The latest patches build upon what I wrote about in February with A Patch That Can Make Btrfs 5~10% Faster. The idea comes down to providing an extent buffer cache for each i-node so that the item doesn't need to be searched from the root of a B+ tree everytime, thereby making searches quicker. The earlier patch wasn't merged but now it's up to a second version consisting of three patches and a stated 20% performance improvement for meta-data operations.
This patchset introduce extent buffer cache to btrfs. The basic idea is to reduce the search time and the contentions of the extent buffer lock by re-using the last search result.Besides providing an extent buffer cache for each i-node, a change now is also having extent buffer caches for delayed i-nodes and delayed reference with the Btrfs file-system. Hopefully the work will be merged soon but it may be too late for being properly vetted for the Linux 3.7 kernel. The patches are currently residing on the kernel mailing list.
I ran stress.sh, xfstests and some other tools to test it, all of them worked well.
As a performance improvement patchset, of course we did performance test. Because this patchset is to improve the b+ tree search, in other words, it improves the performance of the metadata operations, we use file creation test to measure it. So we ran 10 tasks, and all of them created 100000 files in their own directories at the same time. As the result, we found this patchset makes btrfs ~20% faster(98s -> 77s).