volume: sync to disk before copying volume files

address https://github.com/chrislusf/seaweedfs/issues/2976

weed/storage/volume.go

@@ -190,6 +190,21 @@ func (v *Volume) SetStopping() {
 	}
 }
 
+func (v *Volume) SyncToDisk() {
+	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)
+		}
+	}
+	if v.DataBackend != nil {
+		if err := v.DataBackend.Sync(); err != nil {
+			glog.Warningf("Volume Close fail to sync volume %d", v.Id)
+		}
+	}
+}
+
 // Close cleanly shuts down this volume
 func (v *Volume) Close() {
 	v.dataFileAccessLock.Lock()