avoid data loss after restarting a container with a volum server

weed/storage/volume.go

@@ -180,10 +180,16 @@ func (v *Volume) Close() {
 	v.dataFileAccessLock.Lock()
 	defer v.dataFileAccessLock.Unlock()
 	if v.nm != nil {
+		if err := v.nm.Sync(); err != nil {
+			glog.Warningf("Volume Close fail to sync volume idx %d", v.Id)
+		}
 		v.nm.Close()
 		v.nm = nil
 	}
 	if v.DataBackend != nil {
+		if err := v.DataBackend.Sync(); err != nil {
+			glog.Warningf("Volume Close fail to sync volume %d", v.Id)
+		}
 		_ = v.DataBackend.Close()
 		v.DataBackend = nil
 		stats.VolumeServerVolumeCounter.WithLabelValues(v.Collection, "volume").Dec()