We do have Fastly in front of *.github.io, but there's still a significant amount of traffic (on the order of thousands of requests per second throughout the day) that make it through to our own infrastructure.
We don't do any other caching on our own, although the other replies are correct in that the Linux kernel has its own filesystem cache which means not all requests end up hitting the SSD.
We don't do any other caching on our own, although the other replies are correct in that the Linux kernel has its own filesystem cache which means not all requests end up hitting the SSD.