diff --git a/backend/backend_test.go b/backend/backend_test.go
index 36a91cec6..49e151336 100644
--- a/backend/backend_test.go
+++ b/backend/backend_test.go
@@ -13,7 +13,10 @@ import (
 )
 
 func testBackend(b backend.Backend, t *testing.T) {
-	for _, tpe := range []backend.Type{backend.Data, backend.Key, backend.Lock, backend.Snapshot, backend.Tree} {
+	for _, tpe := range []backend.Type{
+		backend.Data, backend.Key, backend.Lock,
+		backend.Snapshot, backend.Tree, backend.Index,
+	} {
 		// detect non-existing files
 		for _, test := range TestStrings {
 			id, err := backend.ParseID(test.id)
diff --git a/backend/interface.go b/backend/interface.go
index e96c6ab7c..2256212a8 100644
--- a/backend/interface.go
+++ b/backend/interface.go
@@ -11,6 +11,7 @@ const (
 	Lock          = "lock"
 	Snapshot      = "snapshot"
 	Tree          = "tree"
+	Index         = "index"
 )
 
 const (
diff --git a/backend/local/local.go b/backend/local/local.go
index bc9cf3b7f..ee1430639 100644
--- a/backend/local/local.go
+++ b/backend/local/local.go
@@ -31,6 +31,7 @@ func Open(dir string) (*Local, error) {
 		filepath.Join(dir, backend.Paths.Data),
 		filepath.Join(dir, backend.Paths.Snapshots),
 		filepath.Join(dir, backend.Paths.Trees),
+		filepath.Join(dir, backend.Paths.Index),
 		filepath.Join(dir, backend.Paths.Locks),
 		filepath.Join(dir, backend.Paths.Keys),
 		filepath.Join(dir, backend.Paths.Temp),
@@ -103,6 +104,7 @@ func Create(dir string) (*Local, error) {
 		filepath.Join(dir, backend.Paths.Data),
 		filepath.Join(dir, backend.Paths.Snapshots),
 		filepath.Join(dir, backend.Paths.Trees),
+		filepath.Join(dir, backend.Paths.Index),
 		filepath.Join(dir, backend.Paths.Locks),
 		filepath.Join(dir, backend.Paths.Keys),
 		filepath.Join(dir, backend.Paths.Temp),
@@ -284,6 +286,8 @@ func dirname(base string, t backend.Type, name string) string {
 		if len(name) > 2 {
 			n = filepath.Join(n, name[:2])
 		}
+	case backend.Index:
+		n = backend.Paths.Index
 	case backend.Lock:
 		n = backend.Paths.Locks
 	case backend.Key:
diff --git a/backend/paths.go b/backend/paths.go
index aa73bf6eb..4b1246e13 100644
--- a/backend/paths.go
+++ b/backend/paths.go
@@ -7,6 +7,7 @@ var Paths = struct {
 	Data      string
 	Snapshots string
 	Trees     string
+	Index     string
 	Locks     string
 	Keys      string
 	Temp      string
@@ -16,6 +17,7 @@ var Paths = struct {
 	"data",
 	"snapshots",
 	"trees",
+	"index",
 	"locks",
 	"keys",
 	"tmp",
diff --git a/backend/sftp/sftp.go b/backend/sftp/sftp.go
index 4237efae4..5a942be50 100644
--- a/backend/sftp/sftp.go
+++ b/backend/sftp/sftp.go
@@ -79,6 +79,7 @@ func Open(dir string, program string, args ...string) (*SFTP, error) {
 		filepath.Join(dir, backend.Paths.Data),
 		filepath.Join(dir, backend.Paths.Snapshots),
 		filepath.Join(dir, backend.Paths.Trees),
+		filepath.Join(dir, backend.Paths.Index),
 		filepath.Join(dir, backend.Paths.Locks),
 		filepath.Join(dir, backend.Paths.Keys),
 		filepath.Join(dir, backend.Paths.Version),
@@ -153,6 +154,7 @@ func Create(dir string, program string, args ...string) (*SFTP, error) {
 		filepath.Join(dir, backend.Paths.Data),
 		filepath.Join(dir, backend.Paths.Snapshots),
 		filepath.Join(dir, backend.Paths.Trees),
+		filepath.Join(dir, backend.Paths.Index),
 		filepath.Join(dir, backend.Paths.Locks),
 		filepath.Join(dir, backend.Paths.Keys),
 		filepath.Join(dir, backend.Paths.Temp),
@@ -408,6 +410,8 @@ func (r *SFTP) dirname(t backend.Type, name string) string {
 		if len(name) > 2 {
 			n = filepath.Join(n, name[:2])
 		}
+	case backend.Index:
+		n = backend.Paths.Index
 	case backend.Lock:
 		n = backend.Paths.Locks
 	case backend.Key: