Couldn’t really find anyone who’d documented setting up a RAID 5 array with Linux and mdadm, so figured I’d jot down the method I used in Ubuntu 6.10. It basically boils down to four commands (I am assuming you have a fresh install):

sudo apt-get install mdadm
sudo mdadm --create /dev/md0 --raid-devices=5 /dev/sd[abcde]1 --level=raid5
sudo mke2fs -j /dev/md0
sudo mount /dev/md0 /mnt/raid

To explain a little:

The first command installs mdadm.

The second command creates the raid array /dev/md0, then sets how many disks – and their respective locations – there’ll be in the array. In my case, we have 5 750GB drives, which are /dev/sda through to /dev/sde. The 1 is the partition identifier. Level sets the raid level, which in this case is raid5.

The third command ‘sudo mke2fs -j /dev/md0 ‘ makes an ext3 filesystem on the array.

And finally ‘sudo mount /dev/md0 /mnt/raid‘ mounts the array to /mnt/raid (you can mount it wherever you like)

Note 1: Make sure you have mdadm installed. If you don’t: sudo apt-get install mdadm

Note 2: If you’re not using Ubuntu, su to root and run the commands without ‘sudo’

Note 3: If you get an error stating ‘mdadm: error opening /dev/md0: No such file or directory’, you need to bypass udev and use this command instead: sudo mdadm --create /dev/md0 --raid-devices=5 /dev/sd[abcde]1 --level=raid5 --auto=yes

Note 4: To view the status of your array: sudo mdadm --detail /dev/md0

Note 5: If you need to view a list of your hard drives, try these commands: ls -l /dev/sd* or ls -l /dev/hd*

I am certainly no Linux expert. Corrections, and suggestions on how to improve this method are encouraged 🙂

Update: There’s a very thorough guide here, but it seems to cover some GUI elements – which wasn’t an option for me.

Written by Phil Wiffen

Phil is an IT Professional working in Cambridge, England. He generally blogs about useful solutions that he comes across in his work/play.

5 Comments

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Johannes

Worked like a charm on Ubuntu 9.10 Server with 4x1TB in Raid 5 using ext4

Zvon

Thanks for this, it works great, but you might need to expand it a little…. to include how to make it automatically come up on reboot.

I followed these instructions, then when i rebooted the server, the raid array wasnt running.

need to add settings into /etc/mdadm/mdadm.conf
and also include an entry in fstab.

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