diff --git a/gobeansdb/web.go b/gobeansdb/web.go
index 2ec5524..4376299 100644
--- a/gobeansdb/web.go
+++ b/gobeansdb/web.go
@@ -10,7 +10,7 @@ import (
 	"runtime/debug"
 	"strconv"
 
-	yaml "gopkg.in/yaml.v2"
+	"gopkg.in/yaml.v2"
 
 	"github.com/douban/gobeansdb/cmem"
 	"github.com/douban/gobeansdb/config"
@@ -55,7 +55,6 @@ func init() {
 
 	http.HandleFunc("/statgetset", handleStatGetSet)
 	http.HandleFunc("/freememory", handleFreeMemory)
-
 }
 
 func initWeb() {
@@ -314,6 +313,15 @@ func handleGC(w http.ResponseWriter, r *http.Request) {
 	var err error
 	var bucketID int64
 	var pretend bool
+
+	isQuery := getGCQuery(r)
+	if isQuery {
+		gcResult := storage.hstore.GCBuckets()
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(gcResult)
+		return
+	}
+
 	defer func() {
 		if err != nil {
 			e := fmt.Sprintf("<p> err : %s </p>", err.Error())
@@ -328,17 +336,16 @@ func handleGC(w http.ResponseWriter, r *http.Request) {
 		}
 	}()
 
-	isQuery := getGCQuery(r)
-	if isQuery {
-		result = storage.hstore.GCBuckets()
+	bucketID, err = getBucket(r)
+	if err != nil {
 		return
 	}
+	bktID := int(bucketID)
 
-	bucketID, err = getBucket(r)
+	err = r.ParseForm()
 	if err != nil {
 		return
 	}
-	r.ParseForm()
 	start, err := getFormValueInt(r, "start", -1)
 	if err != nil {
 		return
@@ -353,16 +360,27 @@ func handleGC(w http.ResponseWriter, r *http.Request) {
 	}
 
 	s := r.FormValue("run")
-	pretend = (s != "true")
+	pretend = s != "true"
 
 	s = r.FormValue("merge")
-	merge := (s == "true")
+	merge := s == "true"
+
+	s = r.FormValue("cancel")
+	gcCancel := s == "true"
 
-	start, end, err = storage.hstore.GC(int(bucketID), start, end, noGCDays, merge, pretend)
-	if err == nil {
+	if gcCancel {
+		src, dst := storage.hstore.CancelGC(bktID)
+		if src == -1 {
+			result = fmt.Sprintf("bucket %d is not gcing", bktID)
+		} else {
+			result = fmt.Sprintf("cancel gc on bucket %d, src: %d -> dst: %d", bktID, src, dst)
+		}
+	} else {
+		start, end, err = storage.hstore.GC(bktID, start, end, noGCDays, merge, pretend)
 		result = fmt.Sprintf("<p/> bucket %d, start %d, end %d, merge %v, pretend %v <p/>",
 			bucketID, start, end, merge, pretend)
 	}
+	return
 }
 
 func handleDU(w http.ResponseWriter, r *http.Request) {
diff --git a/store/data.go b/store/data.go
index d5a6de7..78a348f 100644
--- a/store/data.go
+++ b/store/data.go
@@ -132,7 +132,9 @@ func (ds *dataStore) flush(chunk int, force bool) error {
 			filessize, w.offset, ds.genPath(chunk), &ds.chunks[chunk])
 	}
 	nflushed, err := ds.chunks[chunk].flush(w, false)
+	ds.Lock()
 	ds.wbufSize -= nflushed
+	ds.Unlock()
 	w.Close()
 
 	return nil
diff --git a/store/gc.go b/store/gc.go
index 58b9a3a..59b743e 100644
--- a/store/gc.go
+++ b/store/gc.go
@@ -25,7 +25,8 @@ type GCState struct {
 	Src int
 	Dst int
 
