Before revision:  From FOOwe develop fast, practical methods to compute nearly tight bounds for the optimal hit ratio, which we call practical flow-based offline optimal (P-FOO). P-FOO enables the first analysis of optimal caching on realistic traces with hundreds of millions of requests.
After revision:  Indeed, FOO's error over 10M requests of production CDN and storage traces is negligible: at most 0.3\%. FOO thus reveals, for the first time, the limits of caching with variable object sizes. While FOO is very accurate, it is computationally impractical on traces with hundreds of millions of requests.