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:
/dev/loop0
/dev/loop1
/dev/loop2
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:
--create- Create a new RAID array.--verbose- Print information about whatmdadmdoes./dev/md0- The name and location of the array.--level=5- Create a RAID 5 array.--raid-devices=3- The number of devices in the array./dev/loop0 /dev/loop1 /dev/loop2- The name and path of the devices.
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.
lsblk
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
mdadm
The mdadm command will also print information about the array when it is passed the --detail option:
# mdadm --detail /dev/md0
/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