mdadm - Linux Software RAID Management Tool - cmd Cheatsheets

The mdadm command is a powerful utility for managing Linux software RAID arrays. It allows you to create, assemble, stop, query, and manage RAID devices, providing a flexible and robust solution for data redundancy and performance.

Creating New RAID Arrays

To initialize a new RAID array, you use the --create option. This command specifies the RAID level, the number of devices, and the devices to be included in the array, along with any spare devices.

# Example: Create a RAID 5 array with 4 devices and 1 spare
mdadm --create /dev/md${M} --level=raid5 --raid-devices=4 /dev/sd{a,b,c,d,e}${P} --spare-devices=/dev/sdf1

Assembling and Stopping RAID Arrays

Existing RAID arrays can be manually or automatically assembled. The --assemble command activates an array, while --stop deactivates it.

# Manually assemble an existing array
mdadm --assemble /dev/md${M} /dev/sd{a,b,c,d,e}${P}

# Automatically assemble all existing arrays
mdadm --assemble --scan

# Stop an assembled (active) array
mdadm --stop /dev/md${M}

Querying and Examining RAID Status

You can query the configuration of an array or examine the superblock content of individual components to understand the RAID setup and device status.

# See array configuration
mdadm --query /dev/md${M}

# See array component configuration (dump superblock content)
mdadm --query --examine /dev/sd${D}${P}

# See detailed array configuration/status
mdadm --detail /dev/md${M}

Managing RAID Components

mdadm provides commands to manage individual components of a RAID array, such as marking a device as failed, removing it, or adding a new device to replace a failed one.

# Manually mark a component as failed
mdadm --manage /dev/md${M} --fail /dev/sd${D}${P}

# Remove a failed component
mdadm --manage /dev/md${M} --remove /dev/sd${D}${P}

# Add a new component to an existing array (triggers rebuild)
mdadm --manage /dev/md${M} --add /dev/sd${D,new}${P}

Saving and Zeroing Superblocks

It's crucial to save the existing array configuration for boot-time assembly and to zero the superblock of a device before reusing it for other purposes.

# Save existing arrays configuration
mdadm --detail --scan > /etc/mdadm/mdadm.conf

# Erase array component superblock (MUST do before reusing a partition)
mdadm --zero-superblock /dev/sd${D}${P}

Additional Information

You can view the status of assembled arrays using /proc/mdstat. Renaming a device can be done during assembly with the --name and --update=name options.

# See assembled (active) arrays status
cat /proc/mdstat

# Rename a device during assembly
mdadm --assemble /dev/md${M} /dev/sd{a,b,c,d,e}${P} --name="${name}:${M}" --update=name

mdadm - Linux Software RAID Management Tool

mdadm - Linux Software RAID Management