Creating Software RAID Devices using mdadm

Run the mdadm command to create the MD RAID device as follows:
```
sudo mdadm --create md_device --level=RAID_level [options] --raid-devices=Ndevices
```
md_device

Name of the RAID device, for example, /dev/md0.

RAID_level

Level number of the RAID to create, for example, 5 for a RAID-5 configuration.

--raid-devices=N

Number of devices to become part of the RAID configuration.

devices

Devices to be configured as RAID, for example, /dev/sd[bcd] for 3 devices for the RAID configuration.

The devices you list must total to the number you specified for --raid-devices.

This example creates a RAID-5 device /dev/md1 from /dev/sdb, /dev/sdc, and dev/sdd:
```
sudo mdadm --create /dev/md1 --level=5 --raid-devices=3 /dev/sd[bcd]
```
The previous example creates a RAID-5 device /dev/md1 out of 3 devices. We can use 4 devices where one device is configured as a spare for expansion, reconfiguration, or replacement of failed drives:
```
sudo mdadm --create /dev/md1 --level=5 -raid-devices=3 --spare-devices=1 /dev/sd[bcde]
```
(Optional) Add the RAID configuration to /etc/mdadm.conf:
```
sudo mdadm --examine --scan | sudo tee -a /etc/mdadm.conf
```
Based on the configuration file, mdadm assembles the arrays at boot time.

For example, the following entries define the devices and arrays that correspond to /dev/md0 and /dev/md1:
```
DEVICE /dev/sd[c-g]
ARRAY /dev/md0 devices=/dev/sdf,/dev/sdg
ARRAY /dev/md1 spares=1 devices=/dev/sdb,/dev/sdc,/dev/sdd,/dev/sde
```
For more examples, see the sample configuration file /usr/share/doc/mdadm/mdadm.conf-example.

An MD RAID device is used in the same way as any physical storage device. For example, the RAID device can be configured as an LVM physical volume, a file system, a swap partition, an Automatic Storage Management (ASM) disk, or a raw device.

To check the status of the MD RAID devices, view /proc/mdstat:

cat /proc/mdstat

Personalities : [raid1]
mdo : active raid1 sdg[1] sdf[0]

To display a summary or detailed information about MD RAID devices, use the --query or --detail option, respectively, with mdadm.

For more information, see the md(4), mdadm(8), and mdadm.conf(5) manual pages.