How do I create Software RAID 1 arrays on Linux systems without using GUI tools or installer options? How do I setup RAID 1 array under Linux systems?
You need to install mdadm which is used to create, manage, and monitor Linux software MD (RAID) devices. RAID devices are virtual devices created from two or more real block devices. This allows multiple devices (typically disk drives or partitions) to be combined into a single device to hold (for example) a single filesystem. Some RAID levels include redundancy and can survive some degree of device failure.
Linux Support For Software RAID
Currently, Linux supports the following RAID levels (quoting from the man page):
- LINEAR
- RAID0 (striping)
- RAID1 (mirroring)
- RAID4
- RAID5
- RAID6
- RAID10
- MULTIPATH, and FAULTY.
MULTIPATH is not a Software RAID mechanism, but does involve multiple devices: each device is a path to one common physical storage device. FAULTY is also not true RAID, and it only involves one device. It provides a layer over a true device that can be used to inject faults.
Install mdadm
Type the following command under RHEL / CentOS / Fedora Linux:
# yum install mdadm
Type the following command under Debian / Ubuntu Linux:
# apt-get update && apt-get install mdadm
How Do I Create RAID1 Using mdadm?
Type the following command to create RAID1 using /dev/sdc1 and /dev/sdd1 (20GB size each). First run fdisk on /dev/sdc and /dev/sdd with “Software Raid” type i.e. type 0xfd:
# fdisk /dev/sdc
# fdisk /dev/sdd
See fdisk(8) man page to setup partition type. Do not format partition. Just create the same. Now, create RAID-1 as follows.
If the device contains a valid md superblock, the block is overwritten with zeros:
# mdadm --zero-superblock /dev/sdc /dev/sdd
Create RAID1 using /dev/sdc1 and /dev/sdd1
# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdc1 /dev/sdd1
Format /dev/md0 as ext3:
# mkfs.ext3 /dev/md0
Mount /dev/md0
# mkdir /raid1
# mount /dev/md0 /raid1
# df -H
Edit /etc/fstab
Make sure RAID1 get mounted automatically. Edit /etc/fstab and append the following line:
/dev/md0 /raid1 ext3 noatime,rw 0 0
Save and close the file.
How Do I See RAID Array Building Progress and Current Status?
Type the following command:
# watch -n 2 cat /proc/mdstat
OR
# tail -f /proc/mdstat
Update /etc/mdadm.conf File
Update or edit /etc/mdadm/mdadm.conf or /etc/mdadm.conf (distro specific location) file as follows:
ARRAY /dev/md0 devices=/dev/sdc1,/dev/sdd1 level=1 num-devices=2 auto=yes
This config file lists which devices may be scanned to see if they contain MD super block, and gives identifying information (e.g. UUID) about known MD arrays. Please note that Linux kernel v2.6.xx above can use both /dev/mdX or /dev/md/XX names. You can also create partitions for /dev/md/XX as /dev/md/d1/p2.
How Do I Get Information On Existing Array?
Type the following command
# mdadm --query /dev/md0
This will find out if a given device is a raid array, or is part of one, and will provide brief information about the device.
References:
- See man pages: mdadm(8) and mdadm.conf(5)
- RAID 5 vs RAID 10: Recommended RAID For Safety and Performance
🐧 32 comments so far... add one ↓
| Category | List of Unix and Linux commands |
|---|---|
| File Management | cat |
| Firewall | Alpine Awall • CentOS 8 • OpenSUSE • RHEL 8 • Ubuntu 16.04 • Ubuntu 18.04 • Ubuntu 20.04 |
| Network Utilities | dig • host • ip • nmap |
| OpenVPN | CentOS 7 • CentOS 8 • Debian 10 • Debian 8/9 • Ubuntu 18.04 • Ubuntu 20.04 |
| Package Manager | apk • apt |
| Processes Management | bg • chroot • cron • disown • fg • jobs • killall • kill • pidof • pstree • pwdx • time |
| Searching | grep • whereis • which |
| User Information | groups • id • lastcomm • last • lid/libuser-lid • logname • members • users • whoami • who • w |
| WireGuard VPN | Alpine • CentOS 8 • Debian 10 • Firewall • Ubuntu 20.04 |
Thanks for the article.
