The end of block barriers

The problem is that in general an fsync requires a journal commit. If you have to flush the entire write cache throughput for non-fsync-serialized threads might be fine, but the performance of threads that call fsync will seriously suffer due to the delay.

Flushing the write cache of the device a lot also moves the request ordering / merging efficiency problem down a level. If the device must flush the entire write cache on a regular basis, it has much reduced opportunity to order write operations as optimal for that device.