Introduction to Linux Software RAID1

This is a HTML-ised version of a presentation I gave to Gloucestershire Linux User Group on 17th April 2007. It is also available as a PDF.

RAID Basics

  • RAID = Redundant Array of Inexpensive Disks
  • Many drives to create a single volume
  • Different levels, eg:
  • There are many other variations, I'm only going to talk about RAID1


  • Used to require dedicated hardware card
  • Now also on some motherboards
  • Hardware RAID is set up via extension to BIOS - you get an extra "Press F3 to set up RAID" screen or similar upon boot
  • Linux has supported RAID in OS ("software RAID") since 2.4
  • No need for hardware card or fancy motherboard in Linux!
  • You can use any IDE, SCSI or SATA drives
  • With RAID1 "Mirroring", you will see a volume equal only to the smallest drive size in the array (eg. 60gb & 40gb = 40gb)

    Easy choices

  • I'm only going to talk about RAID1 "Mirroring"
  • Create a single volume, identical copy on each of two disks
  • Either disk can fail and we will keep our data
  • Ideal for:
  • Not ideal for:

    Avoid Legacy Tools

  • Software RAID HOWTO is woefully out of date
  • raidtools are deprecated as of kernel 2.4
  • fd RAID volume type (eg. in fdisk) is also deprecated
  • mdadm is what you want to use, certainly on distros <5 years old
  • mdadm rarely uses config file - don't bother with config file, it will almost always autodetect and ignore it anyway


  • You have a single root partition on /dev/hda1
  • You have a spare drive /dev/hdc that is the same size or bigger
  • You want to create a RAID1 mirror of your root filesystem
  • You want to boot from root partition using GRUB (if not, this demo will still be interesting, but ignore GRUB bits)
  • You have kernel >=2.4 and mdadm installed
  • You have recent verified backups of all important files

    Get Started

  • Create a partition on /dev/hdc1 that is the same size or larger than /dev/hda1
  • Create a new RAID device made of /dev/hdc1 plus a special virtual device called "missing" - we will fill this with hda1 later
    # mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/hdc1 missing
  • Format /dev/md0
    # mkfs.ext3 /dev/md0
  • Mount new /dev/md0
    # mkdir /newroot
    # mount /dev/md0 /newroot
  • Copy existing root filesystem onto /dev/md0
    # cp -axv / /newroot

    Reboot into RAID

  • Change grub (or lilo) to mount root as /dev/md0
    title           Ubuntu, kernel 2.6.15-28-k7
    root            (hd0,0)
    kernel          /boot/vmlinuz-2.6.15-28-k7 root=/dev/md0 ro quiet splash
    initrd          /boot/initrd.img-2.6.15-28-k7
  • Don't forget to copy /boot/grub/menu.lst to /newroot
    # cp /boot/grub/menu.lst /newroot/boot/grub/menu.lst
  • Setup GRUB
    grub> root (hd0,0)
    grub> setup (hd0,0)
    grub> root (hd2,0)
    grub> setup (hd2,0)
    grub> quit

    Check everything

  • Reboot. If it fails to boot, use GRUB menu to go back to previous config.
  • Otherwise you should now be on /dev/md0
    # df
    # cat /proc/mdstat
  • Should show root filesystem is /dev/md0
  • Should show /dev/md0 has one device and one missing [U_]
  • MAKE SURE YOUR SYSTEM WORKS NOW. We are about to add /dev/hda1 to the array, overwriting it - point of no return!
    # mdadm --manage /dev/md0 --add /dev/hda1
    # cat /proc/mdstat

    That's All Folks

  • You're done! Reboot and check everything is okay.
  • If something's gone wrong, you can use rescue disk to boot from /dev/hda1 or /dev/hdc1 - you can boot from either without needing RAID, they are normal ext3 filesystems! (Note: This only applies to RAID1! Very useful feature!)
  • You can observe RAID status with:
    # cat /proc/mdstat
    Personalities : [raid1] 
    md0 : active raid1 hdc1[0] hda1[1]
          38074560 blocks [2/2] [UU]
    unused devices: 
  • mdmonitor can email you when a drive fails or other event action


  • mdadm will ignore it's own config file when booting from RAID (well, duh, how could it read the config file from something it's only just about to boot from?)
  • If you have multiple RAID arrays, mdadm boot autodetect will arrange arrays in order of drive letter: eg. two RAID volumes hda1 & hdc1=md0 , hdb1 & hdd1=md1
  • You can't change it so that hda1 & hdc1=md1 and vice versa - unless you don't boot from RAID
  • Personally I don't see much point having RAID that isn't bootable, if the boot partition fails then the system is dead
  • You can do really cool stuff by taking one drive out of a RAID1 array and putting it in another system! Instant system copy!

    Public Domain - Andrew Oakley - 2007-04-18

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