Very cool! \o/
Thanks for this tutorial. It was exactly the information I searched for and a great help.
This is a nice article. Unfortunately it does not seem to give complete information. I have followed the steps, and have a /dev/md0 created, but I can’t format it. I get an error from mkfs.ext3 saying “Device size reported to be zero.” When I do mdadm –query /dev/md0 I get the following response “/dev/md0: is an md device which is not active”. Googling these errors has not produced anything helpful.
use
mke2fs -f /dev/md0
then use the following command to re size the md
resize2fs /dev/md0
Please tell me the procedure to recover the disk if some disaster happens .
I got the following error,
mdadm: Cannot open /dev/sda1: Device or resource busy
Can any one help me? I am New to Linux
In the rescue mode, while all drives dismounted, I’ve successfully mirrored my /, /home , and /usr, Using mdadm –create /dev/md0 –level=1 –raid-devices=2 /dev/sda1 /dev/sdb1.
But the system is unable to boot from md0( / ). I updated etc/fstabe before I proceed with array creation. Also made changes in grub.conf, but the system unable to boot successfully, during booting process it asks about file system check on md0, I used e2fsck command but there is no result.
two steps more, 1st is to create an initrd (initial ramdisk) with RAID1 module (/sbin/mkinitrd ./boot/grub/initrdname uname -r –preload raid1 –with=raid1 –fstab=/etc/fstab)
2nd step: resize2fs and fsck all md……
and it is bootable
thanks a lot…………………
Thank you very much for detailed tutorials. Your site is the best place for Linux beginners, average and even experts.
Hi, many thanks for this.
I got as far as creating my array using 2 HDDs (2TB each) already in the system. (fedora 14)
I create /dev/md1 mounted at /raid1. (md0 is the operating system array which I created when I set-up the system)
After I rebooted I could no longer mount/dev/md1 so I looked at “disk utility” and found the the raid array “degraded” and called /dev/md127 (which I CAN mount)
/cat /proc/mdstat : show that the array is only using one HDD: /dev/sdd[1]
First time this happened I started from the beginning but now that I’s happened again, I wonder how to repair the raid array.
Any help would be much appreciated.
Thanks
Hi,
Before rebooting your machine try to mount md1 into any mount-point, (don’t make changes into fstab file until you can successfully mount)
It may not mount because you have to resize md1 before you mount it.
First you have to add the second drive into the array then fsck and then resize..
After you done make changes into fstab file(if not done before)
Example:
mdadm –add /dev/md1 /dev/sdx (here sdx is the secondary drive, you need to add in the array because it is degraded)
after synched , first e2fsck /dev/md1
then resize2fs /dev/md1.
If there is any error accrued you need to do from beginning, if accorded kindly let me know.
Regards,
Owais Hyder
Sr.Technical Consultant
Synergy Computers (Pvt.) Ltd.
I was searching a way to make software RAID on my Redhat Linux Advanced Server 5.5 Machine and this article helps me a-lot. very warm thanx for the author.
A million thanks i was trying about 100 times everything went right except for the mdadm.conf file which i did not no what to do
and the /etc/fstab
Thanks for the article
again
Thank u very very much
Awesome, this doc definitely helps the failed partitioner on Fedora 16 installer (for software raid that you want unmounted during install). Thanks!!
In my experience the mdadm.conf array definition needs the level spelled out.
Not this:
ARRAY /dev/md0 devices=/dev/sdc1,/dev/sdd1 level=1 num-devices=2 auto=yes
But rather this:
ARRAY /dev/md0 devices=/dev/sdc1,/dev/sdd1 level=raid1 num-devices=2 auto=yes
This is on Debian. For me, the first variation results in the array definition not being recognized, and the array being auto-mounted as /dev/md128 instead of /dev/md0. Your mileage, as always, may vary.
