Mdadm cheat sheet

I have spent some time over the last few weeks getting familiar with mdadm and software RAID on Linux, so I thought I would write down some of the commands and example syntax that I have used while getting started.

1)If we would like to create a new RAID array from scratch we can use the following example commands:

RAID1-with 2 Drives:

# mdadm --create --verbose /dev/md0 --level=1 /dev/sda1 /dev/sdb1

RAID5-with 5 Drives:

# mdadm --create --verbose /dev/md0 --level=5 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1

RAID6-with 4 Drives with 1 spare:

# mdadm --create --verbose /dev/md0 --level=6 --raid-devices=4 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1

2)If we would like to add a disk to an existing array:

# mdadm --add /dev/md0 /dev/sdf1 (only added as a spare)
# mdadm --grow /dev/md0 -n [new number of active disks -- spares] (grow the size of the array)

3)If we would like to remove a disk from an existing array:

First we need to ‘fail’ the drive:

# mdadm --fail /dev/md0 /dev/sdc1

Next it can be safely removed from the array:

# mdadm --remove /dev/md0 /dev/sdc1

4)In order to make the array survive a reboot, you need to add the details to ‘/etc/mdadm/mdadm.conf’

# mdadm --detail --scan >> /etc/mdadm/mdadm.conf (Debian)
# mdadm --detail --scan >> /etc/mdadm.conf (Everyone else)

5)In order to delete and remove the entire array:

First we need to ‘stop’ the array:

# mdadm --stop /dev/md0

Next it can be removed:

# mdadm --remove /dev/md0

6)Examining the status of your RAID array:

There are two options here:

# cat /proc/mdstat
# mdadm --detail /dev/md0

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