correct assign logic for rack level. Still need to fix data center

level.

go/topology/volume_growth.go

@@ -110,11 +110,21 @@ func (vg *VolumeGrowth) findEmptySlotsForOneVolume(topo *Topology, option *Volum
 		if option.Rack != "" && node.IsRack() && node.Id() != NodeId(option.Rack) {
 			return fmt.Errorf("Not matching preferred rack:%s", option.Rack)
 		}
+		if node.FreeSpace() < rp.SameRackCount+1 {
+			return fmt.Errorf("Free:%d < Expected:%d", node.FreeSpace(), rp.SameRackCount+1)
+		}
 		if len(node.Children()) < rp.SameRackCount+1 {
+			// a bit faster way test failed cases
 			return fmt.Errorf("Only has %d data nodes, not enough for %d.", len(node.Children()), rp.SameRackCount+1)
 		}
-		if node.FreeSpace() < rp.SameRackCount+1 {
-			return fmt.Errorf("Free:%d < Expected:%d", node.FreeSpace(), rp.SameRackCount+1)
+		possibleDataNodeCount := 0
+		for _, n := range node.Children() {
+			if n.FreeSpace() >= 1 {
+				possibleDataNodeCount++
+			}
+		}
+		if possibleDataNodeCount < rp.SameRackCount+1 {
+			return fmt.Errorf("Only has %d data nodes with a slot, not enough for %d.", possibleDataNodeCount, rp.SameRackCount+1)
 		}
 		return nil
 	})