VirtualGraph discarding does not delete associated files.

VirtualGraphSupport.discard() is documented:

/**
  * Removes this graph from the set of active graphs. If the graph is
  * workspace persistent, the contents are deleted from disk.
  * 
  * @return <code>true</code> if the virtual graph was properly discarded
  */

In practice, the implementation does not even attempt to delete the files. Thus, any application logic that works with assumption that discarding virtual graphs would remove the data does not work.