@@ -52,7 +52,7 @@ func Init(home string, options []string) (graphdriver.Driver, error) {

 		home: home,

 	}

-	return graphdriver.NaiveDiffDriver(driver), nil

+	return graphdriver.NewNaiveDiffDriver(driver), nil

 }

 // Driver contains information about the filesystem mounted.

@@ -232,7 +232,7 @@ func (devices *DeviceSet) hasImage(name string) bool {

 // ensureImage creates a sparse file of <size> bytes at the path

 // <root>/devicemapper/<name>.

-// If the file already exists, it does nothing.

+// If the file already exists and new size is larger than its current size, it grows to the new size.

 // Either way it returns the full path.

 func (devices *DeviceSet) ensureImage(name string, size int64) (string, error) {

 	dirname := devices.loopbackDir()

@@ -242,7 +242,7 @@ func (devices *DeviceSet) ensureImage(name string, size int64) (string, error) {

 		return "", err

 	}

-	if _, err := os.Stat(filename); err != nil {

+	if fi, err := os.Stat(filename); err != nil {

 		if !os.IsNotExist(err) {

 			return "", err

 		}

@@ -256,6 +256,19 @@ func (devices *DeviceSet) ensureImage(name string, size int64) (string, error) {

 		if err := file.Truncate(size); err != nil {

 			return "", err

 		}

+	} else {

+		if fi.Size() < size {

+			file, err := os.OpenFile(filename, os.O_RDWR|os.O_CREATE, 0600)

+			if err != nil {

+				return "", err

+			}

+			defer file.Close()

+			if err := file.Truncate(size); err != nil {

+				return "", fmt.Errorf("Unable to grow loopback file %s: %v", filename, err)

+			}

+		} else if fi.Size() > size {

+			logrus.Warnf("Can't shrink loopback file %s", filename)

+		}

 	}

 	return filename, nil

 }

@@ -3,8 +3,10 @@

 package devmapper

 import (

+	"fmt"

 	"testing"

+	"github.com/docker/docker/daemon/graphdriver"

 	"github.com/docker/docker/daemon/graphdriver/graphtest"

 )

@@ -40,3 +42,40 @@ func TestDevmapperCreateSnap(t *testing.T) {

 func TestDevmapperTeardown(t *testing.T) {

 	graphtest.PutDriver(t)

 }

+

+func TestDevmapperReduceLoopBackSize(t *testing.T) {

+	tenMB := int64(10 * 1024 * 1024)

+	testChangeLoopBackSize(t, -tenMB, defaultDataLoopbackSize, defaultMetaDataLoopbackSize)

+}

+

+func TestDevmapperIncreaseLoopBackSize(t *testing.T) {

+	tenMB := int64(10 * 1024 * 1024)

+	testChangeLoopBackSize(t, tenMB, defaultDataLoopbackSize+tenMB, defaultMetaDataLoopbackSize+tenMB)

+}

+

+func testChangeLoopBackSize(t *testing.T, delta, expectDataSize, expectMetaDataSize int64) {

+	driver := graphtest.GetDriver(t, "devicemapper").(*graphtest.Driver).Driver.(*graphdriver.NaiveDiffDriver).ProtoDriver.(*Driver)

+	defer graphtest.PutDriver(t)

+	// make sure data or metadata loopback size are the default size

+	if s := driver.DeviceSet.Status(); s.Data.Total != uint64(defaultDataLoopbackSize) || s.Metadata.Total != uint64(defaultMetaDataLoopbackSize) {

+		t.Fatalf("data or metadata loop back size is incorrect")

+	}

+	if err := driver.Cleanup(); err != nil {

+		t.Fatal(err)

+	}

+	//Reload

+	d, err := Init(driver.home, []string{

+		fmt.Sprintf("dm.loopdatasize=%d", defaultDataLoopbackSize+delta),

+		fmt.Sprintf("dm.loopmetadatasize=%d", defaultMetaDataLoopbackSize+delta),

+	})

+	if err != nil {

+		t.Fatalf("error creating devicemapper driver: %v", err)

+	}

+	driver = d.(*graphdriver.NaiveDiffDriver).ProtoDriver.(*Driver)

+	if s := driver.DeviceSet.Status(); s.Data.Total != uint64(expectDataSize) || s.Metadata.Total != uint64(expectMetaDataSize) {

+		t.Fatalf("data or metadata loop back size is incorrect")

+	}

+	if err := driver.Cleanup(); err != nil {

+		t.Fatal(err)

+	}

+}

@@ -57,7 +57,7 @@ func Init(home string, options []string) (graphdriver.Driver, error) {

 		home:      home,

 	}

-	return graphdriver.NaiveDiffDriver(d), nil

+	return graphdriver.NewNaiveDiffDriver(d), nil

 }

 func (d *Driver) String() string {

@@ -11,29 +11,29 @@ import (

 	"github.com/docker/docker/pkg/ioutils"

 )

-// naiveDiffDriver takes a ProtoDriver and adds the

+// NaiveDiffDriver takes a ProtoDriver and adds the

 // capability of the Diffing methods which it may or may not

 // support on its own. See the comment on the exported

-// NaiveDiffDriver function below.

