@@ -37,10 +37,10 @@ type fileMetadataTransaction struct {

 	ws    *ioutils.AtomicWriteSet

 }

-// NewFSMetadataStore returns an instance of a metadata store

+// newFSMetadataStore returns an instance of a metadata store

 // which is backed by files on disk using the provided root

 // as the root of metadata files.

-func NewFSMetadataStore(root string) (MetadataStore, error) {

+func newFSMetadataStore(root string) (*fileMetadataStore, error) {

 	if err := os.MkdirAll(root, 0700); err != nil {

 		return nil, err

 	}

@@ -66,7 +66,7 @@ func (fms *fileMetadataStore) getMountFilename(mount, filename string) string {

 	return filepath.Join(fms.getMountDirectory(mount), filename)

 }

-func (fms *fileMetadataStore) StartTransaction() (MetadataTransaction, error) {

+func (fms *fileMetadataStore) StartTransaction() (*fileMetadataTransaction, error) {

 	tmpDir := filepath.Join(fms.root, "tmp")

 	if err := os.MkdirAll(tmpDir, 0755); err != nil {

 		return nil, err

@@ -24,12 +24,12 @@ func newFileMetadataStore(t *testing.T) (*fileMetadataStore, string, func()) {

 	if err != nil {

 		t.Fatal(err)

 	}

-	fms, err := NewFSMetadataStore(td)

+	fms, err := newFSMetadataStore(td)

 	if err != nil {

 		t.Fatal(err)

 	}

-	return fms.(*fileMetadataStore), td, func() {

+	return fms, td, func() {

 		if err := os.RemoveAll(td); err != nil {

 			t.Logf("Failed to cleanup %q: %s", td, err)

 		}

@@ -201,54 +201,6 @@ type DescribableStore interface {

 	RegisterWithDescriptor(io.Reader, ChainID, distribution.Descriptor) (Layer, error)

 }

-// MetadataTransaction represents functions for setting layer metadata

-// with a single transaction.

-type MetadataTransaction interface {

-	SetSize(int64) error

-	SetParent(parent ChainID) error

-	SetDiffID(DiffID) error

-	SetCacheID(string) error

-	SetDescriptor(distribution.Descriptor) error

-	setOS(string) error

-	TarSplitWriter(compressInput bool) (io.WriteCloser, error)

-

-	Commit(ChainID) error

-	Cancel() error

-	String() string

-}

-

-// MetadataStore represents a backend for persisting

-// metadata about layers and providing the metadata

-// for restoring a Store.

-type MetadataStore interface {

-	// StartTransaction starts an update for new metadata

-	// which will be used to represent an ID on commit.

-	StartTransaction() (MetadataTransaction, error)

-

-	GetSize(ChainID) (int64, error)

-	GetParent(ChainID) (ChainID, error)

-	GetDiffID(ChainID) (DiffID, error)

-	GetCacheID(ChainID) (string, error)

-	GetDescriptor(ChainID) (distribution.Descriptor, error)

-	getOS(ChainID) (string, error)

-	TarSplitReader(ChainID) (io.ReadCloser, error)

-

-	SetMountID(string, string) error

-	SetInitID(string, string) error

-	SetMountParent(string, ChainID) error

-

-	GetMountID(string) (string, error)

-	GetInitID(string) (string, error)

-	GetMountParent(string) (ChainID, error)

-

-	// List returns the full list of referenced

-	// read-only and read-write layers

-	List() ([]ChainID, []string, error)

-

-	Remove(ChainID) error

-	RemoveMount(string) error

-}

-

 // CreateChainID returns ID for a layerDigest slice

 func CreateChainID(dgsts []DiffID) ChainID {

 	return createChainIDFromParent("", dgsts...)

@@ -27,7 +27,7 @@ import (

 const maxLayerDepth = 125

 type layerStore struct {

-	store       MetadataStore

+	store       *fileMetadataStore

 	driver      graphdriver.Driver

 	useTarSplit bool

@@ -65,18 +65,15 @@ func NewStoreFromOptions(options StoreOptions) (Store, error) {

 	}

 	logrus.Debugf("Initialized graph driver %s", driver)

-	fms, err := NewFSMetadataStore(fmt.Sprintf(options.MetadataStorePathTemplate, driver))

-	if err != nil {

-		return nil, err

-	}

+	root := fmt.Sprintf(options.MetadataStorePathTemplate, driver)

-	return NewStoreFromGraphDriver(fms, driver, options.OS)

+	return newStoreFromGraphDriver(root, driver, options.OS)

 }

-// NewStoreFromGraphDriver creates a new Store instance using the provided

+// newStoreFromGraphDriver creates a new Store instance using the provided

 // metadata store and graph driver. The metadata store will be used to restore

 // the Store.

