diff --git a/br/pkg/lightning/backend/external/BUILD.bazel b/br/pkg/lightning/backend/external/BUILD.bazel
index 0280abc26c41a..7761221e3add8 100644
--- a/br/pkg/lightning/backend/external/BUILD.bazel
+++ b/br/pkg/lightning/backend/external/BUILD.bazel
@@ -65,7 +65,7 @@ go_test(
     ],
     embed = [":external"],
     flaky = True,
-    shard_count = 49,
+    shard_count = 47,
     deps = [
         "//br/pkg/lightning/backend/kv",
         "//br/pkg/lightning/common",
diff --git a/br/pkg/lightning/backend/external/bench_test.go b/br/pkg/lightning/backend/external/bench_test.go
index 548c8a7974d22..26e3dbd2e7d6c 100644
--- a/br/pkg/lightning/backend/external/bench_test.go
+++ b/br/pkg/lightning/backend/external/bench_test.go
@@ -345,6 +345,7 @@ func createEvenlyDistributedFiles(
 	kvCnt := 0
 	for i := 0; i < fileCount; i++ {
 		builder := NewWriterBuilder().
+			SetBlockSize(10 * 1024 * 1024).
 			SetMemorySizeLimit(uint64(float64(fileSize) * 1.1))
 		writer := builder.Build(
 			store,
diff --git a/br/pkg/lightning/backend/external/byte_reader.go b/br/pkg/lightning/backend/external/byte_reader.go
index 1293f6e0336f5..ce87741640f55 100644
--- a/br/pkg/lightning/backend/external/byte_reader.go
+++ b/br/pkg/lightning/backend/external/byte_reader.go
@@ -18,16 +18,18 @@ import (
 	"context"
 	"io"
 
+	"github.com/pingcap/errors"
 	"github.com/pingcap/tidb/br/pkg/membuf"
 	"github.com/pingcap/tidb/br/pkg/storage"
 	"github.com/pingcap/tidb/pkg/util/logutil"
+	"github.com/pingcap/tidb/pkg/util/size"
 	"go.uber.org/zap"
 )
 
 var (
 	// ConcurrentReaderBufferSizePerConc is the buffer size for concurrent reader per
 	// concurrency.
-	ConcurrentReaderBufferSizePerConc = 4 * 1024 * 1024
+	ConcurrentReaderBufferSizePerConc = int(4 * size.MB)
 	// ConcurrentReaderConcurrency is the concurrency for concurrent reader.
 	ConcurrentReaderConcurrency = 8
 )
@@ -44,8 +46,6 @@ type byteReader struct {
 	curBufOffset int
 	smallBuf     []byte
 
-	retPointers []*[]byte
-
 	concurrentReader struct {
 		largeBufferPool *membuf.Buffer
 		store           storage.ExternalStorage
@@ -193,24 +193,23 @@ func (r *byteReader) switchToConcurrentReader() error {
 	return nil
 }
 
-// readNBytes reads the next n bytes from the reader and returns a buffer slice containing those bytes.
-// The returned slice (pointer) can not be used after r.reset. In the same interval of r.reset,
-// byteReader guarantees that the returned slice (pointer) will point to the same content
-// though the slice may be changed.
-func (r *byteReader) readNBytes(n int) (*[]byte, error) {
+// readNBytes reads the next n bytes from the reader and returns a buffer slice
+// containing those bytes. The content of returned slice may be changed after
+// next call.
+func (r *byteReader) readNBytes(n int) ([]byte, error) {
 	b := r.next(n)
 	readLen := len(b)
 	if readLen == n {
-		ret := &b
-		r.retPointers = append(r.retPointers, ret)
-		return ret, nil
+		return b, nil
 	}
 	// If the reader has fewer than n bytes remaining in current buffer,
 	// `auxBuf` is used as a container instead.
+	if n > int(size.GB) {
+		return nil, errors.Errorf("read %d bytes from external storage, exceed max limit %d", n, size.GB)
+	}
 	auxBuf := make([]byte, n)
 	copy(auxBuf, b)
 	for readLen < n {
-		r.cloneSlices()
 		err := r.reload()
 		switch err {
 		case nil:
@@ -226,24 +225,7 @@ func (r *byteReader) readNBytes(n int) (*[]byte, error) {
 		copy(auxBuf[readLen:], b)
 		readLen += len(b)
 	}
-	return &auxBuf, nil
-}
-
-func (r *byteReader) reset() {
-	for i := range r.retPointers {
-		r.retPointers[i] = nil
-	}
-	r.retPointers = r.retPointers[:0]
-}
-
-func (r *byteReader) cloneSlices() {
-	for i := range r.retPointers {
-		copied := make([]byte, len(*r.retPointers[i]))
-		copy(copied, *r.retPointers[i])
-		*r.retPointers[i] = copied
-		r.retPointers[i] = nil
-	}
-	r.retPointers = r.retPointers[:0]
+	return auxBuf, nil
 }
 
