Random read / write speed
Operating systems issue many small, random I/O operations. This is even more true when you put many OS onto one single physical server. In this case, random I/O speed become a crucial parameter.
Please note that the following IOMeter results were taken with cache=none only, as a writeback cache would unrealistically distort the results.
How BTRFS fares here?
Random read speed is very good, even better then EXT4.
However, random write are an entirely different matter: default BTRFS performances are 6x lower then EXT4! While disabling CoW and preallocating all disk blocks give quite an improvement, it remain very slow. As a side note, see how EXT4 preallocated score is way lower than not-preallocated one: it depend on EXT4 excellent sparse-file support.