Configuring RAID Arrays in Linux: A Comprehensive Guide

In the world of Linux, ensuring data redundancy and improving performance can often be achieved through the use of RAID (Redundant Array of Independent Disks) configurations. RAID allows you to manage multiple hard drives, improving their fault tolerance and read/write speeds. In this guide, we'll discuss how to configure RAID arrays in Linux, covering the different types of RAID levels and providing step-by-step instructions for setting up RAID using MDADM, a widely used tool in the Linux ecosystem.

Understanding RAID Levels

Before setting up RAID, it's important to understand the different RAID levels:

RAID 0 (Striping): Splits data across multiple disks, offering increased performance but no redundancy.
RAID 1 (Mirroring): Duplicates the same data on two or more disks; this is ideal for redundancy.
RAID 5 (Striped with parity): Data and parity (used for recovery operations) are distributed across three or more disks. It offers a good balance between performance and data security.
RAID 6 (Striped with double parity): Similar to RAID 5, but with extra parity block allowing survival of two disks failures.
RAID 10 (Combination of RAID 1 and RAID 0): Provides mirroring and striping which delivers high performance and redundancy.

Prerequisites

Before configuring RAID in Linux, you need to install the necessary software, which can be done using the package manager specific to your distribution:

Debian/Ubuntu (apt):
```
sudo apt update
sudo apt install mdadm
```
Fedora (dnf):
```
sudo dnf install mdadm
```
OpenSUSE (zypper):
```
sudo zypper install mdadm
```

Ensure you have at least two unused hard drives or partitions available for setting up RAID.

Setting Up RAID

Step 1: Create the RAID Array

Creating a RAID array involves combining multiple disks into one logical unit based on the desired RAID level. Here’s how to create various RAID levels using mdadm.

Creating RAID 0:

sudo mdadm --create --verbose /dev/md0 --level=0 --raid-devices=2 /dev/sda1 /dev/sdb1

Creating RAID 1:

sudo mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sda1 /dev/sdb1

Creating RAID 5:

sudo mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sda1 /dev/sdb1 /dev/sdc1

Creating RAID 10:

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

Step 2: Configure mdadm.conf

To ensure that your RAID configuration is preserved across reboots, update the mdadm.conf file:

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

Update the initramfs file for the changes to take effect:

sudo update-initramfs -u

Step 3: Creating Filesystems and Mounting

After creating the RAID device, you need to format it and then mount it:

Format the RAID device (assuming RAID 1 from earlier):
```
sudo mkfs.ext4 /dev/md0
```
Mount the RAID device:
```
sudo mount /dev/md0 /mnt
```

Add the following entry to /etc/fstab to mount at boot:

/dev/md0    /mnt   ext4    defaults    0    0

Maintaining RAID Arrays

Regularly check the status of the RAID array with:

cat /proc/mdstat

sudo mdadm --detail /dev/md0

Replace failed disks promptly. Suppose a disk in RAID 1 (/dev/sdb1) needs replacement, you can remove it and add a new disk using:

sudo mdadm /dev/md0 --fail /dev/sdb1
sudo mdadm /dev/md0 --remove /dev/sdb1
sudo mdadm /dev/md0 --add /dev/sdc1

Conclusion

Configuring RAID in Linux isn't overly complex but requires careful planning around the choice of RAID level and diligent maintenance after setup, including regular monitoring and prompt replacement of failed disks. With the right setup, RAID can greatly improve the reliability and performance of your storage infrastructure.