 func (r *byteReader) next(n int) []byte {
diff --git a/br/pkg/lightning/backend/external/byte_reader_test.go b/br/pkg/lightning/backend/external/byte_reader_test.go
index 71d281ec13f3b..b5ef2457073a4 100644
--- a/br/pkg/lightning/backend/external/byte_reader_test.go
+++ b/br/pkg/lightning/backend/external/byte_reader_test.go
@@ -63,11 +63,6 @@ func (s *mockExtStore) GetFileSize() (int64, error) {
 }
 
 func TestByteReader(t *testing.T) {
-	testByteReaderNormal(t, false)
-	testByteReaderNormal(t, true)
-}
-
-func testByteReaderNormal(t *testing.T, useConcurrency bool) {
 	st, clean := NewS3WithBucketAndPrefix(t, "test", "testprefix")
 	defer clean()
 
@@ -96,9 +91,8 @@ func testByteReaderNormal(t *testing.T, useConcurrency bool) {
 	// Test basic readNBytes() usage.
 	br, err = newByteReader(context.Background(), newRsc(), 3)
 	require.NoError(t, err)
-	y, err := br.readNBytes(2)
+	x, err = br.readNBytes(2)
 	require.NoError(t, err)
-	x = *y
 	require.Equal(t, 2, len(x))
 	require.Equal(t, byte('a'), x[0])
 	require.Equal(t, byte('b'), x[1])
@@ -106,9 +100,8 @@ func testByteReaderNormal(t *testing.T, useConcurrency bool) {
 
