Translation(s): English

FileSystem > Btrfs > Btrfs migration

Here is a simple tutorial guide of system migration from ext4 to btrfs. This migration should be safer with dual boot system.

Contents

  1. Overview
  2. Dual boot preparation
  3. Install 2nd OS
  4. Migration of the existing root filesystem to Btrfs
  5. Use snapshot with btrfs

Overview

Although you can boot the system with a USB memory stick with Debian installer and do essentially the same to migrate system to btrfs, it is safer strategy to migrate a system while making system dual boot.

Dual boot preparation

Here is required preparation before running sudo update-grub to set up grub menu.

More through boot system update for UEFI system with NVRAM variables is offered by sudo dpkg-reconfigure grub-efi-amd64.

Semi-manual grub configuration

So far, grub auto OS detection is flaky for btrfs, Let me recap simple semi-manual grub configuration.

I usually add following as /etc/grub.d/40_fixed_linux (The first line should be #!/bin/bash without space. Intentional mistype to avoid missing line): 

# !/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
menuentry 'Debian GNU/Linux --- main SSD' --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-fixed-fe3e1db5-6454-46d6-a14c-071208ebe4b1' {
        load_video
        insmod gzio
        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
        insmod part_gpt
        insmod btrfs
        search --no-floppy --fs-uuid --set=root fe3e1db5-6454-46d6-a14c-071208ebe4b1
        echo    'Loading Linux from /dev/nvme0n1p5 ...'
        linux   /@rootfs/vmlinuz root=UUID=fe3e1db5-6454-46d6-a14c-071208ebe4b1 ro rootflags=subvol=@rootfs  quiet
        echo    'Loading initial ramdisk from /dev/nvme0n1p5 ...'
        initrd  /@rootfs/initrd.img
}
menuentry 'Debian GNU/Linux --- sub SSD' --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-fixed-51f9cd11-30b3-4d99-b2ed-fe411fa22ee6' {
        load_video
        insmod gzio
        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
        insmod part_gpt
        insmod btrfs
        search --no-floppy --fs-uuid --set=root 51f9cd11-30b3-4d99-b2ed-fe411fa22ee6
        echo    'Loading Linux from /dev/nvme1n1p1 ...'
        linux   /@rootfs/vmlinuz root=UUID=51f9cd11-30b3-4d99-b2ed-fe411fa22ee6 ro rootflags=subvol=@rootfs  quiet
        echo    'Loading initial ramdisk from /dev/nvme1n1p1 ...'
        initrd  /@rootfs/initrd.img
}

Here UUID needs to be adjusted accordingly. If one of the system is ext4, adjustment needs to be made.

Note on `/boot/efi/EFI/debian/grub.cfg`

If you install multiple Debian systems to dual boot, this becomes quite tricky.

Install 2nd OS

The debian-installer can installs Debian system to btrfs partition and it uses @rootfs subvolume (bullseye).

Migration of the existing root filesystem to Btrfs

Debian grub auto configuration script (as of 2022/buster) is compatible with this usage.

Keeping the root filesystem in a subvolume makes it more flexible. I usually deploy flat layout to keep my home directory /home/<username> as an independent subvolume /@<username> and explicitly mount it via /etc/fstab. You may alternatively keep the whole /home as a subvolume.

Use snapshot with btrfs

One of the greatest feature of btrfs is snapshot. There are a few interesting packages in Debian:

For maximum flexibility and simplicity, I am using my shell scripts bss .