#+title: Btrfs
#+setupfile: ../org-templates/page.org

** Create systemd.mount unit for Btrfs on external HDD
#+BEGIN_QUOTE
internet_archive is used here as an example.
#+END_QUOTE

Get the UUID of the Btrfs partition.

#+BEGIN_SRC shell
sudo blkid -s UUID -o value /dev/sda1

d3b5b724-a57a-49a5-ad1d-13ccf3acc52f
#+END_SRC

Edit ~/etc/systemd/system/mnt-internet_archive.mount.

#+BEGIN_SRC systemd
[Unit]
Description=internet_archive Btrfs subvolume
DefaultDependencies=yes

[Mount]
What=/dev/disk/by-uuid/d3b5b724-a57a-49a5-ad1d-13ccf3acc52f
Where=/mnt/internet_archive
Type=btrfs
Options=subvol=@internet_archive,compress=zstd:1

[Install]
WantedBy=multi-user.target
#+END_SRC

- ~DefaultDependencies=yes~ : The mount unit automatically acquires ~Before=umount.target~ and ~Conflicts=umount.target~. Local filesystems automatically gain ~After=local-fs-pre.target~ and ~Before=local-fs.target~. Network mounts, such as NFS, automatically acquire ~After=remote-fs-pre.target network.target network-online.target~ and ~Before=remote-fs.target~.
- ~Options=subvol=@internet_archive,compress=zstd:1~ : Use the subvolume ~@internet_archive~ and use zstd compression level 1.

#+BEGIN_QUOTE
Note that the name of the unit file, e.g. ~mnt-internet_archive.mount~, must correspond to the ~Where=/mnt/internet_archive~ directive, such that the filesystem path separator / in the ~Where~ directive is replaced by an en dash in the unit file name.
#+END_QUOTE

Reload the daemons and enable the mount unit.

#+BEGIN_SRC shell
sudo systemctl daemon-reload
sudo systemctl enable --now mnt-internet_archive.mount
#+END_SRC

** Setup encrypted external drive for backups
*** Prepare the external drive
#+begin_src shell
sudo cryptsetup --type luks2 -y -v luksFormat /dev/sda1
sudo cryptsetup -v luksOpen /dev/sda1 cryptbackup
sudo mkfs.btrfs /dev/mapper/cryptbackup
sudo mkdir /srv/backup
sudo mount -o noatime,compress=zstd:1 /dev/mapper/cryptbackup /srv/backup
sudo restorecon -Rv /srv/backup
#+end_src

*** Setup ~/etc/crypttab~
#+begin_src shell
sudo blkid -s UUID -o value /dev/sda1 | sudo tee -a /etc/crypttab
#+end_src

Add the following line to ~/etc/crypttab~:
#+begin_src shell
cryptbackup UUID=<UUID of /dev/sda1> none discard
#+end_src

*** Setup ~/etc/fstab~
#+begin_src shell
sudo blkid -s UUID -o value /dev/mapper/cryptbackup | sudo tee -a /etc/fstab
#+end_src

Add the following line to ~/etc/fstab~:
#+begin_src shell
UUID=<UUID of /dev/mapper/cryptbackup> /srv/backup btrfs compress=zstd:1,nofail 0 0
#+end_src

Reload the daemons:
#+begin_src shell
sudo systemctl daemon-reload
#+end_src

Mount the filesystems:
#+begin_src shell
sudo mount -av
#+end_src

*** btrfs-backup script
#+begin_src bash
#!/usr/bin/env bash

LOGFILE="/var/log/btrfs-backup.log"
SNAP_DATE=$(date '+%Y-%m-%d_%H%M%S')

# Check if device is mounted
if ! grep "/srv/backup" /etc/mtab >/dev/null; then
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] Backup device is not mounted." | tee -a "$LOGFILE"
    notify-send -i computer-fail "Backup device is not mounted"
    exit 1
fi

create_snapshot() {
    if ! btrfs subvolume snapshot -r "$1" "${1}/.snapshots/$2-$SNAP_DATE" >/dev/null; then
        echo "[$(date '+%Y-%m-%d %H:%M:%S')] Error creating snapshot of $1" | tee -a "$LOGFILE"
        notify-send -i computer-fail "Error creating snapshot of $1"
        exit 1
    else
        echo "[$(date '+%Y-%m-%d %H:%M:%S')] Create snapshot of $1: OK" | tee -a "$LOGFILE"
    fi
}

send_snapshot() {
    mkdir -p "/srv/backup/$SNAP_DATE"
    if ! btrfs send -q "${1}/.snapshots/$2-$SNAP_DATE" | btrfs receive -q "/srv/backup/$SNAP_DATE"; then
        echo "[$(date '+%Y-%m-%d %H:%M:%S')] Error sending snapshot of $1 to /srv/backup" | tee -a "$LOGFILE"
        notify-send -i computer-fail "Error sending snapshot of $1 to /srv/backup"
        exit 1
    else
        echo "[$(date '+%Y-%m-%d %H:%M:%S')] Send snapshot of $1 to /srv/backup: OK" | tee -a "$LOGFILE"
    fi
}

# Create root and home snapshots
create_snapshot "/" "root"
create_snapshot "/home" "home"

# Send root and home snapshots
send_snapshot "/" "root"
send_snapshot "/home" "home"
#+end_src

Move/copy the script to ~/etc/cron.daily/btrfs-backup~.