As you known, BTRFS is the new, full-featured filesystem being developed for Linux. While BTRFS remain marked as “experimental”, it catalyze much attention in the filesystem community due to its impressive feature lists, relentless development and performance promises.
Speaking about performances, however, it seems that BTRFS has some significant problems, especially with database files and virtual machine images. With the latter, BTRFS speed is so low  that the official KVM tuning guide advise against using it .
Being granted that you should not use yet BTRFS for backing production-level virtual machine images (remember: it is marked as experimental), why are performances so low? What can we do for increase them? This article try to answer these questions.