How to Configure a RAID HDD Array in Linux
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Got spare hard disk drives that you want to use more efficiently with your Linux computer? RAID can provide a performance boost, or add redundancy, depending on how it’s configured. Let’s take a quick dive into the multi-disk world.

RAID 101

A Redundant Array of Inexpensive (or Independent) Disks (RAID) is a collection of drives working cohesively to provide benefit to a system. These benefits can either be performance, redundancy or both. The usual configurations you will come across are RAID 0, RAID 1, RAID 5, RAID 6, and RAID 10. We’ve summarized them below.

configure raid hdd array linux

Other configurations exist, but these are the most common.

Regardless of which RAID level you would like to use, RAID is not a backup solution.

While it may help you get back up and running quickly and provide another layer to protect your data, it does not replace actual backups. RAID is a great use case where high availability is a must. Our guide to RAID What Is RAID Storage & Can I Use It On My Home PC? [Technology Explained] What Is RAID Storage & Can I Use It On My Home PC? [Technology Explained] RAID is an acronym for Redundant Array of Independent Disks, and it’s a core feature of server hardware that ensures data integrity. It’s also just a fancy word for two or more hard disks connected... Read More explains further.

RAID 0: Non-Critical Storage

configure raid hdd array linux

RAID 0 works by striping data across multiple drives. You need a minimum of two drives for RAID 0, but you can theoretically add as many as you like. Because your computer is writing across multiple drives simultaneously, this provides a performance boost.

You can also use drives of differing sizes. Your array will, however, be limited to the smallest drive in the array. If you have a drive that is 100GB and a drive that is 250GB striped in a RAID 0 array, the total space for the array will be 200GB. That’s 100GB from each disk.

RAID 0 is excellent for non-critical storage that requires higher read and write speeds that a single disk can’t offer. RAID 0 is not fault-tolerant.

If any of the drives in your array fails, you will lose all the data within that array. You have been warned.

RAID 1: Mirror Your HDD

configure raid hdd array linux

RAID 1 is a simple mirror. Whatever happens on one drive will happen on the other drives. While there will be no performance benefit from RAID 1, there is an exact replica of your data on each drive, which means there is a redundancy benefit with RAID 1. As long as one drive in your array is alive, your data will be intact.

The maximum size of your array will be equal to the size of the smallest drive in the array. If you have a drive that is 100GB and a drive that is 250GB in a RAID 1 array, the total space for the array will be 100GB. This cost implication just needs to be kept in mind.

RAID 5 and 6: Performance and Redundancy

configure raid hdd array linux

RAID 5 and 6 will provide both performance and redundancy. Data is striped across the drives along with parity information. RAID 5 uses a total of one drive’s worth of parity with RAID 6 using two. Using the parity data, the computer can recalculate the data of one of the other data blocks, should the data no longer be available. This means RAID 5 can suffer the loss of a single drive while RAID 6 can survive two drives failing at any single point in time.

configure raid hdd array linux

Storage wise this means RAID 5 and 6 will equal total drive size minus one drive and two drives respectively. So if you had four drives each with a capacity of 100GB your array size in RAID 5 will be 300GB, while RAID 6 gives you 200GB.

RAID 5 needs a minimum of three drives and RAID 6 requires four. While you can mix and match hard drive sizes, the array will see all disks as the size of the smallest drive in the array. In the unfortunate event that a drive fails, your array will still be operational, and you will be able to access all the data. At this point, you will need to swap out the dead drive and rebuild the array.

In its degraded state, the array will operate slower than usual, and it’s not a good idea to be using it until the array has been rebuilt.

RAID 10: Striped and Mirrored

configure raid hdd array linux

RAID 10 is basically RAID 1 + 0. It is a combination of these levels. You will need pairs of disks for this to be achieved. Data is striped across two disks, and that is then mirrored on another set of disks. You get the performance benefit from RAID 0, and the redundancy of RAID 1.

Configuring RAID in Linux

Configuring all of this redundant goodness can be done on either hardware or software levels. The hardware flavor requires a RAID controller which is generally found in server grade hardware. Fortunately, Linux has a software version of RAID. The principles are the same, but bear in mind the overhead will be on your CPU as opposed to the RAID controller.

