def open
      @nodes.open
      @cache.clear
      node = @nodes.total_entries == 0 ?
        SpaceTreeNode::create(self) :
        SpaceTreeNode::load(self, @nodes.first_entry)
      @root_address = node.node_address
    end
def open
      @nodes.open
      @cache.clear
      node = @nodes.total_entries == 0 ?
        SpaceTreeNode::create(self) :
        SpaceTreeNode::load(self, @nodes.first_entry)
      @root_address = node.node_address
    end
Manage the free spaces tree in the specified directory
 @param dir [String] directory path of an existing directory
 Open the SpaceTree file.