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MountainX
Asked: 2020-01-22 21:31:37 +0800 CST

Where are my BTRFS subvolumes?

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I just installed Ubuntu 18.04 server on a VPS using the Ubuntu installer (from a mounted ISO image). I selected BTRFS as my filesystem type during the manual partitioning phase of installation.

Now that the installation has completed, I do not see @ or @home like I normally would on the non-server install. I don't care about a @home subvolume here, but I do not want the top level volume mounted at /.

lsblk:

vda    252:0    0   25G  0 disk
|-vda1 252:1    0    1M  0 part
|-vda2 252:2    0   20G  0 part /
`-vda3 252:3    0    5G  0 part [SWAP]

The produces no results:

btrfs su li /

I tried this next:

btrfs filesystem show | awk '/ path /{print $NF}'
/dev/vda2

and:

# findmnt
TARGET    SOURCE     FSTYPE     OPTIONS
/         /dev/vda2  btrfs      rw,relatime,space_cache,subvolid=5,subvol=/

How can I create BTRFS subvolumes during installation of 18.04 server?

I would like to create @ (for /) and @varlog (for /var/log) and maybe others.

I would like to use Snapper for hourly snapshots. Is there a recommended way to set up Ubuntu server with BTRFS and Snapper?

system-installation btrfs filesystem server 18.04

2 Answers

  1. Best Answer

    You can move your / to a subvolume this way:

    1. Create a shapshot of your filesystem.

       btrfs sub snap / /@

    2. Mount the new subvolume to /mnt.

       mount -o subvol=@ /dev/vda2 /mnt

    3. Chroot to the subvolume and update grub.

       cd /mnt
 mount -o bind /dev  dev
 mount -o bind /sys  sys
 mount -o bind /proc proc
 chroot /mnt
 update-grub
 exit

    4. Update /mnt/etc/fstab adding there subvol=@ as an option.

    5. Reboot. You will boot to the subvolume. Make sure that it is the case by

       mount | grep vda2

    It should show something like

       /dev/vda2 on / type btrfs (rw,relatime,space_cache,subvolid=257,subvol=/@)
    1. Now you can mount the top subvolume somewhere and delete its contents except /@.
    • 4

  2. Here is a solution which works on Ubuntu Server 20.04. It creates @ subvolume before the first boot and removes all files from / volume.

    1. Do the installation of Ubuntu 20.04 with BTRFS root partition, but don't reboot after the system installation.

    2. Switch to terminal (Alt+F2).

    3. Switch to root user and umount all devices other than the BTRFS partition:

      sudo -i
umount /target/boot/efi
umount -l /target/run
umount /target/cdrom

      If you have other mount points (i.e. /home) umount them too.

    4. Create @ subvolume and move all files into it:

      cd /target
btrfs subvolume create @
ls | grep -v @ | xargs mv -t @

    5. umount your BTRFS partition and mount it again pointing this time to the @ subvolume. It is also a good time to define some extra mounting options (in my example there are some recommended options for SSD devices). I assume BTRFS volume is under /dev/sda2 (adjust accrdingly)

      cd /
umount /target
mount -o subvol=@,ssd,noatime,space_cache,commit=120,compress=zstd:2 /dev/sda2 /target

    6. Now it's time to complete the system by mounting all necessary devices and then switch to it with chroot (I assume your boot partition is /dev/sda1)

      mount /dev/sda1 /target/boot/efi
mount --bind /proc /target/proc
mount --bind /dev /target/dev
mount --bind /sys /target/sys
chroot /target

      If you've created some other partitions (i.e. /home) mount them here too

    7. Open fstab in editor:

      vi /etc/fstab

      and update the line with BTRFS partition, i.e.:

      UUID=xxx / btrfs default,subvol=@,ssd,noatime,space_cache,commit=120,compress=zstd:2  0 0

    8. Finally, setup the bootloader (I assume /dev/sda)

      update-initramfs -u -k all
grub-install --recheck /dev/sda
update-grub 
exit

    9. Get back to your installer (Alt+F1) and reboot. Ubuntu should boot to your @ subvolume.

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