 	br, err = newByteReader(context.Background(), newRsc(), 3)
 	require.NoError(t, err)
-	y, err = br.readNBytes(5) // Read all the data.
+	x, err = br.readNBytes(5) // Read all the data.
 	require.NoError(t, err)
-	x = *y
 	require.Equal(t, 5, len(x))
 	require.Equal(t, byte('e'), x[4])
 	require.NoError(t, br.Close())
@@ -124,11 +117,10 @@ func testByteReaderNormal(t *testing.T, useConcurrency bool) {
 	ms := &mockExtStore{src: []byte("abcdef")}
 	br, err = newByteReader(context.Background(), ms, 2)
 	require.NoError(t, err)
-	y, err = br.readNBytes(3)
+	x, err = br.readNBytes(3)
 	require.NoError(t, err)
 	// Pollute mockExtStore to verify if the slice is not affected.
-	copy(ms.src, []byte("xyz"))
-	x = *y
+	copy(ms.src, "xyz")
 	require.Equal(t, 3, len(x))
 	require.Equal(t, byte('c'), x[2])
 	require.NoError(t, br.Close())
@@ -136,57 +128,12 @@ func testByteReaderNormal(t *testing.T, useConcurrency bool) {
 	ms = &mockExtStore{src: []byte("abcdef")}
 	br, err = newByteReader(context.Background(), ms, 2)
 	require.NoError(t, err)
-	y, err = br.readNBytes(2)
+	x, err = br.readNBytes(2)
 	require.NoError(t, err)
 	// Pollute mockExtStore to verify if the slice is not affected.
-	copy(ms.src, []byte("xyz"))
-	x = *y
+	copy(ms.src, "xyz")
 	require.Equal(t, 2, len(x))
 	require.Equal(t, byte('b'), x[1])
-	br.reset()
-	require.NoError(t, br.Close())
-}
-
-func TestByteReaderClone(t *testing.T) {
-	ms := &mockExtStore{src: []byte("0123456789")}
-	br, err := newByteReader(context.Background(), ms, 4)
-	require.NoError(t, err)
-	y1, err := br.readNBytes(2)
-	require.NoError(t, err)
-	y2, err := br.readNBytes(1)
-	require.NoError(t, err)
-	x1, x2 := *y1, *y2
-	require.Len(t, x1, 2)
-	require.Len(t, x2, 1)
-	require.Equal(t, byte('0'), x1[0])
-	require.Equal(t, byte('2'), x2[0])
-	require.NoError(t, br.reload()) // Perform a read to overwrite buffer.
-	x1, x2 = *y1, *y2
-	require.Len(t, x1, 2)
-	require.Len(t, x2, 1)
-	require.Equal(t, byte('4'), x1[0]) // Verify if the buffer is overwritten.
-	require.Equal(t, byte('6'), x2[0])
-	require.NoError(t, br.Close())
-
-	ms = &mockExtStore{src: []byte("0123456789")}
-	br, err = newByteReader(context.Background(), ms, 4)
-	require.NoError(t, err)
-	y1, err = br.readNBytes(2)
-	require.NoError(t, err)
-	y2, err = br.readNBytes(1)
-	require.NoError(t, err)
-	x1, x2 = *y1, *y2
-	require.Len(t, x1, 2)
-	require.Len(t, x2, 1)
-	require.Equal(t, byte('0'), x1[0])
-	require.Equal(t, byte('2'), x2[0])
-	br.cloneSlices()
-	require.NoError(t, br.reload()) // Perform a read to overwrite buffer.
-	x1, x2 = *y1, *y2
-	require.Len(t, x1, 2)
-	require.Len(t, x2, 1)
-	require.Equal(t, byte('0'), x1[0]) // Verify if the buffer is NOT overwritten.
-	require.Equal(t, byte('2'), x2[0])
 	require.NoError(t, br.Close())
 }
 
@@ -196,78 +143,17 @@ func TestByteReaderAuxBuf(t *testing.T) {
 	require.NoError(t, err)
 	y1, err := br.readNBytes(1)
 	require.NoError(t, err)
+	require.Equal(t, []byte("0"), y1)
 	y2, err := br.readNBytes(2)
 	require.NoError(t, err)
-	require.Equal(t, []byte("0"), *y1)
-	require.Equal(t, []byte("12"), *y2)
+	require.Equal(t, []byte("12"), y2)
 
 	y3, err := br.readNBytes(1)
 	require.NoError(t, err)
+	require.Equal(t, []byte("3"), y3)
 	y4, err := br.readNBytes(2)
 	require.NoError(t, err)
-	require.Equal(t, []byte("3"), *y3)
-	require.Equal(t, []byte("45"), *y4)
-	require.Equal(t, []byte("0"), *y1)
-	require.Equal(t, []byte("12"), *y2)
-}
-
-func TestReset(t *testing.T) {
-	testReset(t, false)
-	testReset(t, true)
-}
-
-func testReset(t *testing.T, useConcurrency bool) {
-	st, clean := NewS3WithBucketAndPrefix(t, "test", "testprefix")
-	defer func() {
-		clean()
-	}()
-
-	seed := time.Now().Unix()
-	rand.Seed(uint64(seed))
-	t.Logf("seed: %d", seed)
-	src := make([]byte, 256)
-	for i := range src {
-		src[i] = byte(i)
-	}
-	// Prepare
-	err := st.WriteFile(context.Background(), "testfile", src)
-	require.NoError(t, err)
-
-	newRsc := func() storage.ExternalFileReader {
-		rsc, err := st.Open(context.Background(), "testfile", nil)
-		require.NoError(t, err)
-		return rsc
-	}
-	bufSize := rand.Intn(256)
-	br, err := newByteReader(context.Background(), newRsc(), bufSize)
-	require.NoError(t, err)
-	end := 0
-	toCheck := make([]*[]byte, 0, 10)
-	for end < len(src) {
-		n := rand.Intn(len(src) - end)
-		if n == 0 {
-			n = 1
-		}
-		y, err := br.readNBytes(n)
-		require.NoError(t, err)
-		toCheck = append(toCheck, y)
-		end += n
-
-		l := end
-		r := end
-		for i := len(toCheck) - 1; i >= 0; i-- {
-			l -= len(*toCheck[i])
-			require.Equal(t, src[l:r], *toCheck[i])
-			r = l
-		}
-
-		if rand.Intn(2) == 0 {
-			br.reset()
-			toCheck = toCheck[:0]
-		}
-	}
-	_, err = br.readNBytes(1)
-	require.Equal(t, io.EOF, err)
+	require.Equal(t, []byte("45"), y4)
 }
 
