Trying to compare flashcache's efficiency compared to ZFS' is apples to oranges. ZFS is not like any other file system on the planet. As such, I'd trust ZFS's caching to be best for ZFS in all situations. Not to mention the fact that its imperative that ZFS know about all caching(especially write caches). If you start using more caches(such as RAID controller caches) it can be very destructive to your pool. Even if Flashcache existed I'd call you a raging idiot if you actually tried to use it with ZFS. ZFS is very smart, and its dumb to try to circumvent it. It has the ability to handle its own caching with RAM, L2ARC, and SLOG. Trying to go further is very bad joo-joo.