array composed of sets of RAID 1 arrays. This nested array type gives both redundancy and high performance, at the expense of large amounts of disk space. The mdadm utility has its own RAID 10 type that provides the same type of benefits with increased flexibility. It is not created by nesting arrays, but has many of the same characteristics and guarantees. You will be using the mdadm RAID 10 here.

• Requirements: Minimum of 3 storage devices.
• Primary benefit: Performance and redundancy.
• Things to keep in mind: The amount of capacity reduction for the array is defined by the number of data copies you choose to keep. The number of copies that are stored with mdadm style RAID 10 is configurable.

By default, two copies of each data block will be stored in what is called the near layout. The possible layouts that dictate how each data block is stored are as follows:

• near: The default arrangement. Copies of each chunk are written consecutively when striping, meaning that the copies of the data blocks will be written around the same part of multiple disks.
• far: The first and subsequent copies are written to different parts of the storage devices in the array. For instance, the first chunk might be written near the beginning of a disk, while the second chunk would be written halfway down on a different disk. This can give some read performance gains for traditional spinning disks at the expense of write performance.
• offset: Each stripe is copied, and offset by one drive. This means that the copies are offset from one another, but still close together on the disk. This helps minimize excessive seeking during some workloads.

You can find out more about these layouts by checking out the RAID10 section of this man page:

man 4 md

You can also find this man page online.

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

Creating the Array

To create a RAID 10 array with these components, pass them into the mdadm --create command. You have to specify the device name you wish to create, the RAID level, and the number of devices. In this following command example, you will be naming the device /dev/md0 and include the disks that will build the array:

You can set up two copies using the near layout by not specifying a layout and copy number:

sudo mdadm --create --verbose /dev/md0 --level=10 --raid-devices=4 /dev/sda /dev/sdb /dev/sdc /dev/sdd

If you want to use a different layout or change the number of copies, you will have to use the --layout= option, which takes a layout and copy identifier. The layouts are n for near, f for far, and o for offset. The number of copies to store is appended afterward.

For instance, to create an array that has three copies in the offset layout, the command would include the following:

sudo mdadm --create --verbose /dev/md0 --level=10 --layout=o3 --raid-devices=4 /dev/sda /dev/sdb /dev/sdc /dev/sdd

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 : [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid1] [raid10] 
md0 : active raid10 sdd[3] sdc[2] sdb[1] sda[0]
      209584128 blocks super 1.2 512K chunks 2 near-copies [4/4] [UUUU]
      [===>.................]  resync = 18.1% (37959424/209584128) finish=13.8min speed=206120K/sec
      
unused devices: <none>

In the first highlighted line, the /dev/md0 device has been created in the RAID 10 configuration using the /dev/sda, /dev/sdb, /dev/sdc and /dev/sdd devices. The second highlighted area shows the layout that was used for this example (two copies in the near configuration). The third highlighted area shows the progress on 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

Create a mount point to attach the new filesystem:

sudo mkdir -p /mnt/md0

You can mount the filesystem with the following:

sudo mount /dev/md0 /mnt/md0

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 mounted and accessible.

Saving the Array Layout

To make sure that the array is reassembled automatically at boot, you will have to adjust the /etc/mdadm/mdadm.conf file. You can automatically scan the active array and append the file by running the following:

sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf

Afterwards, you can update the initramfs, or initial RAM file system, so that the array will be available during the early boot process:

sudo update-initramfs -u

Add the new filesystem mount options to the /etc/fstab file for automatic mounting at boot:

echo '/dev/md0 /mnt/md0 ext4 defaults,nofail,discard 0 0' | sudo tee -a /etc/fstab

Your RAID 10 array will now automatically assemble and mount each boot.

Conclusion

In this guide, you learned how to create various types of arrays using Linux’s mdadm software RAID utility. RAID arrays offer some compelling redundancy and performance enhancements over using multiple disks individually.

Once you have settled on the type of array needed for your environment and created the device, you can learn how to perform day-to-day management with mdadm. Our guide on how to manage RAID arrays with mdadm on Ubuntu can help get you started.

sudo parted /dev/sda
> mklabel gpt
> print (to get the disk info for the next command)
> mkpart primary 0 1024K 4000GB
> quit

lsblk -o NAME,SIZE,FSTYPE,TYPE,MOUNTPOINT
NAME         SIZE FSTYPE  TYPE MOUNTPOINT
sda          3.7T         disk
└─sda1       3.7T         part
sdb          3.7T         disk
└─sdb1       3.7T         part
sdc          3.7T         disk
└─sdc1       3.7T         part
sdd          3.7T         disk
└─sdd1       3.7T         part
nvme0n1      477G         disk
├─nvme0n1p1  511M vfat    part /boot/efi
├─nvme0n1p2  476G ext4    part /
├─nvme0n1p3  511M swap    part [SWAP]
└─nvme0n1p4    1M iso9660 part

sudo mkfs.ext4 /dev/sda1
[...]

sudo mdadm --create --verbose /dev/md0 --level=10 --raid-devices=4 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1

sudo mdadm -D /dev/md0
/dev/md0:
[...]
    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync set-A   /dev/sda1
       1       8       17        1      active sync set-B   /dev/sdb1
       2       8       33        2      active sync set-A   /dev/sdc1
       3       8       49        3      active sync set-B   /dev/sdd1

update-initramfs: Generating /boot/initrd.img-4.15.0-47-generic
cryptsetup: WARNING: failed to detect canonical device of /dev/md3
cryptsetup: WARNING: could not determine root device from /etc/fstab
W: Possible missing firmware /lib/firmware/ast_dp501_fw.bin for module ast
I: The initramfs will attempt to resume from /dev/nvme1n1p4
I: (UUID=0e1f0b39-1504-44b9-8e2f-5026d566a325)
I: Set the RESUME variable to override this.