Install Ubuntu with BTRFS and more subvolumes

Question

I want to install an Ubuntu Server with a btrfs filesystem (in a RAID1 way).

The problem is that I can't see the option in the installer to make more subvolumes. The installer creates two by default: @ and @home, and I want to separate also @var, @tmp, ...

Is there any option to do this? Maybe with the shell... but I think the installer doesn't understand the subvolumes feature of btrfs, so you can't mount each partition in each subvolume...

Answer 1

This should be easy enough to change after you have finished your initial install.

• One neat way is to take two snapshots of @, calling them @var and @tmp.
• Then delete everything in @var except for @var/var, them move the contents of @var/var/* up to @var/* and delete @var/var. Same for @tmp/tmp/.
• Then modify /etc/fstab with the new subvolume mount points for /var and /tmp.
• Reboot.
• Finally (I'm not so sure about this last bit) you should be able to delete the original contents of @/var and @/tmp by mounting the subvolume @ under /mnt (so the same subvolume is mounted twice, once as / and once as /mnt) and deleting /mnt/var and /mnt/tmp.

Answer 2

You're right, the installer is not aware of BTRFS subvolumes, and this is still the case for 18.04.

Using a separate subvolume for /var/log/ allows to restore a snapshot of / without loosing the logs.

• Do the server setup as usual, at the Finish installation step, select Go Back and Execute a shell.

• List all your target file systems:

  mount | grep target
  
  /dev/md-0 on /target type btrfs (rw,noatime,space_cache,subvolid=257,subvol=/@)
  /dev/md-0 on /target/home type btrfs (rw,noatime,space_cache,subvolid=258,subvol=/@home)
  proc on /target/proc type proc (rw,nosuid,nodev,noexec,relatime)
  devtmpfs on /target/dev type devtmpfs (rw,nosuid,relatime,size=475608k,nr_inodes=118902,mode=755)
  

• Take a note of the btrfs device, in this example /dev/dm-0.

• Now un-mount all of your mounted file systems.

  umount /target/dev
  umount /target/proc
  umount /target/boot/efi
  umount /target/home
  umount /target/
  

• Mount your flat btrfs filesystem :

  cd /tmp
  mkdir work
  mount /dev/dm-0 work
  cd work
  

• Verify the mount is correct (should show @ and @home):

  ls 
  
  @ @home
  

• Create your additional subvolumes (@tmp, @var, @var-log)

  btrfs subvolume create @tmp
  btrfs subvolume create @var
  btrfs subvolume create @var-log
  

• Move the data

  mv @/var/log/* @var-log/
  mv @/var/* @var/
  
  # Remove data from tmp
  rm @/tmp/* @/tmp/.*
  
  # For 18.04, remove the swapfile since it won't work on btrfs
  rm @/swapfile
  

• Add the new subvolumes to fstab, the device part may be different than the previous mount command, copy the device part from the already existing mount points.

  ...
  /dev/mapper/root-root /               btrfs   noatime,subvol=@ 0       1
  /dev/mapper/root-root /home           btrfs   noatime,subvol=@home 0       2
  /dev/mapper/root-root /var            btrfs   noatime,subvol=@var 0       2
  /dev/mapper/root-root /var/log        btrfs   noatime,subvol=@var-log 0       2
  

• Unmount

  cd /tmp
  umount work
  sync
  

• exit, then Finish the installation

• Install and configure snapper, a great tool for automatizing snapshots.