How can I make mdadm auto-assemble RAID after each boot?

NB: You either need to be logged in as root, or use sudo to do all this...

• Use your favourite editor to create or edit /etc/mdadm/mdadm.conf file as follows:

If the file does not even exist, paste the following into the new, empty file:

# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#

DEVICE partitions

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST <system>

# instruct the monitoring daemon where to send mail alerts
MAILADDR root

# definitions of existing MD arrays

• Save the file

• Run the following command to add a reference to your array config at the end of the file:

  mdadm --detail --scan >> /etc/mdadm/mdadm.conf

This should add a line like the following to the end of mdadm.conf:

ARRAY /dev/md0 level=raid5 num-devices=3 metadata=00.90 UUID=a44a52e4:0211e47f:f15bce44:817d167c

If the mdadm command has added any other stuff above the ARRAY line, remove it. For example, on one of my machines, the command returns 'mdadm: metadata format 00.90 unknown, ignored.' before the ARRAY line.

Your array should now auto-build on boot and thus you can add an entry to /etc/fstab to mount it (if it's not already there)

I realize this is an older question, but I had a frustrating time with this on the 32-bit version of Ubuntu Server 12.04.

Running mdadm --detail --scan >> /etc/mdadm/mdadm.conf appended the line

ARRAY /dev/md0 metadata=1.2 name=ubuntu:0 UUID=a8a570c6:96f61865:05abe131:5c2e2f7e

After a reboot I could never see /dev/md0. Running the mdadm --detail --scan again (without putting the result in a file) I would see

ARRAY /dev/md/ubuntu:0 metadata=1.2 name=ubuntu:0 UUID=a8a570c6:96f61865:05abe131:5c2e2f7e

and manually mounting /dev/md/ubuntu:0 would work. In the end, that was what I put in the fstab file too.

I am not sure what I got wrong, if this is how it works in Ubuntu 12.04, or if this is a bad practice. Just wanted to share what worked for me.

I had this problem on my Raspberry Pi 2 running Raspbian GNU/Linux 8 (jessie). I had a RAID array on /dev/sda1 and /dev/sdb1 which failed to assemble at boot. I had in my /etc/mdadm/mdadm.conf file the entry

ARRAY /dev/md/0  metadata=1.2 UUID=53454954:4044eb66:9169d1ed:40905643 name=raspberrypi:0 

(your numbers will be different; see other answers on how to get this.)

I had in my /etc/fstab file the entry

/dev/md0        /data           ext4    defaults          0       0

(and of course /data indeed existed)

Like the OP, I could assemble and mount the RAID array by hand after boot, but I could not get it to happen automatically during boot despite apparently correctly setting it up.

I was able to solve the problem as follows. I investigated the script at /etc/init.d/mdadm-raid and inserted a line of debug code

ls /dev > /home/pi/devices.txt

Rebooting and checking this file I learned that devices /dev/sda and /dev/sdb existed at the time the mdadm-raid initialization happened, but the partitions /dev/sda1 and /dev/sdb1 were missing. I edited the /etc/init.d/mdadm-raid file and inserted the line

partprobe

after the header (i.e. after the ### END INIT INFO but before the script begins). This caused the partitions to be detected and so the mdadm-raid script was able to assemble the RAID array, resolving the problem. Hope this helps someone!