I have a debian installation on a virtual machine with a btrfs filesystem at /dev/vda2. A subvolume named @ is mounted at / and @home is mounted at /home.

My aim is to do a clean reinstall of the OS while keeping the files in @home. The problem is that formatting the btrfs partition in the installer will clear all files, not just those in the @ subvolume. The solution is to pursuade the debian installer to install into a new subvolume on the btrfs partition.

There is probably a better way but this is how I did it. I have only done this with VMs so YMMV, buyer beware etc. All commands should be run as root.

• Prepare the btrfs partition. Create a new empty subvolume at root level, set it as the default subvolume to boot from, and then poweroff.

        mount -o subvolid=5 /dev/vda2 /mnt
        btrfs subvolume create /mnt/@new
        btrfs subvolume set-default /mnt/@new

        umount /mnt
        systemctl poweroff

• Begin a normal reinstall from the debian installer CD/USB. When partioning the disk, select the existing btrfs partition as root. Do not format the btrfs partition. The installer should detect the default @new btrfs subvolume and complete the install as normal.

• Boot into the reinstalled system. You can check that you have booted the @new subvolume by entering the mount command. The output should include a line for the subvolume mounted at root that looks something like the line shown below. However, @home subvolume is not mounted.

        /dev/vda2 at / type btrfs (rw,relatime,space_cache,subvolid=261,subvol=@new)

• At this point, you can delete the original root subvolume and rename the new subvolume. I'm not sure whether it is necessary but I also reset the new subvolume as the default after I had renamed it.

        mount -o subvolid=5 /dev/vda2 /mnt
        btrfs subvolume delete /mnt/@

        mv /mnt/@new /mnt/@
        btrfs subvolume set-default /mnt/@

        umount /mnt

• If you reboot now, you will probably find grub missing or misconfigured. So to avoid problems, reinstall and update grub.

        grub-install /dev/vda
        update-grub

• Finally, remove the home directory and skeleton files that the installer put in /home and add a new line to fstab to mount the @home subvolume, which should look something like the entry below.

        UUID=0123456789-abcdef... /home  btrfs   defaults,subvol=@home   0    0

You can now reboot into your reinstalled system and find your home directory still intact.

This solution achieved my aim of reinstalling while retaining the files in my home directory. However, it does rely on there being enough space in the btrfs partition to temporarily hold two root subvolumes.