Pretty sure it's still UFS+J. They've mentioned in the past that the benefits of ZFS don't really apply to the CDN boxes: if a drive dies, it's simply removed from the local cache, and files are re-downloaded/redistributed as necessary across the remaining cache drives the next time the appliance refreshes from its upstream origin. Additionally, ZFS prefetching was actually less performant in some cases from what I remember -- or at least providing no benefit -- because of how it couldn't really anticipate the I/O patterns of video segment requests from users.

Additional reference:

- https://news.ycombinator.com/item?id=8077594 ("…but caching is futile.")

- https://www.youtube.com/watch?v=FL5U4wr86L4