What sorts of older things did they throw out? And is there not even eventual persistence with VoltDB? Is there a recovery log or some such? Otherwise it seems like serious power event and you're toast.

Well, a quick read of their white paper makes it look like they just copied the kdb design (http://cs.nyu.edu/shasha/papers/hpts.pdf - which they actually reference in one of their papers).

But perhaps they did something new as well?

edit: citeseer link was broken; replaced with link to shasha's website.

It's consistent, not only eventual consistent, it's k-safe, replicated and it's written to disk consistently at intervals you set. http://community.voltdb.com/node/54 (may require login). It's really pretty neat.

Isn't the D in ACID for 'durable'? The blog post implies that this is an in memory store only ... I feel I must be missing something.

"Durability" in that sense has nothing to do with how the data is stored, or how long it's available for.

It simply means that after a transaction has been reported as having been committed successfully, it won't ever be rolled back.

It makes sense why he's saying that. The people responsible for large, important data sets usually have lots of money to spend storing that data. Since they care about their data, they know better than to get involved with NoSQL databases, and actively shun anything that claims to be a NoSQL database. The real money will flow towards those providing real databases.