Linux Create Software RAID 1 (Mirror) Array

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.

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Linux Support For Software RAID

Currently, Linux supports the following RAID levels (quoting from the man page):

  1. LINEAR
  2. RAID0 (striping)
  3. RAID1 (mirroring)
  4. RAID4
  5. RAID5
  6. RAID6
  7. RAID10
  8. 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

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I'm Vivek Gite, and I write about Linux, macOS, Unix, IT, programming, infosec, and open source. Subscribe to my RSS feed or email newsletter for updates.

32 comments… add one
  • mohan Jan 26, 2012 @ 17:28

    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

  • Vitaly Feb 8, 2012 @ 20:47

    Awesome, this doc definitely helps the failed partitioner on Fedora 16 installer (for software raid that you want unmounted during install). Thanks!!

  • dwasifar Jul 15, 2013 @ 3:42

    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.

  • Pruda Sep 25, 2013 @ 15:06

    I have server with
    HardRAID : LSI MegaRAID 9271 6 Gbps FastPath
    Hard drive : 2 x 300GB SSD
    When i check with df -h

    Filesystem            Size  Used Avail Use% Mounted on
    rootfs                9.8G  605M  8.7G   7% /
    /dev/root             9.8G  605M  8.7G   7% /
    devtmpfs              126G  348K  126G   1% /dev
    /dev/sda2             265G   60M  251G   1% /home
    tmpfs                 126G     0  126G   0% /dev/shm
    /dev/root             9.8G  605M  8.7G   7% /var/named/chroot/etc/named
    /dev/root             9.8G  605M  8.7G   7% /var/named/chroot/var/named
    /dev/root             9.8G  605M  8.7G   7% /var/named/chroot/etc/named.conf
    /dev/root             9.8G  605M  8.7G   7% /var/named/chroot/etc/named.rfc1912.zones
    /dev/root             9.8G  605M  8.7G   7% /var/named/chroot/etc/rndc.key
    /dev/root             9.8G  605M  8.7G   7% /var/named/chroot/usr/lib64/bind
    /dev/root             9.8G  605M  8.7G   7% /var/named/chroot/etc/named.iscdlv.key
    /dev/root             9.8G  605M  8.7G   7% /var/named/chroot/etc/named.root.key
    

    Is it RAID or not?
    Why so much /dev/root s?

    • 🛡️ Vivek Gite (Author and Admin) nixCraft Sep 25, 2013 @ 16:45

      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

  • Pruda Sep 25, 2013 @ 18:53

    Thanks. So these 300GB X 2 SSDs in RAID1?

    • Hakim Oct 2, 2013 @ 10:10

      only 20gb for both disk if im not mistaken

  • Hakim Oct 2, 2013 @ 10:09

    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

  • sgentry Oct 11, 2013 @ 22:53

    Has anyone noticed any problems using mdadm with 4tb HDs?

  • karthik Mar 17, 2014 @ 18:07

    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

  • sukhpreet singh Jul 5, 2014 @ 6:56

    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….

  • ramu Sep 2, 2014 @ 18:17

    is there any cheance to extend the disk? if i created sofware traid.

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