mdadm

Glossary:

command	description
`cat /proc/mdstat`	show status of all raids
`mdadm --detail /dev/md0`	detailed status of raid md0
`mdadm --create /dev/md0 -n2 -l1 /dev/sda1 /dev/sdb1`	new raid `/dev/md0` with 2 disks, raid level 1 on `/dev/sda1` and `/dev/sda2`
`mdadm --fail /dev/md0 /dev/sda1 ; mdadm --remove /dev/md0 /dev/sda1`	remove `/dev/sda1` from `/dev/md0`
`mdadm --add /dev/md0 /dev/sda1`	add `/dev/sda1` to `/dev/md0`
`mdadm --grow /dev/md0 -n3`	use 3 disks in raid `/dev/md0` (e.g. add an additional disk, so a damaged drive can be removed later-on)
`mdadm --assemble /dev/md0`	Assemble `/dev/md0` (e.g. when running live system)
`mdadm --detail --scan >> /etc/mdadm/mdadm.conf`	Update list of arrays in `/etc/mdadm/mdadm.conf` ; you should remove old list by hand first!
`mdadm --examine /dev/sda1`	What is this disk / partition?
`sysctl -w dev.raid.speed_limit_min=10000`	Set minimum raid rebuilding speed to 10000 kiB/s (default 1000)
`sfdisk -d /dev/sdX \| sfdisk /dev/sdY`	Copy partition table from sdX to sdY (MBR only)
`sgdisk /dev/sdX -R /dev/sdY ; sgdisk -G /dev/sdY`	Copy partition table from sdX to sdY (GPT)

To boot a machine even with a degraded array, modify /etc/initramfs-tools/conf.d/mdadm and run update-initramfs -c -kall (Use with caution!)