mdadm / software raid

create raid

raid 1:

mdadm --create /dev/md0 --level=1 --raid=devices=2 /dev/sda /dev/sdb

if you get an error like mdadm: invalid number of raid devices: devices=2 run the command like so (with -l for level and -n for devices):

mdadm --verbose --create /dev/md0 -l 1 -n 2 /dev/sdc /dev/sdb

create a file system:

mkfs.ext4 /dev/md0

or whatever filesystem you want.

check raid status

mdadm --detail /dev/md0

check also: /proc/mdstat

save raid config

well, that's somewhat important, right?

mdadm --detail --scan --verbose | sudo tee -a /etc/mdadm/mdadm.conf

Run update-initramfs -u after updating this file.

also, back up the mdadm.conf file to restore it onto a new system

add and remove devices to/from raid

if they're in a faulty state for example

mdadm --remove /dev/md0 /dev/nvme0n1
mdadm --add /dev/md0 /dev/nvme0n1

replace disk

check how to backup and copy your partition table

remove disk:

mdadm /dev/md0 -r /dev/sdb

copy partition table from backup or primary drive:

sfdisk -d /dev/sda | sfdisk /dev/sdb

add disk:

mdadm /dev/md0 -a /dev/sdb

convert single disk to raid 1

... hopefully without losing data!

1. create raid from 2^nd disk with one missing disk
2. copy data from 1^st disk to 2^nd disk (now in raid)
3. add 1^st (original) disk to raid config
4. rebuild raid
5. profit (took 5 steps, I'm sorry)

create raid:

notice the missing bit?

mdadm --create /dev/md0 --level=1 --raid-devices=2 missing /dev/sdb

now copy the data from the original disk to the newly created raid, so create and mount your filesystem on /dev/md0

[...]

prepare original disk:

let's dump (-d the disk layout from the raid disk to the original disk):

sfdisk -d /dev/sdb | sfdisk /dev/sda

add original disk to raid:

mdadm /dev/md0 -a /dev/sda

check raid rebuild status:

...in minutes

grep -oP 'finish\=\d+\.\d+' /proc/mdstat | cut -d'=' -f2

increase mdadm rebuild / resync speed

you can check the current speed limits with:

sysctl dev.raid.speed_limit_min
sysctl dev.raid.speed_limit_max

and, now just increase that number to make it faster (it's displayed in kb)

sysctl -w dev.raid.speed_limit_min=500000

debugging things

if you messed things up, you can try and restore things with testdisk