🖥️mdadm

➡️This is a command-line reference manual for commands and command combinations that you don’t use often enough to remember it. This cheatsheet explains the mdadm command with important options and switches using examples.

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# For the sake of briefness, we use Bash  "group compound" stanza:
#   /dev/sd{a,b,...}1 => /dev/sda1 /dev/sdb1 ...
# Along the following variables:
#   ${M} array identifier (/dev/md${M})
#   ${D} device identifier (/dev/sd${D})
#   ${P} partition identifier (/dev/sd${D}${P})

# Create (initialize) a new array
mdadm --create /dev/md${M} --level=raid5 --raid-devices=4 /dev/sd{a,b,c,d,e}${P} --spare-devices=/dev/sdf1

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

# Automatically assemble (activate) all existing arrays
mdadm --assemble --scan

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

# 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 confiration/status
mdadm --detail /dev/md${M}

# Save existing arrays configuration
# (MAY be required by initrd for successfull boot)
mdadm --detail --scan > /etc/mdadm/mdadm.conf

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

# Manually mark a component as failed
# (SHOULD when a device shows wear-and-tear signs, e.g. through SMART)
mdadm --manage /dev/md${M} --fail /dev/sd${D}${P}

# Remove a failed component
# (SHOULD before preemptively replacing a device, after failing it)
mdadm --manage /dev/md${M} --remove /dev/sd${D}${P}

# Prepare (format) a new device to replace a failed one
sfdisk -d /dev/sd${D,sane} | sfdisk /dev/sd${D,new}

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

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

# Rename a device
# (SHOULD after hostname change; eg. name="$(hostname -s)")
mdadm --assemble /dev/md${M} /dev/sd{a,b,c,d,e}${P} --name="${name}:${M}" --update=name

#==============================##==============================#
# CMD MDADM					       #
#==============================##==============================#

Cheatsheets are an excellent complement to other information sources like Linux man-pages, Linux help, or How-To’s and tutorials, as they provide compact and easily accessible information. While man-pages and detailed tutorials often contain comprehensive explanations and extensive guides, cheatsheets summarize the most important options forthe command mdadm in a clear format. This allows users to quickly access the needed information for mdadm without having to sift through lengthy texts. Especially in stressful situations or for recurring tasks, cheatsheets for mdadm are a valuable resource to work efficiently and purposefully.

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