The tool we use to do raid is called mdadm. Some useful commands are:

• Show information about this raid device

sudo mdadm --detail /dev/md<device number>

• Show information about raid state, according to the kernel

cat /proc/mdstat

sudo mdadm --manage /dev/md0 --re-add /dev/<disk>

add the new disk to the collection and grow the array

mdadm --add /dev/md0 /dev/sd[X]
mdadm --grow /dev/md0 --raid-devices=4 # if there were three disks in the array before, and you've just added the fourth.

mdadm --assemble --scan
mdadm -R /dev/md0

To swap out a disk with a new one in the array run:

mdadm /dev/md0 --add /dev/<new> --fail /dev/<old> --remove /dev/<old>

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

mdadm /dev/md1 --remove <raid element>
mdadm --zero-superblock <raid element>

Note: this only applies to striping such as raid 0, 5, 6.

To ensure maximum speeds from the array do the following.

• Use to obtain the 'RAID stride' and 'RAID stripe width'

dumpe2fs <filesystem device>

• Run the following command and note the 'Chunk Size'

sudo mdadm --detail <raid device>

• https://gryzli.info/2015/02/26/calculating-filesystem-stride_size-and-stripe_width-for-best-performance-under-raid/
  • Stride size = [RAID chunk size] / [Filesystem block size]
  • Stripe width = [ Stride size ] * [ Number of data-bearing disks]

sudo tune2fs -E stride=128,stripe_width=512 /dev/mapper/zoidberg-stuff

• We need a bios boot partition and an Raid partition

$ parted /dev/sdx
(parted) mklabel gpt
(parted) mkpart primary 1049KB 3146KB
(parted) set 1 bios_grub on
(parted) mkpart primary 3146kB 2000GB
(parted) print free                                                    
Model: ATA WDC WD20EARX-00Z (scsi)
Disk /dev/sdd: 2000GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt

Number  Start   End     Size    File system  Name     Flags
        17.4kB  1049kB  1031kB  Free Space
 1      1049kB  3146kB  2097kB               primary  bios_grub
 2      3146kB  2000GB  2000GB               primary  raid
        2000GB  2000GB  73.2kB  Free Space

• Next you need to install grub to the disk.

$ grub-install --recheck /dev/sdx
Installation finished. No error reported.

• Finally we need to add the drive to the array.

$ sudo mdadm /dev/md0 --manage --add /dev/sdd2
mdadm: added /dev/sdx2
$ sudo mdadm --detail --misc /dev/md0
/dev/md0:
        Version : 1.2
  Creation Time : Sat Mar 31 13:09:17 2012
     Raid Level : raid5
     Array Size : 3906776064 (3725.79 GiB 4000.54 GB)
  Used Dev Size : 1953388032 (1862.90 GiB 2000.27 GB)
   Raid Devices : 3
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Sun May 13 03:40:38 2012
          State : clean
 Active Devices : 3
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 1

         Layout : left-symmetric
     Chunk Size : 512K

           Name : zoidberg:0  (local to host zoidberg)
           UUID : a69274c7:b1b48b1f:7ef9d7c9:87f3b729
         Events : 58

    Number   Major   Minor   RaidDevice State
       3       8       34        0      active sync   /dev/sdc2
       1       8        2        1      active sync   /dev/sda2
       2       8       18        2      active sync   /dev/sdb2

       4       8       50        -      spare   /dev/sdx2

• At this point the disk is a hot spare. To expand the array onto it you'll need to run the following.

mdadm --grow /dev/md0 --raid-devices=4 # if there were three disks in the array before and you now want to use four.

This is a guide to reinstall grub on an lvm and raid setup. My example includes a raid5 of 3 disks:

/dev/sda
/dev/sdb
/dev/sdc

Let's start :)

• Run through the live disk install until you reach “Detect Disks”
• Mount the root filesystem

mount /dev/mapper/<hostname>-root /mnt

• If you have a separate boot and usr partitions you will also need to do

mount /dev/mapper/<hostname>-boot /mnt/boot
mount /dev/mapper/<hostname>-usr /mnt/usr

• Bind mount the important filesystems

for i in /dev /dev/pts /proc /sys; do mount --bind $i /mnt$i; done

• Chroot into your now mounted install

chroot /mnt /bin/bash

• Ensure the assembled array matches your old mdadm.conf

cat /etc/mdadm/mdadm.conf | grep ARRAY
mdadm --assemble --scan

• Ensure all you drives are listed in /boot/grub/devices.map

(hd0)  /dev/disk/by-id/ata-<somedisk>
(hd1)  /dev/disk/by-id/ata-<somedisk>
(hd2)  /dev/disk/by-id/ata-<somedisk>

• run update-grub to generate a new grub.cfg

update-grub

• Run grub-install on each disk to reinstall grub onto them. This should ensure that any of the disks can boot the machine

grub-install /dev/sda
grub-install /dev/sdb
grub-install /dev/sdc

• Finally, just to be nice, unmount all the filesystems

for i in /sys /proc /dev/pts /dev /mnt/boot /mnt/usr /mnt; do sudo umount /mnt$i; done

• Cross you fingers and reboot

reboot

• Call grub-install with –debug
• Call grub-mkimage with –verbose
• Call grub-mkdevicemap with –verbose
• Call grub-mkconfig by calling it via bash like so: /bin/bash -x grub-mkconfig

• This is the process grub-install should be doing for you.

grub-mkimage -O i386-pc --output=/boot/grub/core.img --prefix="(<hostname>-root)/boot/grub" \
biosdisk ext2 mdraid raid raid5rec lvm

• The names at the end are modules found is /usr/lib/grub/i386-pc/

• To install the image on /dev/sda run

grub-setup --verbose --directory=/boot/grub --device-map=/boot/grub/device.map /dev/sda

• Increase the target sync speed when the array is in use.

  sudo sysctl -w dev.raid.speed_limit_min=50000

• Increase the read ahead.

  sudo blockdev --setra 65536 /dev/md1

• Increase the cache size to 32MiB

  sudo bash -c 'echo 32768 > /sys/block/md1/md/stripe_cache_size'

• When resizing some arrays you may get

  [ 4780.580972] md/raid:md0: reshape: not enough stripes.  Needed 512
  [ 4780.597961] md: couldn't update array info. -28

  in which case run

  echo 600 > /sys/block/md0/md/stripe_cache_size