diff --git a/pkg/donut/donut-v2.go b/pkg/donut/donut-v2.go
index 54aebd25c..41fbb2fa3 100644
--- a/pkg/donut/donut-v2.go
+++ b/pkg/donut/donut-v2.go
@@ -361,6 +361,7 @@ func (donut API) createObject(bucket, key, contentType, expectedMD5Sum string, s
 		donut.storedBuckets.Set(bucket, storedBucket)
 		return objMetadata, nil
 	}
+
 	// calculate md5
 	hash := md5.New()
 	sha256hash := sha256.New()
diff --git a/pkg/donut/multipart.go b/pkg/donut/multipart.go
index 1c779ac13..e500443bf 100644
--- a/pkg/donut/multipart.go
+++ b/pkg/donut/multipart.go
@@ -272,18 +272,63 @@ func (donut API) cleanupMultipartSession(bucket, key, uploadID string) {
 	donut.storedBuckets.Set(bucket, storedBucket)
 }
 
+func (donut API) mergeMultipart(parts *CompleteMultipartUpload, uploadID string, fullObjectWriter *io.PipeWriter) {
+	for _, part := range parts.Part {
+		recvMD5 := part.ETag
+		object, ok := donut.multiPartObjects[uploadID].Get(part.PartNumber)
+		if ok == false {
+			fullObjectWriter.CloseWithError(probe.WrapError(probe.NewError(InvalidPart{})))
+			return
+		}
+		calcMD5Bytes := md5.Sum(object)
+		// complete multi part request header md5sum per part is hex encoded
+		recvMD5Bytes, err := hex.DecodeString(strings.Trim(recvMD5, "\""))
+		if err != nil {
+			fullObjectWriter.CloseWithError(probe.WrapError(probe.NewError(InvalidDigest{Md5: recvMD5})))
+			return
+		}
+		if !bytes.Equal(recvMD5Bytes, calcMD5Bytes[:]) {
+			fullObjectWriter.CloseWithError(probe.WrapError(probe.NewError(BadDigest{})))
+			return
+		}
+
+		if _, err := io.Copy(fullObjectWriter, bytes.NewReader(object)); err != nil {
+			fullObjectWriter.CloseWithError(probe.WrapError(probe.NewError(err)))
+			return
+		}
+		object = nil
+	}
+	fullObjectWriter.Close()
+	return
+}
+
 // CompleteMultipartUpload - complete a multipart upload and persist the data
 func (donut API) CompleteMultipartUpload(bucket, key, uploadID string, data io.Reader, signature *Signature) (ObjectMetadata, *probe.Error) {
 	donut.lock.Lock()
+	defer donut.lock.Unlock()
+	size := int64(donut.multiPartObjects[uploadID].Stats().Bytes)
+	fullObjectReader, err := donut.completeMultipartUploadV2(bucket, key, uploadID, data, signature)
+	if err != nil {
+		return ObjectMetadata{}, err.Trace()
+	}
+	objectMetadata, err := donut.createObject(bucket, key, "", "", size, fullObjectReader, nil)
+	if err != nil {
+		// No need to call internal cleanup functions here, caller should call AbortMultipartUpload()
+		// which would in-turn cleanup properly in accordance with S3 Spec
+		return ObjectMetadata{}, err.Trace()
+	}
+	donut.cleanupMultipartSession(bucket, key, uploadID)
+	return objectMetadata, nil
+}
 
+func (donut API) completeMultipartUploadV2(bucket, key, uploadID string, data io.Reader, signature *Signature) (io.Reader, *probe.Error) {
 	if !IsValidBucket(bucket) {
-		donut.lock.Unlock()
-		return ObjectMetadata{}, probe.NewError(BucketNameInvalid{Bucket: bucket})
+		return nil, probe.NewError(BucketNameInvalid{Bucket: bucket})
 	}
 	if !IsValidObjectName(key) {
-		donut.lock.Unlock()
-		return ObjectMetadata{}, probe.NewError(ObjectNameInvalid{Object: key})
+		return nil, probe.NewError(ObjectNameInvalid{Object: key})
 	}
+
 	// TODO: multipart support for donut is broken, since we haven't finalized the format in which
 	//       it can be stored, disabling this for now until we get the underlying layout stable.
 	//
@@ -293,88 +338,38 @@ func (donut API) CompleteMultipartUpload(bucket, key, uploadID string, data io.R
 	//	}
 
