mdadm Command in Linux

Introduction to Software RAID with mdadm on Linux

In this practical guide, you'll discover how to leverage the Linux mdadm utility for creating, managing, and monitoring software RAID arrays. Software RAID offers a budget-friendly and adaptable alternative to hardware RAID, enabling you to configure diverse RAID levels without requiring specialized hardware. First, we'll demystify the concept of software RAID and its distinctions from hardware RAID. Subsequently, you'll construct a RAID 5 array utilizing the mdadm command alongside virtual block devices. Finally, you'll acquire the skills to effectively manage and monitor your software RAID array for optimal performance and data protection.

Understanding Software RAID Concepts

This section delves into the core principles of software RAID (Redundant Array of Independent Disks) and highlights its differences from hardware RAID solutions. Software RAID is an operating system feature that facilitates the creation and management of RAID arrays through software, eliminating the need for dedicated RAID controller cards.

The primary advantage of software RAID lies in its cost-effectiveness and flexibility compared to hardware RAID. It can be implemented on virtually any system equipped with multiple storage devices, thus doing away with specialized RAID hardware. However, it's worth noting that software RAID might exhibit slightly lower performance than hardware RAID, as the system's CPU handles the RAID operations.

Several RAID levels can be configured using software RAID, including RAID 0 (striping), RAID 1 (mirroring), RAID 5 (striping with distributed parity), and RAID 6 (striping with dual distributed parity). Each RAID level presents different trade-offs relating to storage capacity, performance, and data redundancy, allowing systemadmins to select the option best suited for their needs.

The following section guides you through the creation of a software RAID array using the mdadm command within a Linux environment.

Creating a Software RAID Array Using mdadm: A Step-by-Step Guide

This section provides a detailed walkthrough on creating a software RAID array on Linux utilizing the mdadm command. Follow each step carefully to successfully build your RAID array.

To begin, let's create four virtual block devices that will serve as the underlying storage for our RAID array:

sudo dd if=/dev/zero of=~/project/disk1.img bs=1M count=100
sudo dd if=/dev/zero of=~/project/disk2.img bs=1M count=100
sudo dd if=/dev/zero of=~/project/disk3.img bs=1M count=100
sudo dd if=/dev/zero of=~/project/disk4.img bs=1M count=100

Example output:

100+0 records in
100+0 records out
104857600 bytes (105 MB, 100 MiB) copied, 0.0730474 s, 1.4 GB/s

Now, proceed to create a RAID 5 array using these four virtual disks:

sudo mdadm --create /dev/md0 --level=5 --raid-devices=4 ~/project/disk1.img ~/project/disk2.img ~/project/disk3.img ~/project/disk4.img

Example output:

mdadm: chunk size defaults to 512K
mdadm: array /dev/md0 started.

The mdadm command will then create a RAID array named /dev/md0 configured to raid level 5 which uses 4 virtual disks.

Next, verify the status of the RAID array:

sudo mdadm --detail /dev/md0

Example output:

/dev/md0:
           Version : 1.2
     Creation Time : Tue Apr 25 15:25:35 2023
        Raid Level : raid5
        Array Size : 307200 (300.00 MiB 314.43 MB)
     Used Dev Size : 100000 (97.66 MiB 102.40 MB)
      Raid Devices : 4
     Total Devices : 4
       Persistence : Superblock is persistent

       Update Time : Tue Apr 25 15:25:35 2023
             State : clean
    Active Devices : 4
   Working Devices : 4
    Failed Devices : 0
     Spare Devices : 0

            Layout : left-symmetric
        Chunk Size : 512K

Consistency Policy : resync

              Name : localhost.localdomain:0  (local to host localhost.localdomain)
              UUID : 6d2d9c1c:b4d2f9c4:8d6f0e8f:e0b0d4d5
            Events : 0

    Number   Major   Minor   RaidDevice State
       0       0        0        0      active sync   /home/labex/project/disk1.img
       1       0        1        1      active sync   /home/labex/project/disk2.img
       2       0        2        2      active sync   /home/labex/project/disk3.img
       3       0        3        3      active sync   /home/labex/project/disk4.img

This output confirms that the RAID 5 array has been successfully created and is in a clean, active state, ready for use.

The next section shows how to manage and monitor the newly created software RAID array.

Managing and Monitoring Your Software RAID Array: Essential Tasks

This final step focuses on equipping you with the knowledge and commands necessary to effectively manage and monitor the software RAID array created in the previous step. Proper management ensures data integrity and optimal performance.