+// NewNaiveDiffDriver function below.

 // Notably, the AUFS driver doesn't need to be wrapped like this.

-type naiveDiffDriver struct {

+type NaiveDiffDriver struct {

 	ProtoDriver

 }

-// NaiveDiffDriver returns a fully functional driver that wraps the

+// NewNaiveDiffDriver returns a fully functional driver that wraps the

 // given ProtoDriver and adds the capability of the following methods which

 // it may or may not support on its own:

 //     Diff(id, parent string) (archive.Archive, error)

 //     Changes(id, parent string) ([]archive.Change, error)

 //     ApplyDiff(id, parent string, diff archive.Reader) (size int64, err error)

 //     DiffSize(id, parent string) (size int64, err error)

-func NaiveDiffDriver(driver ProtoDriver) Driver {

-	return &naiveDiffDriver{ProtoDriver: driver}

+func NewNaiveDiffDriver(driver ProtoDriver) Driver {

+	return &NaiveDiffDriver{ProtoDriver: driver}

 }

 // Diff produces an archive of the changes between the specified

 // layer and its parent layer which may be "".

-func (gdw *naiveDiffDriver) Diff(id, parent string) (arch archive.Archive, err error) {

+func (gdw *NaiveDiffDriver) Diff(id, parent string) (arch archive.Archive, err error) {

 	driver := gdw.ProtoDriver

 	layerFs, err := driver.Get(id, "")

@@ -84,7 +84,7 @@ func (gdw *naiveDiffDriver) Diff(id, parent string) (arch archive.Archive, err e

 // Changes produces a list of changes between the specified layer

 // and its parent layer. If parent is "", then all changes will be ADD changes.

-func (gdw *naiveDiffDriver) Changes(id, parent string) ([]archive.Change, error) {

+func (gdw *NaiveDiffDriver) Changes(id, parent string) ([]archive.Change, error) {

 	driver := gdw.ProtoDriver

 	layerFs, err := driver.Get(id, "")

@@ -109,7 +109,7 @@ func (gdw *naiveDiffDriver) Changes(id, parent string) ([]archive.Change, error)

 // ApplyDiff extracts the changeset from the given diff into the

 // layer with the specified id and parent, returning the size of the

 // new layer in bytes.

-func (gdw *naiveDiffDriver) ApplyDiff(id, parent string, diff archive.Reader) (size int64, err error) {

+func (gdw *NaiveDiffDriver) ApplyDiff(id, parent string, diff archive.Reader) (size int64, err error) {

 	driver := gdw.ProtoDriver

 	// Mount the root filesystem so we can apply the diff/layer.

@@ -132,7 +132,7 @@ func (gdw *naiveDiffDriver) ApplyDiff(id, parent string, diff archive.Reader) (s

 // DiffSize calculates the changes between the specified layer

 // and its parent and returns the size in bytes of the changes

 // relative to its base filesystem directory.

-func (gdw *naiveDiffDriver) DiffSize(id, parent string) (size int64, err error) {

+func (gdw *NaiveDiffDriver) DiffSize(id, parent string) (size int64, err error) {

 	driver := gdw.ProtoDriver

 	changes, err := gdw.Changes(id, parent)

@@ -43,7 +43,7 @@ type naiveDiffDriverWithApply struct {

 // NaiveDiffDriverWithApply returns a NaiveDiff driver with custom ApplyDiff.

 func NaiveDiffDriverWithApply(driver ApplyDiffProtoDriver) graphdriver.Driver {

 	return &naiveDiffDriverWithApply{

-		Driver:    graphdriver.NaiveDiffDriver(driver),

+		Driver:    graphdriver.NewNaiveDiffDriver(driver),

 		applyDiff: driver,

 	}

 }

@@ -23,7 +23,7 @@ func Init(home string, options []string) (graphdriver.Driver, error) {

 	d := &Driver{

 		home: home,

 	}

-	return graphdriver.NaiveDiffDriver(d), nil

+	return graphdriver.NewNaiveDiffDriver(d), nil

 }

 // Driver holds information about the driver, home directory of the driver.

@@ -103,7 +103,7 @@ func Init(base string, opt []string) (graphdriver.Driver, error) {

 		options:          options,

 		filesystemsCache: filesystemsCache,

 	}

-	return graphdriver.NaiveDiffDriver(d), nil

+	return graphdriver.NewNaiveDiffDriver(d), nil

 }

 func parseOptions(opt []string) (zfsOptions, error) {