-// CompareAndTouch returns nil if one of the volumes already has the
-// given content and it successfully updates the relevant block's
-// modification time in order to protect it from premature garbage
-// collection.
-func CompareAndTouch(hash string, buf []byte) error {
+// CompareAndTouch returns the current replication level if one of the
+// volumes already has the given content and it successfully updates
+// the relevant block's modification time in order to protect it from
+// premature garbage collection. Otherwise, it returns a non-nil
+// error.
+func CompareAndTouch(hash string, buf []byte) (int, error) {