NameNode Fine-Grained Locking via Namespace Tree

As we all known, the write performance of NameNode is limited by the global lock. We target to enable fine-grained locking based on the Namespace tree to improve the performance of NameNode write operations.

There are multiple motivations for creating this ticket:

• We have implemented this fine-grained locking and gained nearly 7x performance improvements in our prod environment
• Other companies made similar improvements based on their internal branch. Internal branches are quite different from the community, so few feedback and discussions in the community.
• The topic of fine-grained locking has been discussed for a very long time, but still without any results.

We implemented this fine-gained locking based on the namespace tree to maximize the number of concurrency for disjoint or independent operations.