diff --git a/internal/storage/utils.go b/internal/storage/utils.go
new file mode 100644
index 0000000000..e5d43f98f1
--- /dev/null
+++ b/internal/storage/utils.go
@@ -0,0 +1,66 @@
+// Copyright (C) 2019-2020 Zilliz. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software distributed under the License
+// is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+// or implied. See the License for the specific language governing permissions and limitations under the License.
+
+package storage
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+
+	"github.com/milvus-io/milvus/internal/kv"
+)
+
+// EstimateMemorySize get approximate memory size of a binlog file.
+//		normal binlog file, error = nil;
+//		key not exist, size = 0, error = nil;
+//		key not in binlog format, size = (a not accurate number), error != nil;
+//		failed to read event reader, size = (a not accurate number), error != nil;
+func EstimateMemorySize(kv kv.DataKV, key string) (int64, error) {
+	total := int64(0)
+
+	header := &baseEventHeader{}
+	headerSize := binary.Size(header)
+
+	startPos := binary.Size(MagicNumber)
+	endPos := startPos + headerSize
+
+	for {
+		headerContent, err := kv.LoadPartial(key, int64(startPos), int64(endPos))
+		if err != nil {
+			// case 1: key not exist, total = 0;
+			// case 2: all events have been read, total = (length of all events);
+			// whatever the case is, the return value is reasonable.
+			return total, nil
+		}
+
+		buffer := bytes.NewBuffer(headerContent)
+
+		header, err := readEventHeader(buffer)
+		if err != nil {
+			// FIXME(dragondriver): should we return 0 here?
+			return total, fmt.Errorf("failed to read event reader: %v", err)
+		}
+
+		if header.EventLength <= 0 || header.NextPosition < int32(endPos) {
+			// key not in binlog format
+			// FIXME(dragondriver): should we return 0 here?
+			return total, fmt.Errorf("key not in binlog format")
+		}
+
+		total += int64(header.EventLength)
+		// startPos = startPos + int(header.EventLength)
+		// 		||
+		// 		\/
+		startPos = int(header.NextPosition)
+		endPos = startPos + headerSize
+	}
+}
diff --git a/internal/storage/utils_test.go b/internal/storage/utils_test.go
new file mode 100644
index 0000000000..5a0c2f47d1
--- /dev/null
+++ b/internal/storage/utils_test.go
@@ -0,0 +1,212 @@
+// Copyright (C) 2019-2020 Zilliz. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software distributed under the License
+// is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+// or implied. See the License for the specific language governing permissions and limitations under the License.
+
+package storage
+
+import (
+	"bytes"
+	"crypto/rand"
+	"encoding/binary"
+	"testing"
+
+	"github.com/milvus-io/milvus/internal/util/funcutil"
+	"github.com/milvus-io/milvus/internal/util/uniquegenerator"
+
+	"github.com/milvus-io/milvus/internal/kv"
+
+	"github.com/stretchr/testify/assert"
+
+	memkv "github.com/milvus-io/milvus/internal/kv/mem"
+)
+
+type mockLessHeaderDataKV struct {
+}
+
+func (kv *mockLessHeaderDataKV) Load(key string) (string, error) {
+	panic("implement me")
+}
+
+func (kv *mockLessHeaderDataKV) MultiLoad(keys []string) ([]string, error) {
+	panic("implement me")
+}
+
+func (kv *mockLessHeaderDataKV) LoadWithPrefix(key string) ([]string, []string, error) {
+	panic("implement me")
+}
+
+func (kv *mockLessHeaderDataKV) Save(key, value string) error {
+	panic("implement me")
+}
+
+func (kv *mockLessHeaderDataKV) MultiSave(kvs map[string]string) error {
+	panic("implement me")
+}
+
+func (kv *mockLessHeaderDataKV) Remove(key string) error {
+	panic("implement me")
+}
+
+func (kv *mockLessHeaderDataKV) MultiRemove(keys []string) error {
+	panic("implement me")
+}
+
+func (kv *mockLessHeaderDataKV) RemoveWithPrefix(key string) error {
+	panic("implement me")
+}
+
+func (kv *mockLessHeaderDataKV) Close() {
+}
+
+func (kv *mockLessHeaderDataKV) LoadPartial(key string, start, end int64) ([]byte, error) {
