One of the blogs I value is Roch Bourbonnais's. He hasn't blogged much in some time but he is back to blogging! He listed the main ZFS performance improvements since Oracle took over, and then also provided more details on ReARC. There is more to come from him, hopefully soon.
ps. to make it clear he is describing improvements to Oracle's ZFS as in Solaris 11 and ZFS-SA and not OpenZFS.