mdadm: common commands

Common mdadm commands I found really great (under Debian).

Generate mdadm.conf

cp /etc/mdadm/mdadm.conf /etc/mdadm/mdadm.conf.save
/usr/share/mdadm/mkconf --generate > /etc/mdadm/mdadm.conf

Create RAID

mdadm --create /dev/md2 --raid-devices=3 --spare-devices=0 --level=5 --run /dev/sd[cde]1
Note: search for "Setting GRUB on a drive" if you are setting up a bootable RAID-1

Remove disk from RAID

mdadm --fail /dev/md0 /dev/sda1
mdadm --remove /dev/md0 /dev/sda1

Copy the partition structure (when replacing a failed drive)

sfdisk -d /dev/sda | sfdisk /dev/sdb 
mdadm --zero-superblock /dev/sdb

Add a disk to a RAID array (to replace a removed failed drive)

mdadm --add /dev/md0 /dev/sdf1

Check RAID status

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

Reassemble a group of RAID disks

This works to move an assembly from one physical machine to another.
mdadm --assemble /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1

Convert a RAID 1 array to RAID 5 (follow the steps to add a disk after running this command)

mdadm --create /dev/md0 --level=5 -n 2 /dev/sda1 /dev/sdb1

Stop and remove the RAID device

mdadm --stop /dev/md0
mdadm --remove /dev/md0

Destroy an existing array

mdadm --manage /dev/md2 --fail /dev/sd[cde]1
mdadm --manage /dev/md2 --remove /dev/sd[cde]1
mdadm --manage /dev/md2 --stop
mdadm --zero-superblock /dev/sd[cde]1

Re-use a disk from another RAID set

If a disk has been used in another RAID set, it has a superblock on it that really, really can cause problems. Simply clear the superblock to re-use it
mdadm --zero-superblock /dev/sdb

Speed up a sync (after drive replacement)

cat /proc/sys/dev/raid/speed_limit_max
200000
cat /proc/sys/dev/raid/speed_limit_min
1000

This means you are running a minimum of 1000 KB/sec/disk and a maximum of 200,000. To speed it up:
echo 50000 >/proc/sys/dev/raid/speed_limit_min
which will set the minimum to 50,000 KB/sec/disk (ie, 50 times greater). Expect your system to be a lot slower.