View source for Ubuntu: Migrate to Grub2

This howto has been written to document a migration path from how I used to partition my servers, to my current partitioning scheme.

I used to use a USB flash disk to contain the /boot partition. Usually I would have a RAID-1 with the LVM taking the entire block devices. While using the entire block device is possible, it does not allow any space for a bootloader on the disk. The USB boot had the advantage of keeping the kernel and initial ramdisk on a disk that was outside of the raid and LVM subsystem. This was important while using grub-legacy as it did not understand LVM. Since Grub2 now understands LVM and some raid modes, it is now possible to have everything inside of a LVM, so we can ditch the USB disk requirement.

'''Make sure you have a backup before you attempt this procedure'''

== Collect Information ==

Get the Raid information
 root@server:~# ''cat /proc/mdstat'' 
 Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
 '''md0''' : active '''raid1''' '''sda'''[0] '''sdb'''[1]
      976762496 blocks [2/2] [UU]
      
 unused devices: <none>

This tells us that we have a Raid-1 (mirror) with two disks and the raid array is ''md0''. Since a Raid-1 is made up of two identical disks, we have the option of splitting the raid array, creating a new array with one of the disks and then moving the LVM to the new array.

Get the LVM information
 root@server:~# ''pvscan'' 
  PV /dev/md0   VG '''lvmname'''   lvm2 [931.51 GiB / 714.51 GiB free]
  Total: 1 [931.51 GiB] / in use: 1 [931.51 GiB] / in no VG: 0 [0   ]

This tells us that we have a LVM with the name ''lvnname''. This particular LVM is 931.51GB in size, but of that, 714.51GB is free space. The destination drive therefore needs to be 217GB minimum.


== Split the Raid-1 ==

Fail one of the disks. In this case it will be /dev/sdb
 root@server:~# ''mdadm --manage --fail /dev/md0 /dev/sdb'' 

Remove the failed drive from the array
 root@server:~# ''mdadm --remove /dev/md0 /dev/sdb'' 

== Create a new Raid-1 with the spare disk ==

Sanitise the removed disk by removing the raid superblock
 root@server:~# ''mdadm --misc --zero-superblock /dev/sdb''

Partition the removed disk
 root@server:~# ''fdisk /dev/sdb''
 Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
 Building a new DOS disklabel with disk identifier 0xd85c2e05.
 Changes will remain in memory only, until you decide to write them.
 After that, of course, the previous content won't be recoverable.
 
 Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)
 
 WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
         switch off the mode (command 'c') and change display units to
         sectors (command 'u').
 
 Command (m for help): ''n''
 Command action
   e   extended
   p   primary partition (1-4)
 ''p''
 Partition number (1-4): ''1''
 First cylinder (1-121601, default 1): 
 Using default value 1
 Last cylinder, +cylinders or +size{K,M,G} (1-121601, default 121601): 
 Using default value 121601
 
 Command (m for help): ''t''
 Selected partition 1
 Hex code (type L to list codes): ''fd''
 Changed system type of partition 1 to fd (Linux RAID autodetect)
 
 Command (m for help): ''a''
 Partition number (1-4): ''1''
 
 Command (m for help): ''w''
 The partition table has been altered!
 
 Calling ioctl() to re-read partition table.
 Syncing disks.

Create a new Raid-1 with the removed disk
 root@server:~# ''mdadm --create --level=1 --raid-devices=2 /dev/md1 /dev/sdb1 missing''
 mdadm: array /dev/md1 started.

== Migrate LVM ==
The good news is that you can migrate the entire LVM while the system is running. The bad news is that it is a block level copy of all allocated blocks, so if you have a 1TB logical volume that has an empty filesystem in it, you will still be moving 1TB of data between disks. It can be slow. You have been warned.

Create the new physical volume on the new Raid-1
 root@server:~# ''pvcreate /dev/md1''

Extend the existing LVM onto the new physical volume
 root@server:~# ''vgextend lvmname /dev/md1''

Move all extents from the physical volume ''/dev/md0'' to ''/dev/md1''. '''This will take some time'''. You might consider doing this in a screen session.
 root@server:~# ''pvmove -v /dev/md0 /dev/md1''

Remove the physical volume /dev/md0 from the LVM
 root@server:~# ''vgreduce lvmname /dev/md0''
 root@server:~# ''pvremove /dev/md0''

== Tidying up ==

Stop the old Raid-1 array /dev/md0
 root@server:~# ''mdadm --manage --stop /dev/md0''

Sanitise the other disk by removing the raid superblock
 root@server:~# ''mdadm --misc --zero-superblock /dev/sda''

Partition the removed disk
 root@server:~# ''fdisk /dev/sda''
 Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
 Building a new DOS disklabel with disk identifier 0xd85c2e05.
 Changes will remain in memory only, until you decide to write them.
 After that, of course, the previous content won't be recoverable.
 
 Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)
 
 WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
         switch off the mode (command 'c') and change display units to
         sectors (command 'u').
 
 Command (m for help): ''n''
 Command action
   e   extended
   p   primary partition (1-4)
 ''p''
 Partition number (1-4): ''1''
 First cylinder (1-121601, default 1): 
 Using default value 1
 Last cylinder, +cylinders or +size{K,M,G} (1-121601, default 121601): 
 Using default value 121601
 
 Command (m for help): ''t''
 Selected partition 1
 Hex code (type L to list codes): ''fd''
 Changed system type of partition 1 to fd (Linux RAID autodetect)
 
 Command (m for help): ''a''
 Partition number (1-4): ''1''
 
 Command (m for help): ''w''
 The partition table has been altered!
 
 Calling ioctl() to re-read partition table.
 Syncing disks.

Add other disk to new Raid-1
 root@server:~# ''mdadm --manage --add /dev/md1 /dev/sda1''

Modify /etc/mdadm/mdadm.conf to reflect new UUID '''(Clean me up)'''
  root@server:~# ''mdadm --examine --scan >> /etc/mdadm/mdadm.conf''
  root@server:~# ''vi /etc/mdadm/mdadm.conf''

Update the initial ramdisk for all kernels
 root@server:~# ''update-initramfs -u''

== Remove USB boot device ==

This is a work-in-progress
 mkdir /bootnew
 cd /bootnew
 cp -auxv /boot/* .
 cd /
 umount /boot
 rmdir /boot
 mv /bootnew /boot
 dpkg-reconfigure grub-pc (upgrade-from-grub-legacy)