Some filesystems try to optimize barrier flushes by maintaining
fs-specific generation counters, but if we introduce generic
flush generation counter for block device filesystems may use
it for fdatasync(2) optimization. Optimization should works if
userspace performs mutli-threaded IO with a lot of fdatasync()
Here are graphs for a test where each task performs random buffered writes
to dedicated file and performs fdatasync(2) after each operation.

Axis: x=nr_tasks, y=write_iops