 func TestUnexpectedEOF(t *testing.T) {
@@ -366,7 +252,7 @@ func TestSwitchMode(t *testing.T) {
 			break
 		}
 		require.NoError(t, err)
-		totalCnt += len(*y)
+		totalCnt += len(y)
 	}
 	require.Equal(t, fileSize, totalCnt)
 
diff --git a/br/pkg/lightning/backend/external/engine.go b/br/pkg/lightning/backend/external/engine.go
index 4465534053917..1b60434287b47 100644
--- a/br/pkg/lightning/backend/external/engine.go
+++ b/br/pkg/lightning/backend/external/engine.go
@@ -143,10 +143,10 @@ func split[T any](in []T, groupNum int) [][]T {
 
 func (e *Engine) getAdjustedConcurrency() int {
 	if e.checkHotspot {
-		// estimate we will open at most 1000 files, so if e.dataFiles is small we can
+		// estimate we will open at most 8000 files, so if e.dataFiles is small we can
 		// try to concurrently process ranges.
 		adjusted := maxCloudStorageConnections / len(e.dataFiles)
-		return min(adjusted, 8)
+		return min(adjusted, 16)
 	}
 	adjusted := min(e.workerConcurrency, maxCloudStorageConnections/len(e.dataFiles))
 	return max(adjusted, 1)
diff --git a/br/pkg/lightning/backend/external/engine_test.go b/br/pkg/lightning/backend/external/engine_test.go
index deb33d4b0fde7..156b6807897bb 100644
--- a/br/pkg/lightning/backend/external/engine_test.go
+++ b/br/pkg/lightning/backend/external/engine_test.go
@@ -337,7 +337,7 @@ func TestGetAdjustedConcurrency(t *testing.T) {
 		workerConcurrency: 32,
 		dataFiles:         genFiles(100),
 	}
-	require.Equal(t, 8, e.getAdjustedConcurrency())
+	require.Equal(t, 16, e.getAdjustedConcurrency())
 	e.dataFiles = genFiles(8000)
 	require.Equal(t, 1, e.getAdjustedConcurrency())
 
diff --git a/br/pkg/lightning/backend/external/file.go b/br/pkg/lightning/backend/external/file.go
index fd66938a03a0a..a625248e61f31 100644
--- a/br/pkg/lightning/backend/external/file.go
+++ b/br/pkg/lightning/backend/external/file.go
@@ -50,25 +50,26 @@ func NewKeyValueStore(
 }
 
 // addEncodedData saves encoded key-value pairs to the KeyValueStore.
-// data layout: keyLen + key + valueLen + value. If the accumulated
+// data layout: keyLen + valueLen + key + value. If the accumulated
 // size or key count exceeds the given distance, a new range property will be
 // appended to the rangePropertiesCollector with current status.
 // `key` must be in strictly ascending order for invocations of a KeyValueStore.
-func (s *KeyValueStore) addEncodedData(val []byte) error {
-	_, err := s.dataWriter.Write(s.ctx, val)
+func (s *KeyValueStore) addEncodedData(data []byte) error {
+	_, err := s.dataWriter.Write(s.ctx, data)
 	if err != nil {
 		return err
 	}
 
