Regarding HDFS-15382.Split FsDatasetImpl from blockpool lock to blockpool volume lock

[Xiaoqiao He <he...@apache.org>]

Hi guys,

I always hear that DataNode throughput could not improve recently because
of the global coarse-grain read-write lock on DataNod especially for
federation deployment. Actually about more than one years ago, we split it
into a fine-grain read-write lock for BlockPool/Volume grain in my internal
branch[1], it works well for a long time and the performance is very good
as expected, I invited my colleague Li Mingxiang to contribute this
improvement back to the community and now this work is continuing[2].

Now I try to involve more guys which is interested or who are experts on
DataNode to give some more comments or help to review the PR[3].

Welcome any suggestions or discussion here.

Thanks. Regards

- He Xiaoqiao

[1] https://issues.apache.org/jira/browse/HDFS-15382
[2] https://issues.apache.org/jira/browse/HDFS-15180
[3] https://github.com/apache/hadoop/pull/3941