Ah, so this explains what you guys thought might be an interaction with Nagle?
Yep, turned out to be much simpler
The patch failed on Hudson due to
HDFS-3034 having removed the deprecated method. I'll commit this based on Aaron's +1 and based on my manual stress testing using HBase's HLog class, which uses this method.
No unit tests since it's hard to unit test for performance, and the hflush equivalent is already tested by TestMultithreadedHflush