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RAID

RAID (Redundant Array of Independent Disks) is a data storage virtualization technology.

RAID allows:

  • To spread I/O across multiple disks to increase performance and fault tolerance.
  • Implemetation on the either HW/SW level.
  • Mirroring: writing the same data to more than one disk.
  • Striping: splitting data to more than one disk.
  • Parity: extra data is stored to allow problem detection and repair.

RAID creates filesystems which spans more than one disk. RAID devices are created by combining partitions from several disks together.

To manage RAID devices:

mdadm

Raid device can then be used like any filesystem (/dev/mdX)

Levels​

  • RAID 0: only striping so gives performance boost. no redundancy so any disk fail will cause total lost.
  • RAID 1: only mirroring. each disk has a duplicate so at least two disks required. good for recovery.
  • RAID 5: rotating parity stripe. single disk failure will cause no data loss, only performance drop. must have 3 disks.
  • RAID 6: striping with dual parity. can handle 2 disk failures and requires 4 disk. RAID 6 is preferred to RAID 5.
  • RAID 10: mirrored and striped. at least 4 disks needed.

Configuring RAID​

  1. Create partitions using fdisk ((t)ype fd)

    sudo fdisk /dev/sdb
    sudo fdisk /dev/sdc

  2. Create RAID device using mmdadm.

    sudo mdadm --create /dev/md0 --level=1 --raid-disks=2 /dev/sdb1 /dev/dev/sdc1

  3. Format RAID device.

    sudo mkfs.ext4 /dev/md0

  4. Add device to /etc/fstab and capture configuration to file:

    echo "/dev/md0 /myraid ext4 defaults 0 2" >> /etc/fstab
    sudo bash -c "mdadm --detail --scan >> /etc/mdadm.conf"

  5. Mount RAID device.

    sudo mkdir /myraid
    sudo mount /dev/md0 /myraid

  6. Check RAID device

    cat /proc/mdstat

To stop RAID device:

sudo mdadm -S /dev/md0

Monitoring​

sudo mdadm --detail /dev/md0

# OR 
cat /proc/mdstat

# OR (requires configuring /etc/mdadm.conf, can set MAILADDR me@domain.com to receive email.
# need to turn on 
sudo systemctl start mdmonitor
sudo systemctl enable mdmonitor
mdmonitor

Hot Spares​

Can use hot spares for redundancy to fix an issue ASAP.

To initialize a RAID device with hot spare:

# -x 1 tells mdadm to configure one psare device.
sudo mdadm --create /dev/md0 -l 5 -n3 -x 1 /dev/sda8 /dev/sda9 /dev/sda10 /dev/sd11

To add it later:

sudo mdadm --fail /dev/md0 /dev/sdb2

To replace faulty with spare:

sudo mdadm --remove /dev/md0 /dev/sdb2
sudo mdadm --add /dev/md0 /dev/sdc2