diff --git a/storage/filesystem.go b/storage/filesystem.go
index 6f3a189..24f8465 100644
--- a/storage/filesystem.go
+++ b/storage/filesystem.go
@@ -2,10 +2,11 @@ package storage
 
 import (
 	"context"
-	"crypto/md5"
+	"crypto/sha1"
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -75,11 +76,18 @@ func (b *filesystemBackend) safePath(ctx context.Context, path string) (string,
 }
 
 func etagForFile(path string) (string, error) {
-	data, err := ioutil.ReadFile(path)
+	f, err := os.Open(path)
 	if err != nil {
 		return "", err
 	}
-	csum := md5.Sum(data)
+	defer f.Close()
+
+	h := sha1.New()
+	if _, err := io.Copy(h, f); err != nil {
+		return "", err
+	}
+	csum := h.Sum(nil)
+
 	return base64.StdEncoding.EncodeToString(csum[:]), nil
 }