Initial commit with btrfs delta

btrfs/btrfs-subvolume-delta

@@ -0,0 +1,47 @@
+#!/bin/sh
+set -e
+
+SRC="$1"
+RECV="$2"
+DSTDIR="$SRC/.snap"
+CDATE="$(date '+%s')"
+PREV="prev"
+CUR="cur"
+TMP="tmp"
+
+msg() {
+	echo "$1" >&2
+}
+
+delta() {
+	[ -d "$(realpath "$DSTDIR/$CUR")" ] || return 1
+	msg "Attempting to send delta"
+	btrfs -v send --compressed-data -p "$DSTDIR/$CUR" "$DSTDIR/$TMP" | btrfs -v receive "$RECV"
+}
+
+full() {
+	msg "Attempting to send full"
+	btrfs -v send --compressed-data "$DSTDIR/$TMP" | btrfs -v receive "$RECV"
+}
+
+symdel() {
+	if [ -L "$1" ]; then
+		btrfs subvolume delete "$1" >&2 || true
+		rm -v "$1" >&2
+	fi
+}
+
+mkdir -vp "$DSTDIR" >&2
+
+symdel "$DSTDIR/$TMP"
+symdel "$DSTDIR/$PREV"
+
+ln -sv "$CDATE" "$DSTDIR"/"$TMP" >&2
+btrfs subvolume snapshot -r "$SRC" "$DSTDIR/$CDATE" >&2
+
+delta || full
+
+# Mark snapshot transmission as complete
+[ -L "$DSTDIR/$CUR" ] && \
+	mv -v "$DSTDIR/$CUR" "$DSTDIR/$PREV" >&2
+mv -v "$DSTDIR/$TMP" "$DSTDIR/$CUR" >&2