-func NewStoreFromGraphDriver(store MetadataStore, driver graphdriver.Driver, os string) (Store, error) {

+func newStoreFromGraphDriver(root string, driver graphdriver.Driver, os string) (Store, error) {

 	if !system.IsOSSupported(os) {

 		return nil, fmt.Errorf("failed to initialize layer store as operating system '%s' is not supported", os)

 	}

@@ -85,8 +82,13 @@ func NewStoreFromGraphDriver(store MetadataStore, driver graphdriver.Driver, os

 		caps = capDriver.Capabilities()

 	}

+	ms, err := newFSMetadataStore(root)

+	if err != nil {

+		return nil, err

+	}

+

 	ls := &layerStore{

-		store:       store,

+		store:       ms,

 		driver:      driver,

 		layerMap:    map[ChainID]*roLayer{},

 		mounts:      map[string]*mountedLayer{},

@@ -94,7 +96,7 @@ func NewStoreFromGraphDriver(store MetadataStore, driver graphdriver.Driver, os

 		os:          os,

 	}

-	ids, mounts, err := store.List()

+	ids, mounts, err := ms.List()

 	if err != nil {

 		return nil, err

 	}

@@ -225,7 +227,7 @@ func (ls *layerStore) loadMount(mount string) error {

 	return nil

 }

-func (ls *layerStore) applyTar(tx MetadataTransaction, ts io.Reader, parent string, layer *roLayer) error {

+func (ls *layerStore) applyTar(tx *fileMetadataTransaction, ts io.Reader, parent string, layer *roLayer) error {

 	digester := digest.Canonical.Digester()

 	tr := io.TeeReader(ts, digester.Hash())

@@ -69,11 +69,8 @@ func newTestStore(t *testing.T) (Store, string, func()) {

 	}

 	graph, graphcleanup := newTestGraphDriver(t)

-	fms, err := NewFSMetadataStore(td)

-	if err != nil {

-		t.Fatal(err)

-	}

-	ls, err := NewStoreFromGraphDriver(fms, graph, runtime.GOOS)

+

+	ls, err := newStoreFromGraphDriver(td, graph, runtime.GOOS)

 	if err != nil {

 		t.Fatal(err)

 	}

@@ -403,7 +400,7 @@ func TestStoreRestore(t *testing.T) {

 		t.Fatal(err)

 	}

-	ls2, err := NewStoreFromGraphDriver(ls.(*layerStore).store, ls.(*layerStore).driver, runtime.GOOS)

+	ls2, err := newStoreFromGraphDriver(ls.(*layerStore).store.root, ls.(*layerStore).driver, runtime.GOOS)

 	if err != nil {

 		t.Fatal(err)

 	}

@@ -90,11 +90,8 @@ func TestLayerMigration(t *testing.T) {

 		t.Fatal(err)

 	}

-	fms, err := NewFSMetadataStore(filepath.Join(td, "layers"))

-	if err != nil {

-		t.Fatal(err)

-	}

-	ls, err := NewStoreFromGraphDriver(fms, graph, runtime.GOOS)

+	root := filepath.Join(td, "layers")

+	ls, err := newStoreFromGraphDriver(root, graph, runtime.GOOS)

 	if err != nil {

 		t.Fatal(err)

 	}

@@ -218,11 +215,8 @@ func TestLayerMigrationNoTarsplit(t *testing.T) {

 		t.Fatal(err)

 	}

-	fms, err := NewFSMetadataStore(filepath.Join(td, "layers"))

-	if err != nil {

-		t.Fatal(err)

-	}

-	ls, err := NewStoreFromGraphDriver(fms, graph, runtime.GOOS)

+	root := filepath.Join(td, "layers")

+	ls, err := newStoreFromGraphDriver(root, graph, runtime.GOOS)

 	if err != nil {

 		t.Fatal(err)

 	}

@@ -121,7 +121,7 @@ func (rl *roLayer) depth() int {

 	return rl.parent.depth() + 1

 }

-func storeLayer(tx MetadataTransaction, layer *roLayer) error {

+func storeLayer(tx *fileMetadataTransaction, layer *roLayer) error {

 	if err := tx.SetDiffID(layer.diffID); err != nil {

 		return err

 	}