Create and Manage a RAID Array in Ubuntu

Posted December 30th, 2012 (Updated 31 Dec 2012) in Linux

The below is a collaboration of useful information I’ve found while attempting to build and maintain a RAID5 array consisting of 4 HDDs. This tutorial is a work in progress and I’m learning everything as I go. I’ve left relevant links in each section for more information and if you spot anything that could be done better or that I’m missing let me know in the comments below!

A few useful commands to get us started:

  • lsblk to see mount points
  • df -T to see file systems and
  • sudo fdisk -l to see partition types


Format and partition drives


Make sure all disks are umounted:

First step is to format and partition our drives to prepare them for the array. I’m using four 2TB SATA drives.

# Mine are sdb sdc sdd and sde
sudo umount /media/sdb
sudo umount /media/sdc
sudo umount /media/sdd
sudo umount /media/sde

Delete all partitions and create new Linux RAID autodetect partitions

RAID disks require RAID autodetect partitions. These can be made with fdisk:

sudo fdisk /dev/sdb
d #until all deleted
n #use defaults
t #change partition type
fd #Linux RAID autodetect type

Repeat the above for each drive

Partition to EXT4

I’m using EXT4 for my array.

	sudo mkfs.ext4 sdb1
	sudo mkfs.ext4 sdc1
	sudo mkfs.ext4 sdd1
	sudo mkfs.ext4 sde1


Create the Array


As mentioned above I’m using four drives and making a RAID5 array.

sudo mdadm --create /dev/md0 --level=5 --raid-devices=4 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1
watch -n3 cat /proc/mdstat

This will take several hours depending on number and size of hdds. When done you should see

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid5 sde1[4] sdd1[2] sdc1[1] sdb1[0]
5860145664 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/4] [UUUU]

Reboot and the array will have disappeared. Find the new location:

cat /proc/mdstat

Mine had for some reason been replaced with /dev/md127:

Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid1] [raid10]
md127 : active raid5 sde1[4] sdc1[1] sdd1[2] sdb1[0]
     5860145664 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/4] [UUUU]


sudo mdadm --assemble --scan
sudo mdadm --detail --scan

and paste the output at the bottom of /etc/mdadm/mdadm.conf. Mine looked like

ARRAY /dev/md/xbmc:0 metadata=1.2 name=xbmc:0 UUID=3dcfe843:c2300a40:75190922:f6caf9c7

Add your array from /proc/mdstat (in my case /dev/md127) to fstab like so:

/dev/md127        /media/md127    ext4             defaults 0       0



Failed Arrays

I wanted to test a repair so I pulled a drive and plugged it back in. Alternatively you could use

mdadm -manage -set-faulty /dev/md127 /dev/sdb1

Doing a

sudo mdadm --query --detail /dev/md127

on the newly failed array showed:

Version : 1.2

State : clean, degraded
Active Devices : 3
Working Devices : 3
Failed Devices : 0
Spare Devices : 0

Layout : left-symmetric
Chunk Size : 512K

Name : xbmc:0  (local to host xbmc)
UUID : 3dcfe843:c2300a40:75190922:f6caf9c7
Events : 610

Number   Major   Minor   RaidDevice State
0       8       17        0      active sync   /dev/sdb1
1       8       33        1      active sync   /dev/sdc1
2       0        0        2      removed
4       8       49        3      active sync   /dev/sdd1

Notice the State is now set to ‘clean, degraded’.

Email Notifications

First and foremost you want to be notified by email about failed arrays. Open /etc/mdadm/mdadm.conf and set the MAILADDR line to your email address. When arrays fail you’ll now receive an email like the following:

This is an automatically generated mail message from mdadm
running on xbmc

A DegradedArray event had been detected on md device /dev/md/xbmc:0.

Faithfully yours, etc.

P.S. The /proc/mdstat file currently contains the following:

Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid1] [raid10]
md127 : active raid5 sdf1[5] sdd1[4] sdc1[1] sdb1[0]
5860145664 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [UU_U]
[===========>.........]  recovery = 59.5% (1163564104/1953381888) finish=184.5min speed=71313K/sec

unused devices: <none>

Physically Locating the faulty disk


Always have a diagram written up beforehand showing the location of each drive and their UUID/serial number! This is the best method.

If you haven’t done the above a nice way to determine which disk is faulty if you know the /dev/sdx association (using cat /proc/mdstat should do the trick, or  lsscsi -l might help) is by doing a

cat /dev/sdx >/dev/null

and seeing which LED light blinks madly out front of your machine.

Here’s a script I whipped up to output /dev location, UUID and model numbers of each drive:

for disk in `sudo fdisk -l | grep -Eo '(/dev/[sh]d[a-z]):' | sed -E 's/://'`;
   uuid=`sudo blkid /dev/sdb1 | grep -Eo 'UUID="([^"]+)"'`
   model=`sudo hdparm -i $disk | grep -Eo 'SerialNo=.*' | sed -E 's/SerialNo=//';`
   echo "$disk $uuid model='$model'"


/dev/sdc UUID=”3dcfe843-c230-0a40-7519-0922f6caf9c7″ model=’W -DCWYA00667790′
/dev/sdb UUID=”3dcfe843-c230-0a40-7519-0922f6caf9c7″ model=’W -DCWYA00186517′
/dev/sdd UUID=”3dcfe843-c230-0a40-7519-0922f6caf9c7″ model=’W -DCWYA00677873′
/dev/sde UUID=”3dcfe843-c230-0a40-7519-0922f6caf9c7″ model=’W -DCWYA00879590′
/dev/sdf UUID=”3dcfe843-c230-0a40-7519-0922f6caf9c7″ model=’W -DCWYA00878581′

Repairing a failed Array


Do an

ls /dev | grep sd

to find the newly plugged in drive (in my case /dev/sdf) and add the partition on the drive to your raid array (You may have to format/partition as described in the Format and partition drives section above):

sudo mdadm /dev/md127 -a /dev/sdf1
# mdadm: added /dev/sdf1

Monitor your progress with

watch -n3 cat /proc/mdstat

I made the mistake of reading from the drive before it’d started repairing. This apparently lowers the repair speed to the bare minimum – mine was going to take 22 days! To fix this become root and run

echo 100000 > /proc/sys/dev/raid/speed_limit_min
echo 400000 > /proc/sys/dev/raid/speed_limit_max


GUI Management



While researching I came across webmin. It’s free, easy to install and offers a fair bit of functionality right from the browser. I haven’t used it to do any actual modifications yet but it seems to have all the functionality you need and a bunch of other useful stuff such as a web console, apt updates and apache/proftpd administration. I highly recommend checking it out.

WebMin Raid Management 1

WebMin Raid Management 1



Other Useful Links

udevinfo for Ubuntu

Grow a raid array

Changing /dev locations

Re-adding drives – Write-Intent Bitmaps