Felipe Alfaro Solana

This post roughly describes what I do when I want to non-destructively upgrade my Linux system. By non-destructive means a procedure that allows me to upgrade but also to rollback if something goes wrong. As an example, I wanted to upgrade my Ubuntu system from Jaunty to Karmic. Since Karmic is now Alpha 1, the chances of the upgrade going bad or eating my data were high. Here is where LVM and LVM snapshots come into scene.

Basically, the idea consists of taking a snapshot of the root filesystem using an LVM snapshot, then reboot the system to use the filesystem from the LVM snapshot as the root filesystem, perform an in-place upgrade of Jaunty to Karmic and see what happens. Since the upgrade takes place on a system running with the root filesystem mounted from the LVM snapshot, the original root volume is kept intact. Hence, if something goes wrong I can always reboot in order to use the original root filesystem and the system would behave as if no modifications were done at all during the upgrade.

The LVM snapshot volume should be big enough to, in the worst case, store a completely new Linux installation. The average Ubuntu installation requires less than 8GB of disk, so the LVM snapshot should be about that size plus some slack required to download the packages. In my case, and since I have free enough disk space, I chose 16GB just to be on the safe side.

Resize the root filesystem and root volume

This step is only required if there is no space in the volume group to accommodate for the snapshot volume. In my case, the volume group is full so I need to shrink the root filesystem and the root volume. resize2fs does not currently allow one to shrink a filesystem online, so I booted from the Jaunty LiveCD and entered rescue mode. From there:

# e2fsck -f /dev/root/root
# resize2fs /dev/root/root 80G
# lvresize -L 80G /dev/root/root

Create the LVM snapshot

To create an 16GB LVM snapshot volume of my root volume:

# lvcreate -s -n karmic-root -L 16G /dev/root/root

Prepare the boot environment

Mount the filesystem from the LVM snapshot volume and modify /etc/fstab to replace the device name where the original root filesystem is with the device name where the snapshotted rool filesystem lives:

# mount /dev/root/karmic-root /mnt
# vi /mnt/etc/fstab

In my case, the line for the new root filesystem looks like:

/dev/root/karmic-root /  ext4 defaults 0 1

Reboot into the snapshot

Trigger the grub menu and modify the kernel entry that corresponds to the Ubuntu system. The idea is to use the device name for the LVM snapshot. This is how the new kernel in the menu looks like:

kernel /vmlinuz-2.6.28-11-generic root=/dev/root/karmic-root ro

Then press b to boot the system. The system should boot normally, but instead of using the original root filesystem it should be using the filesystem from the LVM snapshot:

$ grep /dev/mapper /proc/mounts
/dev/mapper/root-karmic--root / ext4 rw,relatime,errors=remount-ro,barrier=1,data=ordered 0 0

In-place upgrade

I won’t describe how to do an in-place upgrade of Ubuntu. There are many resources out there that describe how to do that. The point here is that the upgrade will modify the snapshot while the original root filesystem is kept intact.

Destroy the snapshot

If something goes wrong with the update, and it usually goes wrong when upgrading to an Alpha version, to bring the system back to a usable state is just a matter of rebooting the system and using the right entry listed in grub. In-place upgrades of Ubuntu will typically add a new kernel to the list of entries in grub but won’t modify the existing ones.

After rebooting into the original system, the snapshot can be removed:

# lvremove /dev/root/karmic-root

If you don’t intend to experiment with upgrades, perhaps you want to resize the root LVM volume, then the root filesystem back to their original size.