Ubuntu – Why the @ in Btrfs subvolume names

What is the correct command to create a snapshot of my home partition on Ubuntu 12.04?

Given the fact that home is on a btrfs partition and it is mounted as /home, the correct command is:

sudo btrfs subvolume snapshot /home/ /home/<any_name_you_choose_for_the_snapshot>

Here are my results:

Before running the command today:

$ btrfs subvolume list /home  
ID 256 top level 5 path @home  
ID 257 top level 5 path @home/@snapshot_20120521_1936

The command:

$ sudo btrfs subvolume snapshot /home/ /home/@snapshot_20120611_1307  
Create a snapshot of '/home/' in '/home/@snapshot_20120611_1307'

After running the command today:

$ btrfs subvolume list /home  
ID 256 top level 5 path @home  
ID 257 top level 5 path @home/@snapshot_20120521_1936  
ID 258 top level 5 path @home/@snapshot_20120611_1307

The only issue with this method is that snapshots show up when you ls (list) /home.

The Ubuntu community Wiki has been updated with a solution for this. Here it is:

How to work with snaphots in Ubuntu's layout

In order to work with snapshots of / or /home in the Ubuntu layout it is very convenient to mount the btrfs filesystem at a separate location, and work from the top of the btrfs tree, rather than from the mounted subvolumes. <-- that is the solution. Unfortunately, it is not convenient.

sudo mount /dev/sdX# /mnt

To create a snapshot use the same syntax I used above:

sudo btrfs subvolume snapshot /mnt/@ /mnt/@_snapshot

This will create a snapshot of the @ subvolume named @_snapshot located also in the top of the btrfs tree. Since it is in the top of the tree, it will not show up when listing files in a mounted volume.

To roll back to a snapshot, you simply need to change its name to the name that ubuntu mounts and reboot. Here's how to change the name:

sudo mv /mnt/@ /mnt/@_badroot
sudo mv /mnt/@_snapshot /mnt/@

To delete a snapshot use:

sudo btrfs subvolume delete /mnt/@_badroot

btrfs snapshots are subvolumes in themselves, and self-contained, deleting the old @ subvolume like this is fine, provided we have a replacement.

NOTE: The btrfs-tools command set-default will break Ubuntu's layout.

Sigh, seems I posted a bit too soon, with a bit of help of the almighty Zosma on IRC, we found out how this is supposed to work.

Subvolumes @ and @home are children of the btrfs root, so / is not actually the root of the file system. The UUID of the btrfs root is supplied in /etc/fstab twice, providing the mounts for / and /home. Mounting the actual root of the file system shows the subvolumes created by the installer:

root@box:~# mkdir /mnt/btrfs
root@box:~# mount UUID=f8963df3-1320-4bc0-a125-62be185b029e /mnt/btrfs
root@box:~# ls /mnt/btrfs
@  @home

So that's where they went! Creating a new subvolume in the exact way the ubuntu installer did is now easy:

root@box:~# btrfs subvolume create /mnt/btrfs/@data
Create subvolume '/mnt/btrfs/@data'

Now, as we've created a subvolume in the btrfs root instead of in the root filesystem for the install, /data/ won't show up by itself. Luckily, it's trivial to copy the trick used by the installer by examining /etc/fstab and add another mount for /data:

UUID=f8963df3-1320-4bc0-a125-62be185b029e /data  btrfs  defaults,subvol=@data 0 2

Last step is to create the mount point: mkdir /data. After a reboot, we'll have a /data directory mounted from a separate btrfs subvolume.

… Profit!