 	if !donut.storedBuckets.Exists(bucket) {
-		donut.lock.Unlock()
-		return ObjectMetadata{}, probe.NewError(BucketNotFound{Bucket: bucket})
+		return nil, probe.NewError(BucketNotFound{Bucket: bucket})
 	}
 	storedBucket := donut.storedBuckets.Get(bucket).(storedBucket)
 	// Verify upload id
 	if storedBucket.multiPartSession[key].UploadID != uploadID {
-		donut.lock.Unlock()
-		return ObjectMetadata{}, probe.NewError(InvalidUploadID{UploadID: uploadID})
+		return nil, probe.NewError(InvalidUploadID{UploadID: uploadID})
 	}
 	partBytes, err := ioutil.ReadAll(data)
 	if err != nil {
-		donut.lock.Unlock()
-		return ObjectMetadata{}, probe.NewError(err)
+		return nil, probe.NewError(err)
 	}
 	if signature != nil {
 		ok, err := signature.DoesSignatureMatch(hex.EncodeToString(sha256.Sum256(partBytes)[:]))
 		if err != nil {
-			donut.lock.Unlock()
-			return ObjectMetadata{}, err.Trace()
+			return nil, err.Trace()
 		}
 		if !ok {
-			donut.lock.Unlock()
-			return ObjectMetadata{}, probe.NewError(SignatureDoesNotMatch{})
+			return nil, probe.NewError(SignatureDoesNotMatch{})
 		}
 	}
 	parts := &CompleteMultipartUpload{}
 	if err := xml.Unmarshal(partBytes, parts); err != nil {
-		donut.lock.Unlock()
-		return ObjectMetadata{}, probe.NewError(MalformedXML{})
+		return nil, probe.NewError(MalformedXML{})
 	}
 	if !sort.IsSorted(completedParts(parts.Part)) {
-		donut.lock.Unlock()
-		return ObjectMetadata{}, probe.NewError(InvalidPartOrder{})
+		return nil, probe.NewError(InvalidPartOrder{})
 	}
 
-	var size int64
-	var fullObject bytes.Buffer
-	for i := 0; i < len(parts.Part); i++ {
-		recvMD5 := parts.Part[i].ETag
-		object, ok := donut.multiPartObjects[uploadID].Get(parts.Part[i].PartNumber)
-		if ok == false {
-			donut.lock.Unlock()
-			return ObjectMetadata{}, probe.NewError(InvalidPart{})
-		}
-		size += int64(len(object))
-		calcMD5Bytes := md5.Sum(object)
-		// complete multi part request header md5sum per part is hex encoded
-		recvMD5Bytes, err := hex.DecodeString(strings.Trim(recvMD5, "\""))
-		if err != nil {
-			donut.lock.Unlock()
-			return ObjectMetadata{}, probe.NewError(InvalidDigest{Md5: recvMD5})
-		}
-		if !bytes.Equal(recvMD5Bytes, calcMD5Bytes[:]) {
-			donut.lock.Unlock()
-			return ObjectMetadata{}, probe.NewError(BadDigest{})
-		}
-		if _, err := io.Copy(&fullObject, bytes.NewBuffer(object)); err != nil {
-			donut.lock.Unlock()
-			return ObjectMetadata{}, probe.NewError(err)
-		}
-		object = nil
-		go debug.FreeOSMemory()
-	}
+	fullObjectReader, fullObjectWriter := io.Pipe()
+	go donut.mergeMultipart(parts, uploadID, fullObjectWriter)
 
-	md5sumSlice := md5.Sum(fullObject.Bytes())
-	// this is needed for final verification inside CreateObject, do not convert this to hex
-	md5sum := base64.StdEncoding.EncodeToString(md5sumSlice[:])
-	donut.lock.Unlock()
-	{
-		objectMetadata, err := donut.CreateObject(bucket, key, md5sum, size, &fullObject, nil, nil)
-		if err != nil {
-			// No need to call internal cleanup functions here, caller should call AbortMultipartUpload()
-			// which would in-turn cleanup properly in accordance with S3 Spec
-			return ObjectMetadata{}, err.Trace()
-		}
-		fullObject.Reset()
-
-		donut.lock.Lock()
-		donut.cleanupMultipartSession(bucket, key, uploadID)
-		donut.lock.Unlock()
-		return objectMetadata, nil
-	}
+	return fullObjectReader, nil
 }
 
 // byKey is a sortable interface for UploadMetadata slice