diff --git a/buffer.go b/buffer.go
index 627c0fd..e34c4ba 100644
--- a/buffer.go
+++ b/buffer.go
@@ -19,6 +19,13 @@ type Buffer struct {
 	lastRead readOp // last read operation, so that Unread* can work correctly.
 }
 
+func NewBuffer(length int) *Buffer {
+	return &Buffer{
+		buf:      make([]byte, 0, length),
+		lastRead: opInvalid,
+	}
+}
+
 // The readOp constants describe the last action performed on
 // the buffer, so that UnreadRune and UnreadByte can check for
 // invalid usage. opReadRuneX constants are chosen such that
diff --git a/chunk.go b/chunk.go
index f296a7e..6abed5c 100644
--- a/chunk.go
+++ b/chunk.go
@@ -4,8 +4,10 @@ import (
 	"context"
 	"encoding/csv"
 	"fmt"
+	"io"
 	"os"
 	"path"
+	"path/filepath"
 	"strconv"
 	"time"
 
@@ -37,9 +39,20 @@ func (cc *commPCallback) OnSuccess(buf *Buffer, graphName, payloadCid, fsDetail
 	log.Infof("piece cid: %s, payload size: %d, size: %d ", cpRes.Root.String(), cpRes.PayloadSize, cpRes.Size)
 
 	buf.SeekStart()
-	if err := os.WriteFile(path.Join(cc.carDir, cpRes.Root.String()+".car"), buf.Bytes(), 0o644); err != nil {
+	carFileName := filepath.Join(cc.carDir, cpRes.Root.String())
+	if !cc.rename {
+		carFileName += ".car"
+	}
+	carFile, err := os.OpenFile(carFileName, os.O_RDWR|os.O_CREATE, 0o644)
+	if err != nil {
+		log.Fatalf("failed to create car file: %s", err)
+	}
+	defer carFile.Close()
+
+	if _, err = io.Copy(carFile, buf); err != nil {
 		log.Fatalf("failed to write car file: %s", err)
 	}
+	buf.Reset()
 
 	// Add node inof to manifest.csv
 	manifestPath := path.Join(cc.carDir, "manifest.csv")
@@ -167,7 +180,7 @@ func Chunk(ctx context.Context, sliceSize int64, parentPath, targetPath, carDir,
 			cumuSize += fileSize
 			graphFiles = append(graphFiles, item)
 			// todo build ipld from graphFiles
-			BuildIpldGraph(ctx, graphFiles, GenGraphName(graphName, graphSliceCount, sliceTotal), parentPath, carDir, parallel, cb)
+			BuildIpldGraph(ctx, graphFiles, GenGraphName(graphName, graphSliceCount, sliceTotal), parentPath, carDir, parallel, cb, sliceSize)
 			log.Infof("cumu-size: %d", cumuSize)
 			log.Infof("%s", GenGraphName(graphName, graphSliceCount, sliceTotal))
 			log.Infof("=================")
@@ -193,7 +206,7 @@ func Chunk(ctx context.Context, sliceSize int64, parentPath, targetPath, carDir,
 			})
 			fileSliceCount++
 			// todo build ipld from graphFiles
-			BuildIpldGraph(ctx, graphFiles, GenGraphName(graphName, graphSliceCount, sliceTotal), parentPath, carDir, parallel, cb)
+			BuildIpldGraph(ctx, graphFiles, GenGraphName(graphName, graphSliceCount, sliceTotal), parentPath, carDir, parallel, cb, sliceSize)
 			log.Infof("cumu-size: %d", cumuSize+firstCut)
 			log.Infof("%s", GenGraphName(graphName, graphSliceCount, sliceTotal))
 			log.Infof("=================")
@@ -219,7 +232,7 @@ func Chunk(ctx context.Context, sliceSize int64, parentPath, targetPath, carDir,
 				fileSliceCount++
 				if seekEnd-seekStart == sliceSize-1 {
 					// todo build ipld from graphFiles
-					BuildIpldGraph(ctx, graphFiles, GenGraphName(graphName, graphSliceCount, sliceTotal), parentPath, carDir, parallel, cb)
+					BuildIpldGraph(ctx, graphFiles, GenGraphName(graphName, graphSliceCount, sliceTotal), parentPath, carDir, parallel, cb, sliceSize)
 					log.Infof("cumu-size: %d", sliceSize)
 					log.Infof("%s", GenGraphName(graphName, graphSliceCount, sliceTotal))
 					log.Infof("=================")
@@ -232,7 +245,7 @@ func Chunk(ctx context.Context, sliceSize int64, parentPath, targetPath, carDir,
 	}
 	if cumuSize > 0 {
 		// todo build ipld from graphFiles
-		BuildIpldGraph(ctx, graphFiles, GenGraphName(graphName, graphSliceCount, sliceTotal), parentPath, carDir, parallel, cb)
+		BuildIpldGraph(ctx, graphFiles, GenGraphName(graphName, graphSliceCount, sliceTotal), parentPath, carDir, parallel, cb, sliceSize)
 		log.Infof("cumu-size: %d", cumuSize)
 		log.Infof("%s", GenGraphName(graphName, graphSliceCount, sliceTotal))
 		log.Infof("=================")
diff --git a/utils.go b/utils.go
index 8330c8e..c842743 100644
--- a/utils.go
+++ b/utils.go
@@ -131,8 +131,15 @@ func (b *FSBuilder) getNodeByLink(ln *ipld.Link) (fn fsNode, err error) {
 	return
 }
 
-func BuildIpldGraph(ctx context.Context, fileList []Finfo, graphName, parentPath, carDir string, parallel int, cb GraphBuildCallback) {
-	buf, payloadCid, fsDetail, err := buildIpldGraph(ctx, fileList, parentPath, parallel)
+func BuildIpldGraph(ctx context.Context,
+	fileList []Finfo,
+	graphName, parentPath,
+	carDir string,
+	parallel int,
+	cb GraphBuildCallback,
+	sliceSize int64,
+) {
+	buf, payloadCid, fsDetail, err := buildIpldGraph(ctx, fileList, parentPath, parallel, sliceSize)
 	if err != nil {
 		// log.Fatal(err)
 		cb.OnError(err)
@@ -141,7 +148,7 @@ func BuildIpldGraph(ctx context.Context, fileList []Finfo, graphName, parentPath
 	cb.OnSuccess(buf, graphName, payloadCid, fsDetail)
 }
 
-func buildIpldGraph(ctx context.Context, fileList []Finfo, parentPath string, parallel int) (*Buffer, string, string, error) {
+func buildIpldGraph(ctx context.Context, fileList []Finfo, parentPath string, parallel int, sliceSize int64) (*Buffer, string, string, error) {
 	bs2 := bstore.NewBlockstore(dss.MutexWrap(datastore.NewMapDatastore()))
 	dagServ := dag.NewDAGService(blockservice.New(bs2, offline.Exchange(bs2)))
 
@@ -282,7 +289,7 @@ func buildIpldGraph(ctx context.Context, fileList []Finfo, parentPath string, pa
 	log.Infof("start to generate car for %s", rootNode.Cid())
 	genCarStartTime := time.Now()
 	// car
-	buf := new(Buffer)
+	buf := NewBuffer(int(sliceSize))
 	selector := allSelector()
 	sc := car.NewSelectiveCar(ctx, bs2, []car.Dag{{Root: rootNode.Cid(), Selector: selector}})
 	err = sc.Write(buf)