True, the physically addressable memory is "only" 256TB but the virtual space is still 64 bits. The point in time when the 256TB becomes a problem is when your working set (stuff resident in memory) is ideally more than 256TB. We've got mostly 96GB machines and those are modestly hard to exhaust. I don't think even a 1TB physical memory machine would be exhaustible with our current data sets.

Even today, reading in a 32GB index shard from SSD still takes a couple of minutes.