Simple script for creating space-efficient backups to btrfs using rsync and snapshots

usb-backup.service

# Call the backup script with systemd, and since it is a background service, treat it like that by lowering IO and CPU priority
# so it will work in the background without disturbing your normal workflow.
#
# Author: Kai Krakow <hurikhan77@gmail.com>
# License: GPL3

[Unit]
Description=USB Backup Service

[Service]
Type=oneshot
ExecStart=/usr/local/sbin/usb-backup.sh
IOSchedulingClass=idle
IOSchedulingPriority=7
CPUSchedulingPolicy=batch
Nice=3

usb-backup.sh

#!/bin/bash
#
# Author: Kai Krakow <hurikhan77@gmail.com>
# License: GPL3
#
# Put something like this into your fstab:
# LABEL=usb-backup /mnt/private/usb-backup btrfs noauto,compress-force=zlib,subvolid=0,x-systemd.automount,x-systemd.idle-timeout=300
#
# This script will backup the current system to a scratch area on a btrfs formatted backup drive, then takes a snapshot
# of it named by the timestamp when the backup started.
#
# Todo list (in order of priority):
# * deduplicate backups after snapshotting
# * ensure unmounting the filesystem after btrfs has done all its cleanup work (ie, finished removing old snapshots)
# * use a private mount to mount subvol=0 of all btrfs filesystems and backup these instead (get rid of the exclude patterns)
#   ...that obviously only works right for single btrfs sources, figure out a nice way to support all fs types this way
# * support taking an atomic snapshot of the backup source first and use that as the backup source
# * support backup media rotation (so you could use two drives swapped every day, one attached, one kept off-site)
# * support remote backups

DATE=$(date +%Y%m%d-%H%M)
BASEDIR=/mnt/private/usb-backup

LOG=/var/log/usb-backup.log
KEEP_FREE=$((200*1024*1024))
MIN_AGE=$(date +%Y%m%d-%H%M --date="1 month ago")

clean_snapshots() {
        if [[ $(df "${BASEDIR}" | awk -e'/dev/ { print $4 }') -lt "${KEEP_FREE}" ]]; then
                SNAPSHOT=$(ls ${BASEDIR}/snapshots/ | sort | head -1)
                if [[ -z "${SNAPSHOT}" ]]; then return 0; fi
                if [[ "$(basename "${SNAPSHOT}")" < "system-${MIN_AGE}" ]]; then
                        echo "Removing one old snapshot ${SNAPSHOT}..."
                        btrfs subvolume delete "${BASEDIR}/snapshots/${SNAPSHOT}"
                fi
        fi
}

take_snapshot() {
        btrfs filesystem sync "${BASEDIR}"
        echo "Creating backup snapshot ${DATE} of scratch area..."
        btrfs subvolume snapshot -r "${BASEDIR}/current" "${BASEDIR}/snapshots/system-${DATE}"
        clean_snapshots
        echo "Ensuring backup filesystem sync..."
        btrfs filesystem sync "${BASEDIR}"
        sync
}

if [ -d "${BASEDIR}/snapshots" ]; then
        if [ ! -e "${BASEDIR}/snapshots/system-${DATE}" ]; then
                clean_snapshots
                echo "Starting backup to USB disk scratch area..."
                echo -n >${LOG}
                if /usr/bin/systemd-inhibit --what=sleep rsync -aAXH --timeout=300 --delete --delete-excluded --inplace --no-whole-file --stats --log-file=${LOG} \
                        --exclude "/dev/*" \
                        --exclude "/lost+found/" \
                        --exclude "/media/*" \
                        --exclude "/mnt/*" \
                        --exclude "/proc/*" \
                        --exclude "/run/*" \
                        --exclude "/sys/*" \
                        --exclude "/tmp/*" \
                        --exclude "/var/tmp/*" \
                        --exclude "tmpfs" \
                        --exclude "${LOG}" \
                        --exclude "${BASEDIR}" \
                        / ${BASEDIR}/current/
                then
                        take_snapshot
                else
                        if [ "$?" = 24 ]; then
                                echo "$0: Backup finished with warnings." >&2
                                take_snapshot
                        else
                                echo "$0: Backup failed!" >&2
                                exit 74
                        fi
                fi
                echo "Waiting for snapshot cleanup..."
                btrfs subvolume sync "${BASEDIR}"
        else
                echo "$0: Snapshot directory name already exists - skipping backup" >&2
                exit 69
        fi
else
        echo "$0: USB backup drive not mounted - skipping backup" >&2
        exit 75
fi

usb-backup.timer

# Manual backups are like no backups at all, so here's a systemd timer unit to do automated unattended backups. This pretends that
# you have configured your backup drive for on-access automounting through systemd. This can also wake up your computer if you
# allow systemd to do so. You have to adjust sleep timeout to be longer than the backup takes to run - otherwise you are left with
# a powered-on system in the morning.
#
# Author: Kai Krakow <hurikhan77@gmail.com>
# License: GPL3

[Unit]
Description=Daily USB Backup Timer

[Timer]
OnCalendar=03:00

[Install]
WantedBy=default.target