Create an Array¶

Mirror Boot while installing Ubuntu Server¶

See the ubuntu-os-specific.md file.

Mirror¶

sudo mdadm --create --verbose /dev/md0 --level=mirror --raid-devices=2 /dev/sdc /dev/sdd

Modify an Array¶

Add a new disk to an array¶

mdadm --manage /dev/md127 --add /dev/sde
mdadm: added /dev/sde

Growing an array¶

mdadm --grow --raid-devices=4 /dev/md1

Read Only / Read Write¶

cat /proc/mdstat 
Personalities : [raid1] [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid10] 
md127 : active (auto-read-only) raid6 sdk1[1] sdf[7] sdj[4] sdi[9] sdd[8] sdg[10] sdh[6]
      17580801024 blocks super 1.2 level 6, 512k chunk, algorithm 2 [8/7] [_UUUUUUU]

Run: mdadm --readwrite /dev/md127

cat /proc/mdstat 
Personalities : [raid1] [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid10] 
md127 : active raid6 sdk1[1] sdf[7] sdj[4] sdi[9] sdd[8] sdg[10] sdh[6]
      17580801024 blocks super 1.2 level 6, 512k chunk, algorithm 2 [8/7] [_UUUUUUU]

Convert RAID6 to RAID5¶

sudo mdadm --grow /dev/md127 --level=raid5 --raid-devices=8
mdadm: Need 1 spare to avoid degraded array, and only have 0.
       Use --force to over-ride this check.

admin@example-server:~⟫ sudo mdadm --grow /dev/md127 --level=raid5 --raid-devices=7
raid_disks for /dev/md127 set to 7

Tweaks and other bits¶

Convincing the OS to not call an array md127¶

Update /etc/mdadm/mdadm.conf with the array details.

The fast way:

mdadm --detail --scan >> /etc/mdadm/mdadm.conf

Be warned that this is likely to remove options such as MAILADDR root.

Once the mdadm.conf file is updated you need to run sudo update-initramfs -u.

Make recovery faster¶

sudo sysctl -w dev.raid.speed_limit_max=100000
sudo sysctl -w dev.raid.speed_limit_min=90000

Duplicate a partition table¶

For when you're replacing an OS drive with grub/etc.

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

Remove an array¶

mdadm --stop /dev/md0
mdadm --remove /dev/md0
mdadm --zero-superblock /dev/sda

Repeat zero-superblock for each device that was in the array.