How To Raid In Linux With mdadm?

Hi, today I am gonna show you how to make raid ( redundant array of inexpensive disks ). Why wee need it? Think about what you have customer data stored in a server. Say one day disk is dead and then all of your data about the customer are gone. This is a very bad new.  What if disk dies but the data remains the same. This will be very good. You can think that will disk dies frequently. No, but it is not impossible, especially in the enterprise side or in personal use at home. RAID is a technology to backup disks with extra disks. Say we have 3 disks which are used separate projects. And we want to backup them with high availability in particular working hours. RAID can be done with separate hardware, firmware or software. Making raid with hardware is the best, fast and reliable way but it may be expensive or too much for the project. Firmware raid is generally provided by motherboard manufacturers as a cheap option for hardware raid. Software raid is the cheapest and least reliable way to mail raid. But it can be suitable for home or unprofessional usage. I will not delve into details of raid levels. But you can find more about in wikipedia. In Linux software raid consist of a kernel module and user-space programs.

List Disk Drives

3 disks are used actively to store data but 4. disk the or spare disk is used to backup for 3 disks. It doesn’t store all data of the 3 disks as it is impossible if 3 disks are near to full.

First list disks. We will make an array with vdb,vdc,vdd and vde disks. I make this in my VM so your disk names are maybe different like sda, sdb etc. All disk has 1 GB.

$ ls /dev/vd*
/dev/vda  /dev/vda1  /dev/vdb  /dev/vdc  /dev/vdd  /dev/vde

Install mdadm

Install mdadm in Ubuntu, Debian, Mint, Kali with the following apt command.

$ sudo apt install mdadm
Install mdadm For Ubuntu, Debian, Mint, Kali
Install mdadm For Ubuntu, Debian, Mint, Kali

Load mdadm Kernel Module

mdadm provides services via low-level drivers. It has a kernel module named raid456 and can be loaded with the following command. In order to load a Linux kernel module, we need root privileges which can be provided with the sudo command like below.

$ sudo modprobe raid456

We can check if the raid456 kernel module is loaded properly with the lsmod command which will list all currently loaded kernel modules.

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Load mdadm Kernel Module
Load mdadm Kernel Module

Create RAID

We should give the count of the disk with raid-devices level with level and the disk to be used in the raid. Here metadata is optional it sets raid metadata version After the command is executed we got a message that says our new disk which we named md0 is created. If you want to create raid0 use level=stripe and raid1 level=mirror

$ mdadm --create /dev/md0 --metadata 1.2 --level=4 --raid-devices=4 /dev/vdb /dev/vdc /dev/vdd /dev/vde
mdadm: array /dev/md0 started.

Print mdadm Information

To see raid devices status we will print the /proc/mdstat file like below. Shows total disk count and total usable size which is 3/4 of the total disk size. As you can see the version is 1.2 where we set it while creating the array

$ cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid4 vde[4] vdd[2] vdc[1] vdb[0]
3142656 blocks super 1.2 level 4, 512k chunk, algorithm 0 [4/4] [UUUU]

unused devices: <none>

$ mdadm -D /dev/md0
Version : 1.2
Creation Time : Mon Jul 21 13:48:15 2014
Raid Level : raid4
Array Size : 3142656 (3.00 GiB 3.22 GB)
Used Dev Size : 1047552 (1023.17 MiB 1072.69 MB)
Raid Devices : 4
Total Devices : 4
Persistence : Superblock is persistent

Update Time : Mon Jul 21 13:48:28 2014
State : clean
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0

Chunk Size : 512K

Name : ubuntu:0  (local to host ubuntu)
UUID : 515af24c:86bb12f7:015dfebf:6bd9b35b
Events : 18

Number   Major   Minor   RaidDevice State
0     253       16        0      active sync   /dev/vdb
1     253       32        1      active sync   /dev/vdc
2     253       48        2      active sync   /dev/vdd
4     253       64        3      active sync   /dev/vde

Save/Backup mdadm Configuration

In order to save or backup mdadm with the --detail and --scan options like below.

$ mdadm --detail --scan >> /etc/mdadm/mdadm.conf
$ cat /etc/mdadm/mdadm.conf
# mdadm.conf
# Please refer to mdadm.conf(5) for information about this file.

# by default (built-in), scan all partitions (/proc/partitions) and all
# containers for MD superblocks. alternatively, specify devices to scan, using
# wildcards if desired.
#DEVICE partitions containers

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST <system>

# instruct the monitoring daemon where to send mail alerts

# definitions of existing MD arrays

# This file was auto-generated on Mon, 21 Jul 2014 13:43:42 +0300
# by mkconf $Id$
ARRAY /dev/md/ubuntu:0 metadata=1.2 name=ubuntu:0 UUID=515af24c:86bb12f7:015dfebf:6bd9b35b

List Disk Status and Array

We can list disk and array status with the -E option by providing the disk name.

$ mdadm -E /dev/vdd
Magic : a92b4efc
Version : 1.2
Feature Map : 0x0
Array UUID : 515af24c:86bb12f7:015dfebf:6bd9b35b
Name : ubuntu:0  (local to host ubuntu)
Creation Time : Mon Jul 21 13:48:15 2014
Raid Level : raid4
Raid Devices : 4

Avail Dev Size : 2096128 (1023.67 MiB 1073.22 MB)
Array Size : 3142656 (3.00 GiB 3.22 GB)
Used Dev Size : 2095104 (1023.17 MiB 1072.69 MB)
Data Offset : 1024 sectors
Super Offset : 8 sectors
State : clean
Device UUID : e5cc5bd9:decd247a:16c39662:abeb38bb

Update Time : Mon Jul 21 14:07:48 2014
Checksum : c07c9d94 - correct
Events : 18

Chunk Size : 512K

Device Role : Active device 2
Array State : AAAA ('A' == active, '.' == missing)

Create File System For New Disk

We will use mkfs.ext4 command in order to create a file system to the /dev/md0 disk drive.

$ mkfs.ext4 /dev/md0
mke2fs 1.42.9 (4-Feb-2014)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=128 blocks, Stripe width=384 blocks
196608 inodes, 785664 blocks
39283 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=805306368
24 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

Mount Disk

Mount the disk and create some files. Also, show the status of mounted disks.

$ mount /dev/md0 /mnt
$ mkdir /mnt/ismail
$ ls /mnt/
ismail  lost+found
$ df -lh
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1        18G  1.7G   16G  10% /
none            4.0K     0  4.0K   0% /sys/fs/cgroup
udev            235M  4.0K  235M   1% /dev
tmpfs            49M  268K   49M   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            245M     0  245M   0% /run/shm
none            100M     0  100M   0% /run/user
/dev/md0        2.9G  4.6M  2.8G   1% /mnt

Remove Disk From Array

Say disk vdd is corrupted. And we need to remove it.

$ mdadm --remove /dev/md0 /dev/vde

Add Disk To Disk Array

We will use --add option disk vdf to the mdm drive md0 .

$ mdadm --add /dev/md0 /dev/vdf

Restart or Initialize New Disk

To use raid after the restart. Raid devices must be reassembled every time to use.

mdadm --assemble /dev/md0 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
More easy way to reassembly
mdadm --assemble --scan

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