All,

I stumbled upon a Wikidata Scalability blog post by Magnus Manske (cc'd in on this message) [1] that contains the following excerpt:

"One of theproposed alternatives https://commons.wikimedia.org/wiki/File:WDQS_Backend_Alternatives_working_paper.pdfto blazegraph is Virtuoso. It models the graph structure of Wikidata in MySQL. Even before I read about that, I had a similar thought. But unlike Virtuoso, which is a general solution, Wikidata is a special case, with optimization potential. So I went to do a bit of tinkering. The following was done in the last few days, so it’s not exactly pretty, but initial results are promising."

That's incorrect. Virtuoso is a multi-model DBMS with its own SQL and RDF engines that have nothing to do with MySQL.

Magnus:

Uniprot hosts 100 Billion+ triples, accessed 24/7 globally, handling unpredictable query complexity and solution size unpredictability using a single-server instance of the open source edition of Virtuoso. The same applies to a vast majority of nodes in the  massive LOD Cloud.

Scalability has never been a Virtuoso issue since the very inception of the LOD Cloud bootstrapped by DBpedia (yet another Virtuoso instance).

Links:

[1] http://magnusmanske.de/wordpress/?p=691