@@ -16,6 +16,12 @@ import (

 	"github.com/docker/docker/pkg/log"

 )

+const (

+	buf8K  = 8 * 1024

+	buf16K = 16 * 1024

+	buf32K = 32 * 1024

+)

+

 type TarSum struct {

 	io.Reader

 	tarR               *tar.Reader

@@ -23,7 +29,7 @@ type TarSum struct {

 	gz                 writeCloseFlusher

 	bufTar             *bytes.Buffer

 	bufGz              *bytes.Buffer

-	bufData            [8192]byte

+	bufData            []byte

 	h                  hash.Hash

 	sums               map[string]string

 	currentFile        string

@@ -93,12 +99,19 @@ func (ts *TarSum) Read(buf []byte) (int, error) {

 	if ts.finished {

 		return ts.bufGz.Read(buf)

 	}

-	var buf2 []byte

-	if len(buf) > 8192 {

-		buf2 = make([]byte, len(buf), cap(buf))

-	} else {

-		buf2 = ts.bufData[:len(buf)-1]

+	if ts.bufData == nil {

+		switch {

+		case len(buf) <= buf8K:

+			ts.bufData = make([]byte, buf8K)

+		case len(buf) <= buf16K:

+			ts.bufData = make([]byte, buf16K)

+		case len(buf) <= buf32K:

+			ts.bufData = make([]byte, buf32K)

+		default:

+			ts.bufData = make([]byte, len(buf))

+		}

 	}

+	buf2 := ts.bufData[:len(buf)-1]

 	n, err := ts.tarR.Read(buf2)

 	if err != nil {