I have server with
HardRAID : LSI MegaRAID 9271 6 Gbps FastPath
Hard drive : 2 x 300GB SSD
When i check with df -h
Is it RAID or not?
Why so much /dev/root s?
First /dev/root point to the root file system mounted at /. Rest of /dev/root are used by named server to secure and chroot the server. Use mount command to verify this. Use megactl or megaclisas command to get more info about your LSI raid device.
HTH
Thanks. So these 300GB X 2 SSDs in RAID1?
only 20gb for both disk if im not mistaken
Finally, i manage to do this, thanks for the info, i just skip the #mkdir /raid1 because i have to do another PV, VG and 7 LV on this disk…anywhere,thanks for sharing
Has anyone noticed any problems using mdadm with 4tb HDs?
hi
this is karthik ,here my problem is in my lapy bare material is windows ,and i installed vm in rhel6 ,i added two hard disk /dev/sda, /dev/sdb one partition from /dev/sda, two partitions from /dev/sdb i want create raid but i getting this errors plz tell me how to do ?
error :
mdadm: super1.x cannot open /dev/sdb6: No such file or directory
mdadm: /dev/sdb6 is not suitable for this array.
mdadm: super1.x cannot open /dev/sdb7: No such file or directory
mdadm: /dev/sdb7 is not suitable for this array.
mdadm: create aborted
hi ,,
you write such a good articles..
i want to understand “multipathing” properly. i am not able to understand via different articles over the internet.
could you please send me any doc which can help me understand this topic like how to configure, what is multipathing, types of multipathing,what does it actually mean….
Thanks in advance….
is there any cheance to extend the disk? if i created sofware traid.
I’m not able to find the /etc/mdadm.conf or /etc/mdadm/mdadm.conf. why is it so ?
/dev/md0:
Version : 1.2
Creation Time : Tue Mar 29 21:16:39 2016
Raid Level : raid1
Array Size : 20948288 (19.98 GiB 21.45 GB)
Used Dev Size : 20948288 (19.98 GiB 21.45 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent
Update Time : Tue Mar 29 21:30:07 2016
State : active
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
Name : localhost.localdomain:0 (local to host localhost.localdomain)
UUID : ff2aa97d:6aae2570:0d44f49a:735d94b9
Events : 18
Number Major Minor RaidDevice State
0 8 17 0 active sync /dev/sdb1
1 8 33 1 active sync /dev/sdc1
[root@localhost etc]# ls -ld /etc/md*
ls: cannot access /etc/md*: No such file or directory
[root@localhost etc]#
Personalities : [raid1]
md0 : active raid1 sdc1[1] sdb1[0]
20948288 blocks super 1.2 [2/2] [UU]
unused devices:
You can create it.
I only ran into two problems, one, I had two different size HD’s, so I took the smallest one in the fdisk readout and copied the ‘end’ to the larger when fdisking it, and magic, the sizes are now the same.
Second, I could not fdisk the partition id type fd. I found out, that I had to totally delete the existing partition and use the ‘Create a new label’ and use the ‘o’ to create a new empty DOS partition table, as the partition I was creating was a ‘g’ GPT partition for some unknown reason. Once I got past that, all is well, the finished partition in fdisk should look similar to this: “Device Boot Start End Sectors Size Id Type
/dev/sda1 2048 1953458142 1953456095 931.5G fd Linux raid autodetect” , after getting the raid setup , I then was able to successfull for the first time ever, get DLNA working, woohoo !
Oh, and thanks for the instructions, it would have taken me quite a long time, if ever, for me to figure it all out. I did this on an Raspberry Pi vi win7 putty client.
Hello,
If I do RAID configuration using Setup Utility of my server, will I lose any existing data on the server?
Yes