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I successfully created a RAID (mirroring) by utilizing mdadm. However, I must run the following commands after each boot:

mdadm --stop --scan // to stop /dev/md127 - I don't know where the number 127 even comes from
mdadm --assemble --scan // to start /dev/md0

What am I doing wrong/why do I need to run these commands at boot? What is the right way to auto-start RAID with each (re)boot?

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3 Answers 3

up vote 19 down vote accepted

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

# 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)

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I would maybe add: you should also update your initramfs with update-initramfs -u –  Pablo Montepagano Oct 19 '12 at 3:08
mdadm --detail --scan does not print anything on my system, although I know for sure that I have two partitions that are part of a raid0 (mirror). Any general advice on this is welcome. –  Lennart Rolland Mar 14 at 13:59

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.

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On Debian wheezy one more step is required: in /etc/default/mdadm set autostart from false to true

#   should mdadm start arrays listed in /etc/mdadm/mdadm.conf automatically
#   during boot?

Also I had to use mdadm -Es >>/etc/mdadm/mdadm.conf instead of the --scan option, as that did not work for me.

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