In a previous post I described how I installed Arch Linux on BTRFS and how I decided to use a particular layout that would have come handy in case of problems.

So far I haven’t had any problem during my almost-daily upgrades (yes, Arch is very good to this respect), so the snapshot-before-upgrade technique described in that post didn’t help too much.

But today a huge upgrade was available: it was a combo Kernel + Gnome 3.8 + LibreOffice. I ran my script for taking a snapshot of my ROOT partition and started a pacman -Syu as usual.

However, this time things went wrong. Packages installed correctly but at reboot my laptop froze. I forced a power-off by pressing the on/off button, and when it restarted got a Kernel Panic on… BTRFS!

Things were pretty messed up. However with btrfs utils in the ram disk I was able to do a quick check. Data seemed to be there and a btrfsck was enough to make the Kernel Panic go away. However the laptop continued to freeze at boot, when GDM was started.

This was the perfect situation for reverting things back. I just needed to retrieve the name of the snapshot to restore. Since I name snapshots as ROOT@YYYYMMDD-HHMMSS I couldn’t remember the exact name, and since I wasn’t able to use my laptop I had to boot using an USB key.

Once retrieved the snapshot name, I just edited on the fly the Grub boot command and… Arch booting as usual, as if the upgrade didn’t take place!

Today I have been very glad of having spent some time trying to devise the setup that allowed me to operate in this way. There was only a missing thing… Having a snapshot with an easy-to-remember name.

So I have improved the script I usually run before any upgrade. Now it takes two snapshots of the current state: one whose name has the timestamp, and one that is always called LATEST. In this way booting using the previous state would be straightforward.

If you are interested, here it is the version 2.0 of the script I mentioned before:

#!/bin/bash

DATE=`date "+%Y%m%d-%H%M%S"`
read -p "Comment: " COMMENT

echo "$DATE
$COMMENT" > /SNAPSHOT

btrfs subvolume snapshot -r /run/btrfs-root/__current/ROOT /run/btrfs-root/__snapshot/ROOT@$DATE
btrfs subvolume delete /run/btrfs-root/__snapshot/LATEST
btrfs subvolume snapshot -r /run/btrfs-root/__snapshot/ROOT@$DATE /run/btrfs-root/__snapshot/LATEST

rm /SNAPSHOT

If you think of changing distribution, or re-installing the current one, I highly recommend you to switch to BTRFS. It could really save you the day!