shots which remain in use for a
         long time.  To prevent incorrect results due to cleanup of data which
         would otherwise be visible to the snapshot, an error is generated
         when the snapshot is older than this threshold and the snapshot is
         used to read a page which has been modified since the snapshot was
         built.
        </p><p>
         If this value is specified without units, it is taken as minutes.
         A value of <code class="literal">-1</code> (the default) d