[onetwotree]

It's possible that he refers to a cache miss "going out to disk" by requiring a read from a different data store (a SQL database, for example).

But this is really kind of silly - the entire idea with caching is that misses are much slower than hits. The statement is something like "you need a stupid good cache because misses hurt". Software engineering is always about tradeoffs, and antirez does a fantastic job of describing them in this post.