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Linux RAID (Redundant Array of Independent Disks)

RAID is a method of combining multiple physical disks into a single logical unit for the purposes of redundancy, performance, or both.

It can help protect against hardware failure or improve read/write speeds depending on how it's configured.

Linux supports RAID through the mdadm utility, which allows you to create, manage, and monitor software RAID arrays.

Table of Contents

RAID Levels

RAID has 5 common levels:

  • RAID 0 - Striping
  • RAID 1 - Mirroring
  • RAID 5 - Striping with Parity
  • RAID 6 - Striping with Double Parity
  • RAID 10 (1 + 0) - Striping and Mirroring

Raid 6 with 10 disks in a 8/2 configuration is the most common.

RAID 0 (Striping)

RAID 0 combines multiple disks for better performance by splitting (striping) data across them. However, this that there is no redundancy.

RAID 1 (Mirroring)

RAID 1 duplicates data across multiple disks, which offers redundancy.
If one disk fails, the data is still available on the other.

RAID 5 (Striping with Parity)

With RAID 5, data is striped across multiple disks, and parity information is also stored. This offers both performance and redundancy, but requires at least 3 disks.

RAID 6 (Striping with Double Parity)

Like RAID 5, but with two sets of parity information instead of 1 set.
This allows for the failure of two disks without data loss.
Requires at least 4 disks.

RAID 10 (1+0)

A combination of RAID 1 and RAID 0.
This mirrors data (RAID 1) and then stripes it (RAID 0) for both performance and redundancy.
This also requires at least 4 disks.

mdadm - The Utility for Managing Linux RAID

mdadm (multiple disk admin) is the primary tool used to manage software RAID arrays.

It can be used to create, assemble, and monitore RAID ararys.

Creating a RAID Array

To create a RAID array, the general command syntax is

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

  • --create: Specifies that you're creating a new RAID array.
  • /dev/md0: The name of the new RAID device. It will live under /dev/.
  • --level=1: Specifies the RAID level. In this instance, RAID 1 (mirroring).
  • --raid-devices=2: Specifies that the RAID array will consist of 2 devices.
  • /dev/sdb /dev/sdc: The physical disks that will be used for the RAID array.

Example: Creating a RAID 0 (Striping) Array

For performance, a RAID 0 array can be used.

sudo mdadm --create /dev/md0 --level=0 --raid-devices=2 /dev/sdb /dev/sdc
This creates a RAID 0 array called /dev/md0, striping data across /dev/sdb and /dev/sdc.

Example: Creating a RAID 5 (Striping with Parity) Array

sudo mdadm --create --raid-level=5 --raid-devices=3 /dev/md0 /dev/sdb /dev/sdc /dev/sdd
This creates a RAID 5 array called /dev/md0, using the disks /dev/sdb, /dev/sdb, and /dev/sdd.

Verifying the RAID Array

Once the RAID array is c reated, you can verify its status with mdadm: 

sudo mdadm --detail /dev/md0
--detail: This prints detailed information about the RAID array.

How Striping/RAID 0 Improves Performance

Striping improves performance by utilizing multiple disks at a time.
This means, with 2 disks that have the same speeds, it will essentially double the
amount of data throughput.

  • With striping, data is broken into small chunks called "strip units".
  • These strip units usually have a specific size (e.g., 64KB, 128KB).
  • Each strip unit is then written to a different disk.
  • This allows the system to read or write multiple strip units at one time.
  • This is called "parallel data access".

This setup provides scalability.

If a single disk can transfer data at 100 MB/s, a RAID 0 array with two disks could, in
theory, provide up to 200 MB/s.
With four disks, it could scale up to 400 MB/s, assuming no bottlenecks exist elsewhere (like the disk controller or the interface).

Removing a RAID Array

You can remove a RAID array to convert devices given to RAID back to normal ones:

  • Stop the RAID device and remove it: 

    sudo mdadm --stop /dev/md0    # Stop the device
sudo mdadm --remove /dev/md0  # Remove the device

  • Zero out the "superblocks". 

    # Specify whichever disks need to be cleared
sudo mdadm --zero-superblock /dev/sdb /dev/sdc

    • This is to clear the metadata from the individual devices that were part of the RAID array.
    • This make sure there are no labels that will confuse the OS into thinking it's still a RAID device.
    • You can also do this with dd if you want.
      sudo dd if=/dev/zero of=/dev/sdb bs=1024K count=10

Optionally, check /etc/mdadm/mdadm.conf and remove the entry.

  • Verify that the RAID device was removed: 
    cat /proc/mdstat