Recover LVM Boot partition within a live CD (installing GRUB)

This allows you to recover the Master Boot record when a RAID 5 array fails.

1. Activate LVM partitions

   sudo mdadm --assemble --scan
   sudo vgchange -a y [name of volume group]

2. Mount all volumes to a single mount-point

   sudo mount /dev/mapper/[volume group]/[root volume] /mnt/
   sudo mount /dev/mapper/[volume group]/[usr volume] /mnt/usr/
   sudo mount /dev/mapper/[volume group]/[home volume] /mnt/home/
   ... etc

3. Mount and bind directories for grub to detect the underlying hardware

   sudo mount --bind /dev /mnt/dev
   sudo mount --bind /dev/pts /mnt/dev/pts
   sudo mount --bind /proc /mnt/proc
   sudo mount --bind /sys /mnt/sys

4. Jump into the new mounted system

   sudo chroot /mnt

5. Install GRUB

   grub-install /dev/sda
   grub-install --recheck /dev/sda
   update-grub

6. Exit the environment and restart

   exit
   sudo umount /mnt/dev/pts
   sudo umount /mnt/*
   sudo shutdown -r now

Recovering a RAID 5 array

Recently had a RAID 5 array fail on me. These are the steps I took to recover the data.

NOTE! The order of the disks in /dev/sdX follows the numbering of the ports on the motherboard. So if a disk is plugged into port 0, it will show up as /dev/sda. Keep this in mind as if you remove a failed disk, it might mess up the references in the array!.

1. Boot up from a Live CD
2. Install the software RAID management software

   sudo apt-get install mdadm

3. Make sure RAID and LVM are unmounted

   sudo vgchange -a n [name of your volume group]  sudo mdadm -S /dev/md0

4. Copy the failed disk to the new disk (in case the disk was the boot disk, you need to copy that flag across so that your system boots)

   sudo sfdisk -d /dev/sdx | sudo sfdisk /dev/sdy

5. Check that the disks are the same

   sudo fdisk -l

6. Mount the RAID array, and remove the failed disk

   sudo mdadm --assemble --scan  sudo mdadm --manage --remove /dev/sdx

7. Add the new disk

   sudo mdadm --manage --add /dev/sdy

8. Watch the progress as mdadm rebuilds your RAID array

   sudo cat /proc/mdstat