-	keyLen := binary.BigEndian.Uint64(val)
-	key := val[lengthBytes : lengthBytes+keyLen]
+	keyLen := binary.BigEndian.Uint64(data)
+	key := data[2*lengthBytes : 2*lengthBytes+keyLen]
+
 	if len(s.rc.currProp.firstKey) == 0 {
 		s.rc.currProp.firstKey = key
 	}
 	s.rc.currProp.lastKey = key
 
-	s.offset += uint64(len(val))
-	s.rc.currProp.size += uint64(len(val) - lengthBytes*2)
+	s.offset += uint64(len(data))
+	s.rc.currProp.size += uint64(len(data) - 2*lengthBytes)
 	s.rc.currProp.keys++
 
 	if s.rc.currProp.size >= s.rc.propSizeDist ||
diff --git a/br/pkg/lightning/backend/external/file_test.go b/br/pkg/lightning/backend/external/file_test.go
index 65ab999b17476..f58be194c0988 100644
--- a/br/pkg/lightning/backend/external/file_test.go
+++ b/br/pkg/lightning/backend/external/file_test.go
@@ -16,6 +16,7 @@ package external
 
 import (
 	"context"
+	"encoding/binary"
 	"io"
 	"testing"
 	"time"
@@ -25,6 +26,15 @@ import (
 	"golang.org/x/exp/rand"
 )
 
+func getEncodedData(key, value []byte) []byte {
+	buf := make([]byte, 8*2+len(key)+len(value))
+	binary.BigEndian.PutUint64(buf, uint64(len(key)))
+	binary.BigEndian.PutUint64(buf[8:], uint64(len(value)))
+	copy(buf[8*2:], key)
+	copy(buf[8*2+len(key):], value)
+	return buf
+}
+
 func TestAddKeyValueMaintainRangeProperty(t *testing.T) {
 	ctx := context.Background()
 	memStore := storage.NewMemStorage()
@@ -71,9 +81,9 @@ func TestAddKeyValueMaintainRangeProperty(t *testing.T) {
 	require.NoError(t, err)
 	require.Len(t, rc.props, 1)
 
+	kvStore.Close()
 	err = writer.Close(ctx)
 	require.NoError(t, err)
-	kvStore.Close()
 	expected = &rangeProperty{
 		firstKey: k3,
 		lastKey:  k3,
@@ -152,6 +162,7 @@ func TestKVReadWrite(t *testing.T) {
 		err = kvStore.addEncodedData(getEncodedData(keys[i], values[i]))
 		require.NoError(t, err)
 	}
+	kvStore.Close()
 	err = writer.Close(ctx)
 	require.NoError(t, err)
 
diff --git a/br/pkg/lightning/backend/external/iter.go b/br/pkg/lightning/backend/external/iter.go
index 547d147132f1d..0847e7bad1f98 100644
--- a/br/pkg/lightning/backend/external/iter.go
+++ b/br/pkg/lightning/backend/external/iter.go
@@ -290,6 +290,9 @@ func (i *mergeIter[T, R]) next() bool {
 			i.readers[i.lastReaderIdx] = nil
 			delete(i.hotspotMap, i.lastReaderIdx)
 		default:
+			i.logger.Error("failed to read next element",
+				zap.String("path", rd.path()),
+				zap.Error(err))
 			i.err = err
 			return false
 		}
@@ -366,7 +369,7 @@ func NewMergeKVIter(
 	readerOpeners := make([]readerOpenerFn[*kvPair, kvReaderProxy], 0, len(paths))
 	largeBufSize := ConcurrentReaderBufferSizePerConc * ConcurrentReaderConcurrency
 	memPool := membuf.NewPool(
-		membuf.WithPoolSize(1), // currently only one reader will become hotspot
+		membuf.WithBlockNum(1), // currently only one reader will become hotspot
 		membuf.WithBlockSize(largeBufSize),
 		membuf.WithLargeAllocThreshold(largeBufSize),
 	)
diff --git a/br/pkg/lightning/backend/external/iter_test.go b/br/pkg/lightning/backend/external/iter_test.go
index e9b27f3793b6e..3a1db464ba827 100644
--- a/br/pkg/lightning/backend/external/iter_test.go
+++ b/br/pkg/lightning/backend/external/iter_test.go
@@ -60,15 +60,6 @@ func (r *trackOpenFileReader) Close() error {
 	return nil
 }
 
