Assuming you need to work with quorum in a non-vnode scenario. That means that if 2 nodes in a row in the ring are down some number of quorum operations will fail with UnavailableException (TimeoutException right after the failures). This is because the for a given range of tokens quorum will be impossible, but quorum will be possible for others.
In a vnode world if any two nodes are down, then the intersection of vnode token ranges they have are unavailable.
I think it is two sides of the same coin.
On Mon, Dec 10, 2012 at 7:41 AM, Richard Low <email@example.com>
You're right, the math does assume independence which is unlikely to be accurate. But if you do have correlated failure modes e.g. same power, racks, DC, etc. then you can still use Cassandra's rack-aware or DC-aware features to ensure replicas are spread around so your cluster can survive the correlated failure mode. So I would expect vnodes to improve uptime in all scenarios, but haven't done the math to prove it.