Let’s walk through a RAID 5 configuration using nothing but a terminal window, a few drives, and some determination. When ready, open a terminal window with your favorite shell Which of These 5 Linux Shells Is Best? Which of These 5 Linux Shells Is Best? Happy using the bash shell in Linux? Or would you prefer to try out an alternative? There's tsch, fish, KornShell and Z Shell to choose from. But which of these Linux shells is best? Read More , and type:

sudo apt install mdadm

Preparing the Drives

In our example, we’ll use three 1GB drives, for simplicity (in reality, these will be larger). Check which disks are attached to your system with these terminal commands 5 Networking Commands for the Linux Terminal 5 Networking Commands for the Linux Terminal You can get a lot done in the command line in Linux, including managing and troubleshooting network issues. Find out about the new ip command, wget, ping, and more. Read More :

sudo fdisk -l

configure raid hdd array linux

From the output, we can see sda as the boot drive and sdb, sdd, and sdc just attached to the system.

Now we need to partition these disks. Make sure your cousin’s graduation photos are backed up and not on these drives because this is a destructive process. In a terminal, enter:

sudo fdisk /dev/sdb

configure raid hdd array linux

We then need to answer with the following inputs:

  1. n: Adds a new partition
  2. p: Makes the partition the primary one on the disk
  3. 1: Assigns this number to the partition
  4. t: To change the partition type
  5. fd: This is the RAID partition type
  6. w: Saves the changes and exits

Carry out the exact same steps for the remaining two drives. Namely /dev/sdc and /dev/sdd. We now need to inform our operating system of the changes we just made:

sudo partprobe /dev/sdb

Follow this with:

sudo partprobe /dev/sdc
sudo partprobe /dev/sdd

configure raid hdd array linux

Setting Up RAID 5

Let’s take a quick look at the partition table now. Again, run:

fdisk -l

configure raid hdd array linux

Awesome! Our drives and their partitions are ready to be RAID-ed!

To set them up in RAID 5 run:

mdadm -C /dev/md0 --level=raid5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1

Taking a closer look at the syntax:

  • mdadm: The tool we’re using
  • C: This is the switch to create a RAID array
  • /dev/md0: Where the array will be pooled
  • level: Desired RAID level
  • raid-device: the number of devices and their locations

We can view the details of our RAID by typing:

sudo mdadm --detail /dev/md0

configure raid hdd array linux

The final steps will be to create a file system for the array, and mounting it so we can actually use it! To format the array and assign a place it can be accessed, type:

sudo mkfs.ext4 /dev/md0
sudo mkdir /data

Mounting the Array

There are two options for mounting the newly created array. The first is temporarily, which will require it to be mounted each time the computer is started. Or you could mount it permanently so that it is mounted with each restart. To mount temporarily type:

mount /dev/md0 /data/

If you prefer to persist the storage, you need to edit your /etc/fstab file and make sure you add in the line like the picture below:

sudo nano /etc/fstab

configure raid hdd array linux

Once you’ve saved and closed the file, refresh the mounting table:

sudo mount -a

We can then view our mounted devices by typing:

df -h

configure raid hdd array linux

Congratulations! You’ve successfully created a RAID array, formatted it and mounted it. You may now use that directory as you would any other, and reap the benefits!

Troubleshooting RAID

Remember the redundancy benefits we spoke about? Well, what happens if a drive fails? Using mdadm, you can remove the failed drive with the mdadm -r switch. Hopefully, your motherboard supports hot-swapping of drives and you can plug a replacement drive in.

Following the fdisk command above, you can set up the new drive. Simply add the new drive to the array using the mdadm -a switch. Your array will now start rebuilding. Because this is RAID 5 all your data should be there, and even available while the drive was unavailable.

Do You Need RAID?

The table above lists some possible use cases where RAID might be of benefit to you. If you have a business need that is driving this requirement, it may be worth looking at hardware RAID controllers or options like FreeNAS 10 Reasons Why You Should Store Your Data on a FreeNAS Box 10 Reasons Why You Should Store Your Data on a FreeNAS Box Which company should you entrust your sensitive data to? Want to access your media across all your devices anywhere in the world? Here's why FreeNAS might be the right pick for you. Read More to better suit your need.

If you are looking for a cost-effective way to squeeze some extra performance or provide another layer of redundancy for home, mdadm may be a worthy candidate.

Do you currently use RAID? How often do you go through hard drives? Do you have a data-loss horror story?

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  1. Kannon Y
    December 9, 2017 at 9:15 pm

    Great article Yusuf! Thanks for writing this one!

    • Yusuf Limalia
      December 10, 2017 at 1:04 pm

      Thanks Kannon! Very kind of you =)