Script to conditionally run `btrfs balance`, suitable as a cron job

btrfs-balance.bash

#!/bin/bash

allocated_threshold="95" # Don't run balance if less than this much is allocated (percentage)
fs="/"

# The limit (in chunks) is set to be:
# 75% of unused allocated data (in GiB), rounded up to nearest integer
# This may need to be adjusted depending on the setup
balance_awk_formula='{printf("-dlimit=%d\n",($1 - $2) * 0.75 + 1)}'

unallocated_awk_formula='{printf("%d\n",$2 / $1 * 100)}'

df_sed_expr='s/Data,\s+\w+:\s+total=([0-9.]+)GiB,\s+used=([0-9.]+)GiB/\1 \2/p'
show_sed_expr='s/.*devid.*size\s+([0-9.]+)GiB\s+used\s+([0-9.]+)GiB.*/\1 \2/p'

btrfs="/sbin/btrfs"
awk="/usr/bin/awk"
sed=(/bin/sed -rn -e)

btrfs_df=($btrfs filesystem df --gbytes $fs)
btrfs_show=($btrfs filesystem show --gbytes $fs)
btrfs_balance=($btrfs balance start --enqueue)

allocated_percentage() {
        total="$1"
        allocated="$2"
        echo "$total" "$allocated" | $awk "$unallocated_awk_formula"
}

balance_filter() {
        allocated="$1"
        used="$2"
        echo "$allocated" "$used" | $awk "$balance_awk_formula"
}

df_nums() {
        "${btrfs_df[@]}" | "${sed[@]}" "$df_sed_expr"
}

show_nums() {
        "${btrfs_show[@]}" | "${sed[@]}" "$show_sed_expr"
}

if [[ $(allocated_percentage $(show_nums)) -ge $allocated_threshold ]]; then
        "${btrfs_balance[@]}" "$(balance_filter $(df_nums))" "$fs"
fi