RAID - mdadm (Software RAID)

Grow RAID1 to RAID5

  1. Existing RAID1 array is md127, array to be removed is md126
  2. Backup all data
  3. Make sure the devices are unmounted
    sudo umount /dev/md126
  4. Check the details
    sudo mdadm --detail /dev/md126
  5. Stop the device
    mdadm -S /dev/md126
  6. Zero out the superblocks of physical devices that belong to the MD device
    sudo mdadm --zero-superblock /dev/sde1
  7. Change the array from RAID1 to RAID5 (still only 2 drives)
    sudo mdadm /dev/md127 --grow --level=5
  8. Add the new drive to the array
    sudo mdadm /dev/md127 --add /dev/sde1
  9. Start the sync for all 3 devices
    sudo mdadm --grow /dev/md127 --raid-devices 3
  10. Watch the progress with
    watch -n 15 -d cat /proc/mdstat

Desired configuration of the disks

hdd partition raid1 lvm mount point
sda sda1 md0 - /
sdb sdb1 md0
sda sda2 - - swap
sdb sdb2 - swap
sdc sdc1 md1 vg1 /home, /srv,
sdd sdd1 md1 vg1 /media
sde sde1 md2 vg1
sdf sdf1 md2 vg1
sdg sdg1 md3 vg2 /backup
sdh sdh1 md3 vg2 /backup

see also Hardware configuration

Replace a failed RAID disk

Replace the disk, then copy the partition information from the good disk, randomize the UUID, and re-read the partition information into the system. First, install gdisk from the Debian Universe repositories.

apt-get install gdisk
sgdisk -R=/dev/sdb /dev/sda
sgdisk -G /dev/sdb

Taken from How can I quickly copy a GPT partition scheme from one hard drive to another?

Other resources on mdadm and How do I rename an mdadm raid array?:

Setup configuration:

mdadm -Es >> /etc/mdadm/mdadm.conf

Install Base

  1. Install Ubuntu Hardy (8.04) Alternate Install: F6-F6-Expert Mode
  2. Install Raid1 with MD package activated and assign mount points

Check wether all volumes get mounted during system boot

to check wether root and swap are mounted, enter:

free -m -t

to check mismatching uuid's, enter:

ls -la /dev/disk/by-uuid
cat /etc/fstab

to fix, do:

vim /etc/fstab

replace the uuid's found in fstab with the ones found in /dev/disk. Make sure you copy the correct uuid (md0, md1) to the respective entry in fstab.


Most Debian and Debian-derived distributions create a cron job which issues an array check at 0106 hours each first Sunday of the month in /etc/cron.d/mdadm. This task appears as resync in /proc/mdstat and syslog. So if you suddenly see RAID-resyncing for no apparent reason, this might be a place to take a look.

Normally the kernel will throttle the resync activity (c.f. nice) to avoid impacting the raid device performance.

However, it is a good idea to manage the resync parameters to get optimal performance.

Raid 1, 5, 6

Rebuild speed

  • Get current system values:
    sudo sysctl
    sudo sysctl
  • Default system values on Debian 10: = 1000 = 200,000
  • Reduce max limit to make server more responsive during resync (2021-12-05):
    sudo sysctl -w,000
    sudo sysctl -w,000


  • Get current read-ahead (in 512-byte sectors) per Raid device (default value is 512 on Debian 10):
    blockdev --getra /dev/mdX
  • Set to 32 MB:
    blockdev --setra 65536 /dev/mdX
  • Set to 65536 on a server with 32GB memory, 32768 on a server with 8GB memory (2021-12-05)

Disable NCQ

  • Get NCQ depth on each physical Drive in Raid (default value is 31):
    cat /sys/block/sdX/device/queue_depth
  • Disable NCQ:
    echo 1 > /sys/block/sdX/device/queue_depth

Raid 5, 6 only


It records the size (in pages per device) of the stripe cache which is used for synchronising all write operations to the array and all read operations if the array is degraded. The default is 256 which equals to 3MB memory consumption. Valid values are 17 to 32768. Make sure your system has enough memory available: memory_consumed = system_page_size * nr_disks * stripe_cache_size.

  • Find system page size, on Debian 10 this is 4096:
    getconf PAGESIZE
  • Set to 384MB memory consumption on a 3 disk:
    sudo echo 32768> /sys/block/md0/md/stripe_cache_size
  • Set to 32768 on a server with 32 GB memory, set to 16384 on a server with 8 GB memory (2021-12-05)

Prepare RAID with single disk

Prepare new disk

  1. If the new disk contains partitions
    1. Stop any Raid partitions with
      mdadm --stop /dev/md1
      mdadm --remove /dev/md1
    2. Remove the superblocks
      mdadm --zero-superblock /dev/sdX1
    3. Remove existing partitions with fdisk /dev/sdX
  2. Create a new partition utilizing the full disk space. When asked, remove the existing signature. Change partition type to Linux RAID
    sudo fdisk /dev/sdX
    Command (m for help): d
    Command (m for help): n
    Command (m for help): t,29
  3. Create the RAID
    mdadm --create /dev/mdX --level=raid1 --raid-devices=2 /dev/sdX1 missing
  4. Check the RAID was created
    cat /proc/mdstat
    ls /dev/md*

Move RAID to a new machine

  1. Scan for the old raid disks
    sudo mdadm --assemble --scan
  2. Mount the raid manually to confirm
    sudo mount /dev/md0 /mnt
  3. Append info to mdadm.conf
    mdadm --detail --scan >> /etc/mdadm/mdadm.conf
  4. Update initramfs
    update-initramfs -u


  • Make sure the output of “mdadm –detail –scan” matches your /etc/mdadm/mdadm.conf
  • Examine /etc/fstab