Practice Using mdadm With Virtual Block Devices

mdadm is the tried and trusted software RAID tool for Linux. You might think that getting some practice with mdadm is difficult because you need several storage devices.

That’s not true as you can simple create some virtual devices to practice on.

Here’s how.

Create the virtual block devices

Use dd to create three 32MB files:

dd if=/dev/zero of=disk1 bs=1M count=32
dd if=/dev/zero of=disk2 bs=1M count=32
dd if=/dev/zero of=disk3 bs=1M count=32

Next, map them to loopback block devices using losetup:

losetup --show -f disk0
losetup --show -f disk1
losetup --show -f disk2

There are now three virtual block devices at:


These can be treated like any other block storage device e.g. given a file system and added into a RAID array.

You can remove the devices when you’re done using the following command for each device:

losetup -d /dev/loop0

Create a RAID array

Use the mdadm command to create a RAID 5 array using the the /dev/loop devices:

# mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/loop0 /dev/loop1 /dev/loop2
mdadm: layout defaults to left-symmetric
mdadm: layout defaults to left-symmetric
mdadm: chunk size defaults to 512K
mdadm: size set to 30720K
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.

The options here mean as follows:

You have a fully functional RAID array!

You can now put a file system onto /dev/md0 as you would any other device e.g.:

mkfs.ext4 /dev/md0

Get details about the array

Now that you have an array it is very useful to find information about it. This will work for any system with any RAID array on it.

Inspect the contents of /proc/mdstat

The system keeps a file at /proc/mdstat that retains information about mdadm arrays. Use cat to view its contents e.g.:

# cat /proc/mdstat 
Personalities : [raid6] [raid5] [raid4] 
md0 : active raid5 loop2[3] loop1[1] loop0[0]
      61440 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU]

This will tell you the current status of the RAID device and the block devices that it using.


The lsblk command (list block devices) will show you useful information about the block devices on your system including the RAID device e.g.:

# lsblk -fs
NAME    FSTYPE            LABEL    UUID                                 FSAVAIL FSUSE% MOUNTPOINT
md0     ext4                       31f0183b-633c-4c65-9f61-d72b070d0851
├─loop0 linux_raid_member TEST:0   10224bb8-4f1a-7621-d5f9-8dc2c95693ad
├─loop1 linux_raid_member TEST:0   10224bb8-4f1a-7621-d5f9-8dc2c95693ad
└─loop2 linux_raid_member TEST:0   10224bb8-4f1a-7621-d5f9-8dc2c95693ad


The mdadm command will also print information about the array when it is passed the --detail option:

# mdadm --detail /dev/md0
           Version : 1.2
     Creation Time : Mon Sep  6 05:44:38 2021
        Raid Level : raid5
        Array Size : 61440 (60.00 MiB 62.91 MB)
     Used Dev Size : 30720 (30.00 MiB 31.46 MB)
      Raid Devices : 3
     Total Devices : 3
       Persistence : Superblock is persistent

       Update Time : Mon Sep  6 05:49:02 2021
             State : clean
    Active Devices : 3
   Working Devices : 3
    Failed Devices : 0
     Spare Devices : 0

            Layout : left-symmetric
        Chunk Size : 512K

Consistency Policy : resync

              Name : TEST:0  (local to host TEST)
              UUID : 10224bb8:4f1a7621:d5f98dc2:c95693ad
            Events : 19

    Number   Major   Minor   RaidDevice State
       0       7        0        0      active sync   /dev/loop0
       1       7        1        1      active sync   /dev/loop1
       3       7        2        2      active sync   /dev/loop2