title: What's the difference between getUsableSpace and getUnallocatedSpace of FileStore class
body: <p>I've read the definition in the documentation and performed some searches on the Internet, but it is still not clear to me.  What's the difference between <code>getUsableSpace()</code> and <code>getUnallocatedSpace()</code> in the <code>FileStore</code> class?</p>