A recent post on the Sesame developers list about persistent blank node maps got me into action as well as all this talk about XA2 of course (JRDF's modified RIO parser in Kowari/Mulgara uses a persistent StringToLongMap).
One of the reasons to use Derby is because it has an XAResourceManager. Though I don't expect it to scale or be as fast as most stores (even if the table size is supposed to be unlimited).
I couldn't find an easy way to create a DiskHashtable (although the TestDiskHashtable gave some clues). Here's how I managed to get a persistent DiskHashtable going called derbyDB (it may not be quite right of course):
String driverStr = "org.apache.derby.jdbc.EmbeddedDriver";
EmbeddedDriver driver = (EmbeddedDriver) Class.forName(driverStr).newInstance();
final EmbedConnection30 connection = (EmbedConnection30) DriverManager.getConnection(
final LanguageConnectionContext languageConnectionContext = connection.getLanguageConnection();
TransactionController controller = languageConnectionContext.getTransactionExecute();
ContextService service = ContextService.getFactory();
DiskHashtable diskHashtable = new DiskHashtable(controller, TEMPLATE, INDEXES, true, true);
I have some ideas that it might be possible to take the relational RDF operations and put them into Derby (or vice-versa). The datatype support would be nice to leverage (especially the XML datatype for example). It's very preliminary at the moment and I may ditch it in the future.