I would like to reopen an old topic, which is to introduce a Read Write lock to the datanode.
In the current trunk, a RentrentLock is used, so it is always exclusive. However there are many code paths in the DN where an exclusive lock is not necessary and a read lock would suffice. We know the ReentrantReadWrite lock scales fine, as it is used extensively in the namenode, so the performance of the lock should not be a concern. My proposal in https://issues.apache.org/jira/browse/HDFS-15150 is to start small on this, and simply replace the Reentrant lock with a ReentrantReadWrite lock, and then make all lock acquisitions take the write lock. That would keep the locking exactly as it is now, and hopefully result in a patch that is relatively easy to review. If we can agree on a patch for that, we can create followup Jiras to switch various code paths to use the readlock over time. I have a patch available on HDFS-15150, so I would appreciate any thoughts on the patch and this idea in general. Thanks, Stephen.