-func getEncodedData(key, value []byte) []byte {
-	buf := make([]byte, 8*2+len(key)+len(value))
-	binary.BigEndian.PutUint64(buf, uint64(len(key)))
-	copy(buf[8:], key)
-	binary.BigEndian.PutUint64(buf[8+len(key):], uint64(len(value)))
-	copy(buf[8*2+len(key):], value)
-	return buf
-}
-
 func TestMergeKVIter(t *testing.T) {
 	ctx := context.Background()
 	memStore := storage.NewMemStorage()
@@ -92,6 +83,7 @@ func TestMergeKVIter(t *testing.T) {
 			err = kvStore.addEncodedData(getEncodedData([]byte(kv[0]), []byte(kv[1])))
 			require.NoError(t, err)
 		}
+		kvStore.Close()
 		err = writer.Close(ctx)
 		require.NoError(t, err)
 	}
@@ -144,6 +136,7 @@ func TestOneUpstream(t *testing.T) {
 			err = kvStore.addEncodedData(getEncodedData([]byte(kv[0]), []byte(kv[1])))
 			require.NoError(t, err)
 		}
+		kvStore.Close()
 		err = writer.Close(ctx)
 		require.NoError(t, err)
 	}
@@ -222,6 +215,7 @@ func TestCorruptContent(t *testing.T) {
 			err = kvStore.addEncodedData(getEncodedData([]byte(kv[0]), []byte(kv[1])))
 			require.NoError(t, err)
 		}
+		kvStore.Close()
 		if i == 0 {
 			_, err = writer.Write(ctx, []byte("corrupt"))
 			require.NoError(t, err)
@@ -390,6 +384,7 @@ func TestHotspot(t *testing.T) {
 			err = kvStore.addEncodedData(getEncodedData([]byte(k), value))
 			require.NoError(t, err)
 		}
+		kvStore.Close()
 		err = writer.Close(ctx)
 		require.NoError(t, err)
 	}
diff --git a/br/pkg/lightning/backend/external/kv_reader.go b/br/pkg/lightning/backend/external/kv_reader.go
index 3f295f3f4252c..def354b18f884 100644
--- a/br/pkg/lightning/backend/external/kv_reader.go
+++ b/br/pkg/lightning/backend/external/kv_reader.go
@@ -51,26 +51,21 @@ func newKVReader(
 }
 
 func (r *kvReader) nextKV() (key, val []byte, err error) {
-	r.byteReader.reset()
 	lenBytes, err := r.byteReader.readNBytes(8)
 	if err != nil {
 		return nil, nil, err
 	}
-	keyLen := int(binary.BigEndian.Uint64(*lenBytes))
-	keyPtr, err := r.byteReader.readNBytes(keyLen)
-	if err != nil {
-		return nil, nil, noEOF(err)
-	}
+	keyLen := int(binary.BigEndian.Uint64(lenBytes))
 	lenBytes, err = r.byteReader.readNBytes(8)
 	if err != nil {
 		return nil, nil, noEOF(err)
 	}
-	valLen := int(binary.BigEndian.Uint64(*lenBytes))
-	valPtr, err := r.byteReader.readNBytes(valLen)
+	valLen := int(binary.BigEndian.Uint64(lenBytes))
+	keyAndValue, err := r.byteReader.readNBytes(keyLen + valLen)
 	if err != nil {
 		return nil, nil, noEOF(err)
 	}
-	return *keyPtr, *valPtr, nil
+	return keyAndValue[:keyLen], keyAndValue[keyLen:], nil
 }
 
