Edit logging is a major source of contention within the NN. LogEdit is called within the namespace write log, while logSync is called outside of the lock to allow greater concurrency. The handler thread remains busy until logSync returns to provide the client with a durability guarantee for the response.
Write heavy RPC load and/or slow IO causes handlers to stall in logSync. Although the write lock is not held, readers are limited/starved and the call queue fills. Combining an edit log thread with postponed RPC responses from
HADOOP-10300 will provide the same durability guarantee but immediately free up the handlers.