diff --git a/storage/metric/index.go b/storage/metric/index.go
index 46e590e042..986dbde36e 100644
--- a/storage/metric/index.go
+++ b/storage/metric/index.go
@@ -14,7 +14,9 @@
 package metric
 
 import (
+	"io"
 	"sort"
+	"sync"
 
 	"code.google.com/p/goprotobuf/proto"