That's why Varnish is more effective, because you can configure it to cache the results of things that WP Supercache misses by design including the Recent Comments Widget and its data. If you need SSDs to run Wordpress, there's a flaw somewhere. Every time I've scaled Wordpress, caching has been the answer. Apply liberally.
WP Supercache is a hack anyway, for folks running WP on shared hosts without root. If you have root, there's a plethora of better things for caching, even as ancient as Squid as a reverse. You can get your MySQL traffic down to <1 QPS fairly trivially, no matter what kind of traffic is hitting the frontend.
Don't forget wordpress.com is a huge MU installation, and they've existed since before SSDs became popular. The disk is not your issue here.
WP Supercache is a hack anyway, for folks running WP on shared hosts without root. If you have root, there's a plethora of better things for caching, even as ancient as Squid as a reverse. You can get your MySQL traffic down to <1 QPS fairly trivially, no matter what kind of traffic is hitting the frontend.
Don't forget wordpress.com is a huge MU installation, and they've existed since before SSDs became popular. The disk is not your issue here.