 // noEOF converts the EOF error to io.ErrUnexpectedEOF.
diff --git a/br/pkg/lightning/backend/external/onefile_writer.go b/br/pkg/lightning/backend/external/onefile_writer.go
index b0dea86661744..996f336909fbc 100644
--- a/br/pkg/lightning/backend/external/onefile_writer.go
+++ b/br/pkg/lightning/backend/external/onefile_writer.go
@@ -106,10 +106,13 @@ func (w *OneFileWriter) WriteRow(ctx context.Context, idxKey, idxVal []byte) err
 		w.rc.reset()
 	}
 	binary.BigEndian.AppendUint64(buf[:0], uint64(keyLen))
-	copy(buf[lengthBytes:], idxKey)
-	binary.BigEndian.AppendUint64(buf[lengthBytes+keyLen:lengthBytes+keyLen], uint64(len(idxVal)))
+	binary.BigEndian.AppendUint64(buf[lengthBytes:lengthBytes], uint64(len(idxVal)))
+	copy(buf[lengthBytes*2:], idxKey)
 	copy(buf[lengthBytes*2+keyLen:], idxVal)
-	w.kvStore.addEncodedData(buf[:length])
+	err := w.kvStore.addEncodedData(buf[:length])
+	if err != nil {
+		return err
+	}
 	w.totalSize += uint64(keyLen + len(idxVal))
 	return nil
 }
diff --git a/br/pkg/lightning/backend/external/split_test.go b/br/pkg/lightning/backend/external/split_test.go
index a49f697b46116..738e0a021ec9f 100644
--- a/br/pkg/lightning/backend/external/split_test.go
+++ b/br/pkg/lightning/backend/external/split_test.go
@@ -102,13 +102,15 @@ func TestOnlyOneGroup(t *testing.T) {
 	subDir := "/mock-test"
 
 	writer := NewWriterBuilder().
-		SetMemorySizeLimit(15).
+		SetMemorySizeLimit(20).
 		SetPropSizeDistance(1).
 		SetPropKeysDistance(1).
 		Build(memStore, subDir, "5")
 
 	dataFiles, statFiles, err := MockExternalEngineWithWriter(memStore, writer, subDir, [][]byte{{1}, {2}}, [][]byte{{1}, {2}})
 	require.NoError(t, err)
+	require.Len(t, dataFiles, 1)
+	require.Len(t, statFiles, 1)
 
 	splitter, err := NewRangeSplitter(
 		ctx, dataFiles, statFiles, memStore, 1000, 30, 1000, 10, true,
diff --git a/br/pkg/lightning/backend/external/stat_reader.go b/br/pkg/lightning/backend/external/stat_reader.go
index a921a7f7a098b..5aaea5988973e 100644
--- a/br/pkg/lightning/backend/external/stat_reader.go
+++ b/br/pkg/lightning/backend/external/stat_reader.go
@@ -40,17 +40,16 @@ func newStatsReader(ctx context.Context, store storage.ExternalStorage, name str
 }
 
 func (r *statsReader) nextProp() (*rangeProperty, error) {
-	r.byteReader.reset()
 	lenBytes, err := r.byteReader.readNBytes(4)
 	if err != nil {
 		return nil, err
 	}
-	propLen := int(binary.BigEndian.Uint32(*lenBytes))
+	propLen := int(binary.BigEndian.Uint32(lenBytes))
 	propBytes, err := r.byteReader.readNBytes(propLen)
 	if err != nil {
 		return nil, noEOF(err)
 	}
-	return decodeProp(*propBytes), nil
+	return decodeProp(propBytes), nil
 }
 
 func (r *statsReader) Close() error {
diff --git a/br/pkg/lightning/backend/external/writer.go b/br/pkg/lightning/backend/external/writer.go
index 6556da432b784..6468e1413a8dd 100644
--- a/br/pkg/lightning/backend/external/writer.go
+++ b/br/pkg/lightning/backend/external/writer.go
@@ -179,7 +179,7 @@ func (b *WriterBuilder) Build(
 	if b.keyDupeEncoding {
 		keyAdapter = common.DupDetectKeyAdapter{}
 	}
-	p := membuf.NewPool(membuf.WithPoolSize(0), membuf.WithBlockSize(b.blockSize))
+	p := membuf.NewPool(membuf.WithBlockNum(0), membuf.WithBlockSize(b.blockSize))
 	ret := &Writer{
 		rc: &rangePropertiesCollector{
 			props:        make([]*rangeProperty, 0, 1024),
@@ -214,7 +214,7 @@ func (b *WriterBuilder) BuildOneFile(
 	writerID string,
 ) *OneFileWriter {
 	filenamePrefix := filepath.Join(prefix, writerID)
-	p := membuf.NewPool(membuf.WithPoolSize(0), membuf.WithBlockSize(b.blockSize))
+	p := membuf.NewPool(membuf.WithBlockNum(0), membuf.WithBlockSize(b.blockSize))
 
