So BTRFS is very efficient for big sequential read (which you generally don't care for much, because they're pretty fast in any case) and dies when subjected to small random read (which are the bane of platters in the first place)... isn't that dumb for a general-purpose FS?
What I got from that is that BtrFS sucks for doing lots of small synchronous writes, something that's relatively unique to VMs and is a major improvement over ext4 in just about everything else (feature set and performance). In fact, it's so unique it never popped up on the tests they do regularly on every patch.