Let's start by adding a new disk to the existing RAID 5 array:

sudo dd if=/dev/zero of=~/project/disk5.img bs=1M count=100
sudo mdadm /dev/md0 --add ~/project/disk5.img

Example output:

mdadm: added /home/labex/project/disk5.img

Recheck the RAID array status:

sudo mdadm --detail /dev/md0

Example output:

/dev/md0:
           Version : 1.2
     Creation Time : Tue Apr 25 15:25:35 2023
        Raid Level : raid5
        Array Size : 407200 (397.66 MiB 417.23 MB)
     Used Dev Size : 100000 (97.66 MiB 102.40 MB)
      Raid Devices : 5
     Total Devices : 5
       Persistence : Superblock is persistent

       Update Time : Tue Apr 25 15:26:16 2023
             State : clean, degraded, recovering
    Active Devices : 4
   Working Devices : 5
    Failed Devices : 0
     Spare Devices : 1

            Layout : left-symmetric
        Chunk Size : 512K

Consistency Policy : resync

              Name : localhost.localdomain:0  (local to host localhost.localdomain)
              UUID : 6d2d9c1c:b4d2f9c4:8d6f0e8f:e0b0d4d5
            Events : 6

    Number   Major   Minor   RaidDevice State
       0       0        0        0      active sync   /home/labex/project/disk1.img
       1       0        1        1      active sync   /home/labex/project/disk2.img
       2       0        2        2      active sync   /home/labex/project/disk3.img
       3       0        3        3      active sync   /home/labex/project/disk4.img
       4       0        4        -      spare   /home/labex/project/disk5.img

This output shows that the newly added disk, /home/labex/project/disk5.img, is configured as a spare device within the RAID 5 array. In the event of a failure, the spare disk automatically begins to rebuild the array.

Next, let's simulate a disk failure by intentionally removing one of the devices from the RAID array. This step demonstrates the array's resilience and automated recovery process:

sudo mdadm /dev/md0 --fail /home/labex/project/disk1.img
sudo mdadm /dev/md0 --remove /home/labex/project/disk1.img

Example output:

mdadm: set /home/labex/project/disk1.img faulty in /dev/md0
mdadm: hot removed /home/labex/project/disk1.img from /dev/md0

Now, check the status of the RAID array to reflect the removal of the disk:

sudo mdadm --detail /dev/md0

Example output:

/dev/md0:
           Version : 1.2
     Creation Time : Tue Apr 25 15:25:35 2023
        Raid Level : raid5
        Array Size : 407200 (397.66 MiB 417.23 MB)
     Used Dev Size : 100000 (97.66 MiB 102.40 MB)
      Raid Devices : 5
     Total Devices : 4
       Persistence : Superblock is persistent

       Update Time : Tue Apr 25 15:26:56 2023
             State : clean, degraded
    Active Devices : 4
   Working Devices : 4
    Failed Devices : 1
     Spare Devices : 1

            Layout : left-symmetric
        Chunk Size : 512K

Consistency Policy : resync

              Name : localhost.localdomain:0  (local to host localhost.localdomain)
              UUID : 6d2d9c1c:b4d2f9c4:8d6f0e8f:e0b0d4d5
            Events : 8

    Number   Major   Minor   RaidDevice State
       1       0        1        1      active sync   /home/labex/project/disk2.img
       2       0        2        2      active sync   /home/labex/project/disk3.img
       3       0        3        3      active sync   /home/labex/project/disk4.img
       4       0        4        4      spare   /home/labex/project/disk5.img
       0       0        0        -      removed

The output confirms that the RAID 5 array now has a degraded state. One disk has failed, and a spare disk is now rebuilding the array.

By following this section, systemadmin users will have successfully managed and monitored a software RAID array using the mdadm command, including adding a new disk, simulating a disk failure, and observing the array's state.

Summary of Software RAID Implementation

This lab has explored the concept of software RAID, which enables the creation and management of RAID arrays using software rather than dedicated hardware. We discussed the advantages of software RAID, such as cost-effectiveness and adaptability, and then explored different RAID levels. You then learned how to create a software RAID array using the mdadm command in Linux by creating four virtual block devices and set up a RAID 5 array to get data redundancy and improved performance. With this knowledge, any system admin can leverage the power of software RAID to create reliable and affordable storage solutions.

400+ Linux Commands