-	Err error
+	Err        error
+	CancelFlag bool
 	// sum
 	GCFileState
 }
@@ -210,10 +211,20 @@ func (mgr *GCMgr) gc(bkt *Bucket, startChunkID, endChunkID int, merge bool) {
 	defer mgr.AfterBucket(bkt)
 
 	gc.Dst = startChunkID
-	for i := 0; i < startChunkID; i++ {
+	// try to find the nearest chunk that small than start chunk
+	for i := startChunkID - 1; i >= 0; i-- {
 		sz := bkt.datas.chunks[i].size
-		if sz > 0 && (int64(sz) < Conf.DataFileMax-config.MCConf.BodyMax) {
-			gc.Dst = i
+		if sz > 0 {
+			if int64(sz) < Conf.DataFileMax-config.MCConf.BodyMax {
+				// previous one available and not full, use it.
+				gc.Dst = i
+				break
+			} else {
+				if i < startChunkID-1 { // not previous one
+					gc.Dst = i + 1
+				}
+				break
+			}
 		}
 	}
 
@@ -230,6 +241,10 @@ func (mgr *GCMgr) gc(bkt *Bucket, startChunkID, endChunkID int, merge bool) {
 	}()
 
 	for gc.Src = gc.Begin; gc.Src <= gc.End; gc.Src++ {
+		if gc.CancelFlag {
+			logger.Infof("GC canceled: src %d dst %d", gc.Src, gc.Dst)
+			return
+		}
 		if bkt.datas.chunks[gc.Src].size <= 0 {
 			logger.Infof("skip empty chunk %d", gc.Src)
 			continue
diff --git a/store/hstore.go b/store/hstore.go
index dac7999..553478c 100644
--- a/store/hstore.go
+++ b/store/hstore.go
@@ -6,7 +6,6 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
-	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -20,6 +19,7 @@ import (
 var (
 	logger    = loghub.ErrorLogger
 	mergeChan chan int
+	gcLock    sync.Mutex
 )
 
 type HStore struct {
@@ -246,15 +246,15 @@ func (store *HStore) ListDir(ki *KeyInfo) ([]byte, error) {
 	return store.ListUpper(ki)
 }
 
-func (store *HStore) GCBuckets() string {
-	var result strings.Builder
+func (store *HStore) GCBuckets() []string {
+
 	store.gcMgr.mu.Lock()
+	result := make([]string, 0, len(store.gcMgr.stat))
 	for k := range store.gcMgr.stat {
-		result.WriteString(strconv.FormatInt(int64(k), 16))
-		result.WriteString(",")
+		result = append(result, strconv.FormatInt(int64(k), 16))
 	}
 	store.gcMgr.mu.Unlock()
-	return strings.TrimSuffix(result.String(), ",")
+	return result
 }
 
 func (store *HStore) GC(bucketID, beginChunkID, endChunkID, noGCDays int, merge, pretend bool) (begin, end int, err error) {
@@ -288,10 +288,30 @@ func (store *HStore) GC(bucketID, beginChunkID, endChunkID, noGCDays int, merge,
 	if pretend {
 		return
 	}
+
 	go store.gcMgr.gc(bkt, begin, end, merge)
 	return
 }
 
+func (store *HStore) CancelGC(bucketID int) (src, dst int) {
+	// prevent same bucket concurrent request
+	gcLock.Lock()
+	defer gcLock.Unlock()
+
+	// will delete key at goroutine in store/gc.go
+	store.gcMgr.mu.RLock()
+	stat, exists := store.gcMgr.stat[bucketID]
+	store.gcMgr.mu.RUnlock()
+
+	if exists {
+		stat.CancelFlag = true
+		src, dst = stat.Src, stat.Dst
+	} else {
+		src, dst = -1, -1
+	}
+	return
+}
+
 func (store *HStore) IsGCRunning() bool {
 	var result bool
 	store.gcMgr.mu.RLock()
diff --git a/store/hstore_test.go b/store/hstore_test.go
index 63bbcf8..3b8d2af 100644
--- a/store/hstore_test.go
+++ b/store/hstore_test.go
@@ -1001,6 +1001,7 @@ func testGC(t *testing.T, casefunc testGCFunc, name string, numRecPerFile int) {
 	store.Close()
 	checkAllDataWithHints(bucketDir)
 }
+
 func TestGCConcurrency(t *testing.T) {
 	testGCConcurrency(t, 10000)
 }