 	ret := &OneFileWriter{
 		rc: &rangePropertiesCollector{
@@ -339,9 +339,9 @@ func (w *Writer) WriteRow(ctx context.Context, idxKey, idxVal []byte, handle tid
 		}
 	}
 	binary.BigEndian.AppendUint64(dataBuf[:0], uint64(encodedKeyLen))
-	keyAdapter.Encode(dataBuf[lengthBytes:lengthBytes:lengthBytes+encodedKeyLen], idxKey, rowID)
-	binary.BigEndian.AppendUint64(dataBuf[lengthBytes+encodedKeyLen:lengthBytes+encodedKeyLen], uint64(len(idxVal)))
-	copy(dataBuf[lengthBytes*2+encodedKeyLen:], idxVal)
+	binary.BigEndian.AppendUint64(dataBuf[:lengthBytes], uint64(len(idxVal)))
+	keyAdapter.Encode(dataBuf[2*lengthBytes:2*lengthBytes:2*lengthBytes+encodedKeyLen], idxKey, rowID)
+	copy(dataBuf[2*lengthBytes+encodedKeyLen:], idxVal)
 
 	w.kvLocations = append(w.kvLocations, loc)
 	w.kvSize += int64(encodedKeyLen + len(idxVal))
@@ -377,7 +377,6 @@ func (w *Writer) Close(ctx context.Context) error {
 		zap.String("maxKey", hex.EncodeToString(w.maxKey)))
 
 	w.kvLocations = nil
-
 	w.onClose(&WriterSummary{
 		WriterID:           w.writerID,
 		Seq:                w.currentSeq,
@@ -513,7 +512,7 @@ func (w *Writer) flushKVs(ctx context.Context, fromClose bool) (err error) {
 func (w *Writer) getKeyByLoc(loc membuf.SliceLocation) []byte {
 	block := w.kvBuffer.GetSlice(loc)
 	keyLen := binary.BigEndian.Uint64(block[:lengthBytes])
-	return block[lengthBytes : lengthBytes+keyLen]
+	return block[2*lengthBytes : 2*lengthBytes+keyLen]
 }
 
 func (w *Writer) createStorageWriter(ctx context.Context) (
diff --git a/br/pkg/membuf/buffer.go b/br/pkg/membuf/buffer.go
index 3a999d75e7404..58252f692e9d2 100644
--- a/br/pkg/membuf/buffer.go
+++ b/br/pkg/membuf/buffer.go
@@ -49,17 +49,17 @@ type Pool struct {
 // Option configures a pool.
 type Option func(p *Pool)
 
-// WithPoolSize configures how many blocks cached by this pool.
-func WithPoolSize(size int) Option {
+// WithBlockNum configures how many blocks cached by this pool.
+func WithBlockNum(num int) Option {
 	return func(p *Pool) {
-		p.blockCache = make(chan []byte, size)
+		p.blockCache = make(chan []byte, num)
 	}
 }
 
 // WithBlockSize configures the size of each block.
-func WithBlockSize(size int) Option {
+func WithBlockSize(bytes int) Option {
 	return func(p *Pool) {
-		p.blockSize = size
+		p.blockSize = bytes
 	}
 }
 
diff --git a/br/pkg/membuf/buffer_test.go b/br/pkg/membuf/buffer_test.go
index 44e8441066985..1051e43e11b0e 100644
--- a/br/pkg/membuf/buffer_test.go
+++ b/br/pkg/membuf/buffer_test.go
@@ -42,7 +42,7 @@ func (t *testAllocator) Free(_ []byte) {
 func TestBufferPool(t *testing.T) {
 	allocator := &testAllocator{}
 	pool := NewPool(
-		WithPoolSize(2),
+		WithBlockNum(2),
 		WithAllocator(allocator),
 		WithBlockSize(1024),
 		WithLargeAllocThreshold(512),