Btrfs for Linux 4.5 includes fixes and clean-ups with over 70 commits being sent in for landing during the Linux 4.5 merge window. The big feature for Linux 4.5 is a new free space tree, but it's not yet enabled by default -- though testing it out is easy via the space_cache=v2 mount option. While experimental, it is possible to re-generate the tree in case of problems.
Btrfs lead developer Chris Mason explained that this forthcoming free space cache is tree-based and is faster with less overall work for updating as the commit progresses.
Development on this free space tree was led by Omar Sandoval, a computer engineering student that was interning at Facebook over the summer. This space cache is being redone as Facebook found that for large file-systems the free space caching code was taking a long time during commits. Fortunately, most users won't see these current limitations as the slowdowns were reported for file-systems of around 30+ Terabytes.