add writable checking

fix https://github.com/chrislusf/seaweedfs/issues/405

weed/topology/topology_vacuum.go

@@ -86,7 +86,14 @@ func (t *Topology) Vacuum(garbageThreshold string) int {
 		for _, vl := range c.storageType2VolumeLayout.Items() {
 			if vl != nil {
 				volumeLayout := vl.(*VolumeLayout)
+				writableSet := make(map[storage.VolumeId]bool)
+				for _, id := range volumeLayout.writables {
+					writableSet[id] = true
+				}
 				for vid, locationlist := range volumeLayout.vid2location {
+					if _, isWritable := writableSet[vid]; !isWritable {
+						continue
+					}
 					glog.V(0).Infof("check vacuum on collection:%s volume:%d", c.Name, vid)
 					if batchVacuumVolumeCheck(volumeLayout, vid, locationlist, garbageThreshold) {
 						if batchVacuumVolumeCompact(volumeLayout, vid, locationlist) {