+	header := &baseEventHeader{}
+
+	headerSize := binary.Size(header)
+	mockSize := headerSize - 1
+
+	ret := make([]byte, mockSize)
+	_, _ = rand.Read(ret)
+	return ret, nil
+}
+
+func newMockLessHeaderDataKV() *mockLessHeaderDataKV {
+	return &mockLessHeaderDataKV{}
+}
+
+type mockWrongHeaderDataKV struct {
+}
+
+func (kv *mockWrongHeaderDataKV) Load(key string) (string, error) {
+	panic("implement me")
+}
+
+func (kv *mockWrongHeaderDataKV) MultiLoad(keys []string) ([]string, error) {
+	panic("implement me")
+}
+
+func (kv *mockWrongHeaderDataKV) LoadWithPrefix(key string) ([]string, []string, error) {
+	panic("implement me")
+}
+
+func (kv *mockWrongHeaderDataKV) Save(key, value string) error {
+	panic("implement me")
+}
+
+func (kv *mockWrongHeaderDataKV) MultiSave(kvs map[string]string) error {
+	panic("implement me")
+}
+
+func (kv *mockWrongHeaderDataKV) Remove(key string) error {
+	panic("implement me")
+}
+
+func (kv *mockWrongHeaderDataKV) MultiRemove(keys []string) error {
+	panic("implement me")
+}
+
+func (kv *mockWrongHeaderDataKV) RemoveWithPrefix(key string) error {
+	panic("implement me")
+}
+
+func (kv *mockWrongHeaderDataKV) Close() {
+	panic("implement me")
+}
+
+func (kv *mockWrongHeaderDataKV) LoadPartial(key string, start, end int64) ([]byte, error) {
+	header := &baseEventHeader{}
+
+	header.EventLength = -1
+	header.NextPosition = -1
+
+	buffer := bytes.Buffer{}
+	_ = binary.Write(&buffer, binary.LittleEndian, header)
+
+	return buffer.Bytes(), nil
+}
+
+func newMockWrongHeaderDataKV() kv.DataKV {
+	return &mockWrongHeaderDataKV{}
+}
+
+func TestEstimateMemorySize(t *testing.T) {
+	memoryKV := memkv.NewMemoryKV()
+	defer memoryKV.Close()
+
+	key := "TestEstimateMemorySize"
+
+	var size int64
+	var err error
+
+	// key not in memoryKV
+	size, err = EstimateMemorySize(memoryKV, key)
+	assert.NoError(t, err)
+	assert.Zero(t, size)
+
+	// normal binlog key, for example, index binlog
+	indexBuildID := UniqueID(uniquegenerator.GetUniqueIntGeneratorIns().GetInt())
+	version := int64(uniquegenerator.GetUniqueIntGeneratorIns().GetInt())
+	collectionID := UniqueID(uniquegenerator.GetUniqueIntGeneratorIns().GetInt())
+	partitionID := UniqueID(uniquegenerator.GetUniqueIntGeneratorIns().GetInt())
+	segmentID := UniqueID(uniquegenerator.GetUniqueIntGeneratorIns().GetInt())
+	fieldID := UniqueID(uniquegenerator.GetUniqueIntGeneratorIns().GetInt())
+	indexName := funcutil.GenRandomStr()
+	indexID := UniqueID(uniquegenerator.GetUniqueIntGeneratorIns().GetInt())
+	indexParams := make(map[string]string)
+	indexParams["index_type"] = "IVF_FLAT"
+	datas := []*Blob{
+		{
+			Key:   "ivf1",
+			Value: []byte{1, 2, 3},
+		},
+		{
+			Key:   "ivf2",
+			Value: []byte{4, 5, 6},
+		},
+		{
+			Key:   "large",
+			Value: []byte(funcutil.RandomString(maxLengthPerRowOfIndexFile + 1)),
+		},
+	}
+
+	codec := NewIndexFileBinlogCodec()
+	defer codec.Close()
+
+	serializedBlobs, err := codec.Serialize(indexBuildID, version, collectionID, partitionID, segmentID, fieldID, indexParams, indexName, indexID, datas)
+	assert.Nil(t, err)
+
+	for _, blob := range serializedBlobs {
+		err = memoryKV.Save(blob.Key, string(blob.Value))
+		assert.Nil(t, err)
+
+		size, err = EstimateMemorySize(memoryKV, blob.Key)
+		assert.Nil(t, err)
+		assert.Equal(t, size+int64(binary.Size(MagicNumber)), int64(len(blob.Value)))
+	}
+}
+
+// cover case that failed to read event header
+func TestEstimateMemorySize_less_header(t *testing.T) {
+	mockKV := newMockLessHeaderDataKV()
+
+	key := "TestEstimateMemorySize_less_header"
+
+	_, err := EstimateMemorySize(mockKV, key)
+	assert.Error(t, err)
+}
+
+// cover case that file not in binlog format
+func TestEstimateMemorySize_not_in_binlog_format(t *testing.T) {
+	mockKV := newMockWrongHeaderDataKV()
+
+	key := "TestEstimateMemorySize_not_in_binlog_format"
+
+	_, err := EstimateMemorySize(mockKV, key)
+	assert.Error(t, err)
+}