--- Revision None
+++ Revision 396463363338
@@ -0,0 +1,50 @@
+#!/bin/bash
+
+# Write a 4096 byte block of something
+block ()
+{
+	head -c 4096 /dev/zero | tr '\0' "\\$1";
+}
+
+# Here is some test data with holes in it:
+for y in $(seq 0 2); do
+	for x in 0 1; do
+		block 0;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+		block 21;
+	done
+done > am
+sync
+
+echo "file size: $(du -h am)"
+
+echo "digests after file creation:   $(sha1sum am)"
+sysctl -q vm.drop_caches=1
+echo "digests after file creation 2: $(sha1sum am)"
+
+# Now replace those 101 distinct extents with 101 references to the first extent
+btrfs-extent-same 131072 $(for x in $(seq 0 2); do echo am $((x * 131072)); done) 2>&1 | tail -1
+
+echo "digests after dedupe:          $(sha1sum am)"
+sysctl -q vm.drop_caches=1
+echo "digests after dedupe 2:        $(sha1sum am)"
+
+# Punch holes into the extent refs
+fallocate -v -d am
+
+echo "digests after hole punching:   $(sha1sum am)"
+sysctl -q vm.drop_caches=1
+echo "digests after hole punching 2: $(sha1sum am)"