In debugging Zygo's huge commit delays I noticed we were burning a bunch of time doing READA in cases where we don't need to. The way READA works in btrfs is we'll load up adjacent nodes and leaves as we walk down. This is useful for operations where we're going to be reading sequentially across the tree. But for delayed refs we're looking up one bytenr, and then another one which could be elsewhere in the tree. With large enough extent trees this results in a lot of unneeded latency. The same applies to build_backref_tree, but that's even worse because we're looking up backrefs, which are essentially randomly spread out across the extent root. Thanks, Josef
