First version (not tested)

etc/systemd/system/btrfs-maintenance.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=Run BTRFS maintenance (scrubs and rebalance)
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/bin/btrfs-maintenance
+RemainAfterExit=yes
+
+[Install]
+WantedBy=default.target

etc/systemd/system/btrfs-maintenance.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Run BTRFS maintenance stuff (scrub and rebalance) monthly
+
+[Timer]
+OnCalendar=*-*-01 02:00:00
+Persistent=true
+
+[Install]
+WantedBy=timers.target

usr/local/bin/btrfs-maintenance

@@ -0,0 +1,30 @@
+#!/bin/bash
+set -e
+
+# Find all BTRFS formatted partitions
+PARTITIONS=()
+for i in $(lsblk -nrpo "name" -e 1,7,11); do
+    if [[ "$(lsblk -no FSTYPE "$i")" == "btrfs" ]]; then
+        PARTITIONS+=("$i")
+    fi
+done
+
+# Reallocate blocks that are less than 95% used
+for i in ${PARTITIONS[*]}; do
+    echo "Balance: $i"
+    btrfs balance start -dusage=95 "$i" &
+done
+
+# Wait for balance operations to finish
+wait
+
+# Scrub all partitions
+for i in ${PARTITIONS[*]}; do
+    echo "Scrub: $i"
+    btrfs scrub start -B "$i" &
+done
+
+# Wait for scrub operations to finish
+wait
+
+exit 0