Mdadm

Overview

Several physical disks (/dev/sdX) or partitions (/dev/sdX1) of equal size are joined into a single array.

Creating a RAID array

• (Recommended) Create a partition on each disk with the following attributes:
  • Use the "GPT" partition table format (to handle disks > 2TB)
  • Use optimal alignment for partition start (this will normally mean that the partition start will be at the 1MB boundary)
  • End 100MB before the end of the disk (this is to allow for slight variances in exact size of similar disks)
  • Set partition type to raid (0xFD00); this is optional, but may encourage some tools to avoid writing directly to the disk (and avoid corrupting the array)

parted -a optimal -- /dev/sdX mklabel gpt mkpart primary 0MB -100MB set 1 raid on

• Create a RAID 5 array over 3 partitions:
  • Note, the default metadata version is now 1.2 for create commands

mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdX1 /dev/sdY1 /dev/sdZ1

• Wait (potentially several days) for the array to be built

• Once built, save the current raid setup to /etc, to allow for automounting on startup:

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

• Update the initial boot image for all current kernel versions to include the new mdadm.conf:

update-initramfs -u -k all

• Start the array:

mdadm --assemble /dev/md0 /dev/sdX1 /dev/sdY1 /dev/sdZ1

Useful Commands

cat /proc/mdstat 

Display a summary of current raid status

mdadm --detail /dev/md0 

Display raid information on array md0

mdadm --examine /dev/sdf 

Display raid information on device/partition sdf