diff --git a/auto-rules-io/usr/bin/big-btrfs-balance b/auto-rules-io/usr/bin/big-btrfs-balance
index 08b8280..a3ce899 100755
--- a/auto-rules-io/usr/bin/big-btrfs-balance
+++ b/auto-rules-io/usr/bin/big-btrfs-balance
@@ -7,12 +7,45 @@ FolderRoot=$(btrfs device stats / | tail -n1 | cut -f1 -d] | cut -f2 -d[)
 
 FolderHome=$(btrfs device stats /home | tail -n1 | cut -f1 -d] | cut -f2 -d[)
 
+# A usage filter also picks the last, partly filled chunk that the previous
+# balance just packed, so a filesystem that changed little since then has the
+# same data rewritten on every boot and frees nothing. Freeing a chunk needs
+# at least a chunk of allocated-but-unused space. With single, DUP or RAID1
+# profiles, data chunks are at most 1 GiB, and the kernel reports the
+# metadata chunk size, which is at least 256 MiB (its data chunk size there
+# is only the upper limit). On smaller filesystems both are 10% of the size,
+# rounded up to 16 MiB.
+balance() {
+    local type slack size tenth data_chunk=1073741824 meta_chunk filters=()
+    meta_chunk=$(cat "/sys/fs/btrfs/$(findmnt -no UUID "$1")/allocation/metadata/chunk_size" 2>/dev/null)
+    meta_chunk=${meta_chunk:-268435456}
+    size=$(btrfs filesystem usage -b "$1" 2>/dev/null | awk '/Device size:/ {print $3; exit}')
+    if (( size )); then
+        tenth=$(( (size / 10 + 16777215) / 16777216 * 16777216 ))
+        (( tenth < data_chunk )) && data_chunk=$tenth
+        (( tenth < meta_chunk )) && meta_chunk=$tenth
+    fi
+    while read -r type slack; do
+        case $type in
+            Data) (( slack >= data_chunk )) && filters+=(-dusage=50) ;;
+            Metadata) (( slack >= meta_chunk )) && filters+=(-musage=50) ;;
+            Data+Metadata) filters+=(-dusage=50 -musage=50) ;;
+        esac
+    done < <(btrfs filesystem df -b "$1" | awk -F'[ ,=]+' '$1 == "Data" || $1 == "Metadata" || $1 == "Data+Metadata" {
+        for (i = 2; i < NF; i++) { if ($i == "total") t[$1] += $(i+1); if ($i == "used") u[$1] += $(i+1) }
+    } END { for (k in t) print k, t[k] - u[k] }')
+
+    if (( ${#filters[@]} )); then
+        nice -n+20 ionice -c 3 btrfs balance start "${filters[@]}" "$1"
+    fi
+}
+
 if [ "$FolderRoot" != "" ]; then
-    nice -n+20 ionice -c 3 btrfs balance start -dusage=50 -musage=50 /
+    balance /
 fi
 
 if [ "$FolderHome" != "" ] && [ "$FolderHome" != "$FolderRoot" ]; then
-    nice -n+20 ionice -c 3 btrfs balance start -dusage=50 -musage=50 /home
+    balance /home
 fi